What is electric bike 18650 BMS? What do they do
You hear a lot about BMS and it is kind of mysterious part in your electric bike. You know it must be something very important as everyone talks about it but you never see it. It is like gluten, even you don’t fully understand what it is, you just know it is something to pay attention to.
Let’s explain what is Gluten ? lol
BMS is short of Battery Management System. As you may guess they manage and control the battery pack in a way that all cells inside your battery work smoothly and in coordination.
Generally, if you have a dead electric bike battery, most common cause of it is using a cheap BMS. We hear much lower end electric bikes and conversion kits face it.
Before we dig deeper let me explain certain things about your battery.
Your battery consists of several battery cells which are produced by companies like Samsung, Panasonic, LG or similar manufacturers. So when an electric bike company says they use Samsung Battery actually they mean they use Samsung Cells in your battery pack.
So your battery pack is made from parallel and serial connection of those cells. Each of these cells has 3.6V nominal voltage. Recommended Max voltage per cell is 4.2 V and the lowest is 3.0V. If your battery cell charge goes lower than 3.0 V it’s internal chemistry changes and won’t be able to keep charge like before. We call it lazy battery. If it goes very low it is called a dead battery and you won’t be able to charge at all.
Even it is possible to charge your cells to 4.2 V each, it is not recommended as fully charged battery cells create stress inside the cell chemistry. The sweet spot for full charge battery cell is 4.05V. In this way you can use your battery for longer durations.
To summarize we need to keep each of our cells within 3.0V. 4.05 V range.
Ebike BMS’s main role is arranging each cell to work within these safe ranges. An electric bike battery will have around 52 or 65 cells inside like below one.
If one of those cells goes above 4.05 V or below 3.0V it shuts off the battery for protection. Because if you have one deadcell or lazy cell it affects the whole battery pack. Ebike batteries won’t tolerate any weak links.
What are other functions of E bike Battery BMS?
Besides protecting your cells being overcharged and over-discharged, A 18650 bms will also try to keep all your cells voltage to be at the same value. This is called cell balancing. At each charge, a good quality BMS like we use at Ariel Rider will balance your cells.
If your BMS don’t have cell balancing feature and one of your cells higher voltage than others it will reach 4.05 V earlier than other cells. In that case, BMS will stop charging to prevent overcharging. AS you can see your battery isn’t fully charged and you will start to see a decrease in your range. In the long run that can cause an upto 50% drop in your range just because your BMS don’t work well.
Also, BMS will check these parameters too;
- Short Circuit
- Over Current: Won’t let your batteries to be charged at un-safe current.
- Discharge Over Current: Won’t let your motor suck juice at high rates which may damage inside chemistry of the battery.
- Arrange communication between the controller of your electric bike and battery. It is done in two common ways; UART communication protocol and CANBUS communication protocol. You can learn more about them here
For the communication part, BMS always send SOH (State of Health) and SOP (State of Power) information to your e-bike controller too. In this way, it can know how much capacity is left in your battery pack and how long you can use it based on your current power consumption.
Where is BMS?
Bms is located inside your battery pack.
WHY BMS LIMIT CURRENT DISCHARGE?
This may sound little weird as your controller has a certain max limit of current that can suck from the battery. But in certain cases, some users bypass it and they manage to suck more juice or current from the battery. If it gets too many amps from the battery it can damage inside chemistry of your batteries and can cause serious problems.
We at Ariel Rider take battery subject very seriously and get every protection possible so you can ride your e-bike safely.
About the batteries, you can read more about What is Cell Balancing Article
How To Build A DIY Electric Bicycle Lithium Battery From 18650 Cells
Building a battery pack out of 18650 batteries is not easy, but it ain’t hard with websites like EBIKE SCHOOL. We could never write an article like this, so we teamed up with Ebike School and ask them to e of their articles about making proper 18650 packs. Note that article is complete copy of Ebike School article, we haven’t changed anything
Lithium 18650 Battery pack making
A lithium battery is the heart of any electric bicycle. Your motor is useless without all of that energy stored in your battery. Unfortunately though, a good ebike battery is often the hardest part to come by – and the most expensive. With a limited number of electric bicycle battery suppliers and a myriad of different factors including size, weight, capacity, voltage, and discharge rates, finding the exact battery you are looking for can be challenging and lead to unwanted compromises.
But what if you didn’t have to compromise? What if you could build your own ebike battery to your exact specifications? What if you could build a battery the perfect size for your bike, with all of the features you want, and do it for cheaper than retail? It’s easier than you think, and I’ll show you how below.
Now buckle up, grab a drink and get ready for some serious reading, because this isn’t a short article. But it will definitely be worth it in the end when you’re cruising around on your very own DIY ebike battery!
Safety disclaimer: Before we begin, it’s important to note that lithium batteries inherently contain a large amount of energy, and it is therefore crucial to handle them with the highest levels of caution. Building a DIY lithium battery requires a basic understanding of battery principles and should not be attempted by anyone lacking confidence in his or her electrical and technical skills. Please read this article in its entirety before attempting to build your own ebike battery. Always seek professional assistance if needed.
Note: At multiple points along this article I have inserted videos that I made demonstrating the steps involved in building a battery. The battery used in the videos is the same voltage but slightly larger capacity. The same techniques all still apply. If you don’t understand something in the text, try watching it in the video.
Tools and materials required:
- 18650 cells (more info on these below)
- Pure nickel strip
- Spot welder
- Hot glue gun
- Digital voltmeter
- Soldering iron and solder
- Kapton non-static tape
- BMS (battery management system)
- Short length of silicone wire (12-16 awg)
- Foam padding (optional)
- Large diameter shrink wrap or tape (optional, sort of)
- Heat gun or hair dryer (if using heat shrink tube)
- Electrical connectors
- Work gloves or latex gloves
- Safety goggles
650 lithium cell options
18650 cells, which are used in many different consumer electronics from laptops to power tools, are one of the most common battery cells employed in electric bicycle battery packs. For many years there were only mediocre 18650 cells available, but the demand by power tool makers and even some electric vehicle manufacturers for strong, high quality cells has led to the development of a number of great 18650 options in the last few years.
These cells are distinctive due to their cylindrical shape and are about the size of a finger. Depending on the size of the battery you plan to build, you’ll need anywhere from a few dozen to a few hundred of them.
There are many different types of 18650 cells out there to choose from. I prefer to use name brand cells from companies like Panasonic, Samsung, Sony and LG. These cells have well documented performance characteristics and come from reputable factories with excellent quality control standards. Name brand 18650’s cost a bit more, but trust me, they are worth it. A great entry-level cell is the Samsung ICR18650-26F cell. These 2,600 mAh cells should cost somewhere around 3-4 in any decent quantity and can handle up to 2C continuous discharge (5.2 A continuous per cell). I get my Samsung 26F cells from Aliexpress, usually from this seller but sometimes I’ve seen a better price here.
Name brand Samsung cells (INR18650-29E cells)
Many people are tempted to use cheaper 18650’s sold under names like Ultrafire, Surefire and Trustfire. Don’t be one of those people. These cells are often marketed as up to 5,000 mAh but struggle to get more than 2,000 mAh. In actuality, these cells are just factory rejects, purchased by companies like Ultrafire and repackaged in their own branded shrink wrap. These B-quality cells are then resold for use in low power devices like flashlights where their weaker performance is less of an issue. If a cell costs less than 2, it simply isn’t worth it. Stick to the name brand cells, like my favorite Samsung cells, if you want to build a safe, quality ebike battery.
Samsung ICR18650-26F cells straight from the factory
When it comes to buying your cells, you might be able to find a local source, or you can order them straight from Asia. I prefer the second option, as you’ll usually get a much better price going straight to the source, even when paying for international shipping. One caveat though: do your best to ensure that your source sells genuine cells and not knock-offs. Do this by checking feedback and using a payment method that ensures you can get your money back if the product isn’t as described. For this reason, I like to buy my cells on Alibaba.com and AliExpress.com.
For this tutorial, I’ll be using the green Panasonic 18650PF cells shown above. Lately though I’ve been using 18650GA cells like these, which are a little bit more energy dense, meaning more battery in less space.
Make sure to use only pure nickel strip
When it comes to the nickel strip you’ll be using to connect the 18650 batteries together, you will have two options: nickel-plated steel strips and pure nickel strips. Go for the pure nickel. It costs a little bit more than nickel plated steel but it has much lower resistance. That will translate into less wasted heat, more range from your battery, and a longer useful battery lifetime due to less heat damage to the cells.
Be warned: some less-than-honest vendors try to pass off nickel plated steel for the pure stuff. They often get away with it because it’s nearly impossible to distinguish between to the two with the naked eye. I wrote a whole article on some methods I developed for testing nickel strip to make sure you get what you paid for. Check it out here.
When it comes to nickel strip, I also like to use Aliexpress. You can also find it on ebay or even a local source if you’re lucky. Once I started building lots of batteries I began buying pure nickel strip by the kilogram here, but in the beginning I recommend you pick up a smaller amount. You can get pure nickel strip for a good price in smaller amounts from a seller like this one, but you’ll still get the best price by buying it in kilo or half kilo quanitites.
As far as dimensions, I prefer to use 0.1 or 0.15 mm thick nickel, and usually use a 7 or 8 mm wide strip. A stronger welder can do thicker strip, but will cost a lot more. If your welder can do 0.15 mm nickel strip then go for it; thicker is always better. If you have thinner strips then that’s fine too, just lay down a couple layers on top of each other when necessary to create connections that can carry more current.
Author’s note: Hi guys, Micah here. I run this site and wrote this article. I just wanted to let you know real quick about my new book, “DIY Lithium Batteries: How To Build Your Own Battery Packs” which is available in both ebook and paperback format on Amazon and is available in most countries. It goes into much deeper detail than this article and has dozens of drawings and illustrations showing you every step of designing and building a battery. If you find this free site helpful, then taking a look at my book can help support the work I do here to benefit everyone. Thanks! Ok, now back to the article.
Do I HAVE To Use a Spot Welder?
Well, let me put it differently: Yes, if you don’t want to damage your cells.
The first thing to know about lithium battery cells is that heat kills them. The reason we spot weld them is to securely join the cells together without adding much heat.
Sure, it is possible to solder directly to the cells (though it can be tricky without the right tools). The problem with soldering is that you add a lot of heat to the cell and it doesn’t dissipate very quickly. This speeds up a chemical reaction in the cell which robs the cell of its performance. The result is a cell that delivers less capacity and dies an earlier life.
Spot welders for batteries aren’t the same as most home spot welders. Unlike the large jaw spot welders for home workshops, battery spot welders have the electrodes on the same side. I’ve never seen them for sale in the US, but they can be found pretty easily on eBay and other international commerce websites. My full time use welder is a fairly simple model that I got here. A highly recommended source for a slightly nicer spot welder design (pictured below) with both mounted and handheld electrodes can be found here.
There are two main levels of spot welders currently available: hobby level and professional. A good hobby model should run about 200, while a good professional one can easily be ten times that price. I’ve never had a professional welder because I just can’t justify the cost, but I do own three different hobby models and have played around with many more. Their quality is very hit or miss, even on identical models from the same seller. Unfortunately the lemon ratio is quite high, meaning you could fork over a couple hundred bucks for a machine that just won’t work right (like my first welder!). Again, this is a good reason to use a site with buyer protection like Aliexpress.com.
I use my welders on 220V, though 110V versions are available. If you have access to 220V in your home (many 110V countries have 220V lines for clothes dryers and other high power appliances) then I’d recommend sticking with 220V. In my experience the 110V models seem to have more problems than their 220V brothers. Your mileage may vary.
The purchase price is often a turnoff for many people, but in reality 200 for a good hobby-level spot welder isn’t bad. All together, the supplies for my first battery, including the cost of the tools like the spot welder, ending up costing me about the same as if I had bought a retail battery of equal performance. That meant that in the end I had a new battery and I considered all the tools as free. Since then I’ve used them to build countless more batteries and made some huge savings!
Before you begin
A few tips before you get started:
Work in a clean area free of clutter. When you have exposed contacts of many battery cells all wired together, the last thing you want is to accidentally lay the battery down on a screwdriver or other metallic object. I once nearly spilled a box of paperclips on the top of an exposed battery pack while trying to move it out of the way. I can only imagine the fireworks show that would have caused.
Wear gloves. Work gloves, mechanic gloves, welding gloves, even latex gloves – just wear something. High enough voltage can conduct on the surface of your skin, especially if you have even slightly sweaty palms. I’ve felt the tingle enough times to always wear gloves now. In fact, my pair of choice for battery work are some old pink dish gloves. They are thin and provide great dexterity while protecting me from short circuits and sparks.
Remove all metallic jewelry. This is another tip that I can give from experience. Arcing the contacts on your battery is not something you want to happen ever, and especially not against your bare skin. I’ve had it happen on my wedding ring and once even had a burn mark in the shape of my watch’s clasp on my wrist for a week. Now I take everything off.
Wear safety goggles. Seriously. Don’t skip this one. During the process of spot welding it is not at all uncommon for sparks to fly. Skip the safety glasses and head for chemistry lab style goggles if you have them – you’ll want the wrap around protection when the sparks start bouncing. You’ve only got two eyes; protect them. I’d rather lose an arm than an eye. Oh, speaking of arms, I’d recommend long sleeves. Those sparks hurt when they come to rest on your wrists and forearms.
Ok, let’s build an electric bicycle battery!
You’re probably excited to start welding, but the first step is to plan out the configuration of your battery.
Most electric bicycle batteries fall into the 24V to 48V range, usually in 12V increments. Some people use batteries as high as 100 volts, but we’re going to stick to a medium sized 36V battery today. Of course the same principles apply for any voltage battery, so you can just scale up the battery I show you here today and build your own 48V, 60V or even higher voltage battery.
To reach our intended voltage of 36V, we have to connect a number of 18650 cells in series. Lithium-ion battery cells are nominally rated at 3.6 or 3.7V, meaning to reach 36V nominal, we’ll need 10 cells in series. The industry abbreviation for series is ‘s’, so this pack will be known as a “10S pack” or 10 cells in series for a final pack voltage of 36V.
Next, we’ll need to wire multiple 18650 cells in parallel to reach our desired pack capacity. Each of the cells I’m using are rated at 2,900 mAh. I plan to put 3 cells in parallel, for a combined capacity of 2.9Ah x 3 cells = 8.7 Ah. The industry abbreviation for parallel cells is ‘p’, meaning that my final pack configuration is considered a “10S3P pack” with a final specification of 36V 8.7AH.
Most commercially available 36V packs are around 10Ah, meaning our pack will be just a bit smaller. We could have also gone with a 4p configuration giving us 11.6 Ah, which would have been a slightly bigger and more expensive pack. The final capacity is totally defined by your own needs. Bigger isn’t always better, especially if you’re fitting a battery into tight spaces.
Next, plan out your cell configuration on your computer or even with a pencil and paper. This will help ensure you are laying out your pack correctly and show you the final dimensions of the pack. In my top-down drawing below I’ve designated the positive end of the cells in red and the negative end of the cells in white.
This is a very simple layout where each column of 3 cells is connected in parallel and then the 10 columns are connected across in series from left to right. The BMS board is shown at the far right end of the pack. You’ll see how the pack represented in the drawing will come together in real life shortly.
Below is a video I made showing how to design the cell layout of a battery.
Prepare your cells
Now that we’ve got all that pesky planning out of the way, let’s get started on the actual battery. Our work space is clear, all our tools are on hand, we’ve got our safety equipment on and we’re ready to go. We’ll begin by preparing our individual 18650 battery cells.
Test the voltage of each cell to make sure that they are all identical. If your cells came straight from the factory, they shouldn’t vary by more than a few percentage points from one to the next. They will likely fall in the range of 3.6-3.8 volts per cell as most factories ship their cells partially discharged to extend their shelf lives.
If any one battery cell varies significantly from the others, do NOT connect it to the other cells. Paralleling two or more cells of different voltages will cause an instantaneous and massive current flow in the direction of the lower voltage cell(s). This can damage the cells and even result in fire on rare occasions. Either individually charge or discharge the cell to match the others, or more likely, just don’t use it in your pack at all. The reason for the voltage difference could have something to do with an issue in the cell, and you don’t want a bad cell in your pack.
This is why I always use name brand cells now. The only time I’ve ever received factory direct cells with non-matched voltages is when buying unbranded cells.
Once I’ve got all the cells I need checked out and ensured they have matching voltages, I like to arrange them on my work surface in the orientation of the intended pack. This gives me one final check to make sure the orientation will work as planned, and a chance to see the real-life size of the pack, minus a little bit of padding and heat shrink wrap.
This is approximately how the pack should look when the battery is finished
Prepare your nickel
I like to cut most of my nickel strip in advance so I can just weld straight through without breaking my flow to stop and cut more nickel. I measured out the width of three cells and cut enough nickel strip to weld the top and bottoms of 10 sets of 3 cells, meaning 20 strips of nickel that were each 3 cells wide, plus a couple spares in case I messed anything up.
Nickel strips cut from the roll
The nickel is surprisingly soft, which means you can use an ordinary pair of scissors to cut it. Try not to bend it too much though, as you want it to remain as flat as possible. If you do bend the corners with the scissors, you can easily bend them back down with your finger.
Prepare your parallel groups for welding
You’ll need someway to hold your cells in a straight line while welding, as free-handing is harder than it looks. I have a nice jig (that I received as a free ‘gift’ with the purchase of one of my welders) for holding my cells in a straight line while welding. However, before I received it I used a simple wooden jig I made to hold the cells while I hot glued them into a straight line.
My “real” 18650 spot welding jig
My old wooden 18650 hot gluing jig
Either way works, but my orange jig saves me one hot glue step which just makes for a cleaner looking pack. Of course it’s all the same after the pack gets covered with shrink wrap, so you can use any method you’d like. I’ve even found that some of those cylindrical ice cube trays are perfectly sized to hold 18650 cells. Cutting off the top would leave it clear for welding. I’d add some strong neodymium magnets to the backside to hold the cells in place like my orange jig has, but other than that it’s a perfect jig almost as-is.
An ice cube tray that makes a perfect 18650 spot welding jig
Time to start welding!
Alright, here’s the moment everyone’s been itching for. Let’s weld up our cells.
Now the game plan here is to weld parallel groups of 3 cells (or more or less for your pack depending on how much total capacity you want). To weld the cells in parallel, we’ll need to weld the tops and the bottoms of the cells together so all 3 cells share common positive and negative terminals.
There are different models of welders out there but most of them work in a similar way. You should have two copper electrodes spaced a few millimeters apart on two arms, or you might have handheld probes. My machine has welding arms.
Lay your nickel strip over the tops of your cells and lift up against the welding probes to initiate a weld
Lay your nickel strip on top of the three cells, ensuring that it covers all three terminals. Turn your welder on and adjust the current to a fairly low setting (if it’s your first time using the welder). Perform a test weld by placing the battery cells and copper strip below the probes and lifting up until the welding arms raise high enough to initiate the weld.
You’ll see two dots where the weld was performed. Test the weld by pulling on the nickel strip (if it’s your first time using the welder). If it doesn’t come off with hand pressure, or requires a lot of strength, then it’s a good weld. If you can easily peel it off, turn the current up. If the surface looks burnt or is overly hot to the touch, turn the current down. It helps to have a spare cell or two for dialing in the power of your machine.
This is how your cells should look after the first set of welds
Continue down the row of cells placing a weld on each cell. Then go back and do another set of welds on each cell. I like to do 2-3 welds (4-6 weld points) per cell. Any less and the weld isn’t as secure; any more and you’re just unnecessarily heating the cell. and more welds won’t increase the current carrying ability of the nickel strip very much. The actual weld point isn’t the only place where current flows from the cell to the strip. A flat piece of nickel will be touching the whole surface of the cell cap, not just at the points of the weld. So 6 weld points is plenty to ensure good contact and connection.
Here are the cells with a couple more welds
Once you’ve got 2-3 welds on the top of each cell, turn the 3 cells over and do the same thing to the bottom of the 3 cells with a new piece of nickel. Once you’ve completed the bottom welds you’ll have one complete parallel group, ready to go. This is technically a 1S3P battery already (1 cell in series, 3 cells in parallel). That means I’ve just created a 3.6V 8.7Ah battery. Only nine more of these and I’ll have enough to complete my entire pack.
Now weld the same way on the opposite side of the cells
Next, grab another 3 cells (or however many you are putting in your parallel groups) and perform the same operation to make another parallel group just like the first one. Then keep going. I’m making eight more parallel groups for a total of 10 parallel groups.
Below is a video I made showing how to perform the spot welding steps on a battery.
Assembling parallel groups in series
Now I’ve got 10 individual parallel groups and I’m going to assemble them in series to make a single ebike battery pack.
10 parallel groups all welded up with nowhere to go…
When it comes to layout, there are two ways to assemble cells in straight packs (rectangular packs like I am building). I don’t know if there are industry terms for this, but I call the two methods “offset packing” and “linear packing”.
Offset packing results in a shorter pack because the parallel groups are offset by half a cell, taking up part of the space between the cells of the previous parallel group. However, this results in a somewhat wider pack as the offset parallel groups extend to each side by a quarter of a cell more than they would have in linear packing. Offset packing is handy for times where you need to fit the pack into a shorter area (such as the frame triangle) and don’t care about the width penalty.
Linear packing, on the other hand, will result in a narrower pack that ends up a bit longer than offset packing. Some people say offset packing is more efficient because you can fit more cells in a smaller area by taking advantage of the space between cells. However, offset packing creates wasted space on the ends of parallel group rows where gaps form between the edge of the pack and the ‘shorter’ rows. The larger the battery pack, the less wasted space is taken up compared to the overall pack size, but the difference is negligible for most packs. For my battery, I decided to go with offset packing to make the pack shorter and fit easier into a small triangle bag.
When it comes to welding your parallel groups in series, you’ll have to plan out the welds based on your welder’s physical limits. The stubby arms on my welder can only reach about two rows of cells deep, meaning I will need to add a single parallel group at a time, weld it, then add another one. If you have handheld welding probes then you could theoretically weld up your whole pack at once.
And I’d be theoretically jealous of you.
Since most welders have arms like mine, I’ll show you how I did it. I started by hot gluing two parallel groups together in an offset fashion, making sure the ends were opposite (one positive and one negative at each end, as shown in the picture). Then I snipped a pile of nickel strips long enough to bridge just two cells.
Note that the parallel groups are aligned with opposite poles
I placed the first parallel group positive side up, and the second parallel group negative side up. I laid the nickel strips on top of each of the three sets of cells, bridging the positive caps of the first parallel group with the negative terminal of the second parallel group, as shown in the picture.
I then put one set of welds on each cell end of the first parallel group, effectively tacking the three nickel strips in place. Then I added another set of welds on each of the negative terminals of the second parallel group. This gave me 6 weld sets, or one weld set for each cell. Lastly, I followed up those single weld sets with another couple welds per cell to ensure good contact and connection.
Next, I added the third parallel group after the second, hot gluing it in place in the same orientation as the first, so the top of the pack alternates from positive terminals to negative terminals and back to positive terminals along the first three parallel groups.
Now this step is very important: I’m going to turn the pack upside-down and perform this set of welds between the positive caps on the second parallel group and negative terminals on the third parallel group. Essentially, I’m welding on the opposite side of the pack as I did when I connected the first two parallel groups. Skip down a few pictures to see the completely welded pack to understand how the alternating side system works.
Why do we alternate sides of the pack during the welding process? We do it because in this way we connect the positive terminal of each parallel group to the negative terminal of the next group in line. That’s how series connections work: always positive to negative to positive to negative, alternating between the two.
When we add the fourth parallel group, we’ll again hot glue it in place in the opposite orientation of the third parallel group (and the same orientation of the second parallel group) and then weld it on the opposite side as we welded between the second and third group (and the same side as we welded between the first and second group).
This pattern continues until we’ve got all 10 parallel groups connected. In my case, you can see that the first and last parallel groups aren’t welded on the top side of the pack. That is because they are the “ends” of the pack, or the main positive and negative terminals of the entire 36V pack.
Each of the cell groups not connected at the top are connected underneath
Adding the BMS (Battery Management System)
The battery cells have now been assembled into a larger 36V pack, but I still have to add a BMS to control the charging and discharging of the pack. The BMS monitors all of the parallel groups in the pack to safely cut off power at the end of charging, balance all the cells identically and keep the pack from being over-discharged.
A BMS isn’t necessarily strictly required – it is possible to use the pack as is, without a BMS. But that requires very careful monitoring of the cells of the battery to avoid damaging them or creating a dangerous scenario during charging or discharging. It also requires buying a more complicated and expensive charger that can balance all of the cells individually. It’s much better to go with a BMS unless you have specific reasons to want to monitor your cells by yourself.
The BMS I chose is a 30A maximum constant discharge BMS, which is more than I’ll need. It’s good to be conservative and over-spec your BMS if possible, so you aren’t running it near its limit. My BMS also has a balance feature that keeps all of my cells balanced on every charge. Not all BMS’s do this, though most do. Be wary of extremely cheap BMS’s because that’s when you’re likely to encounter a non-balancing BMS.
To wire the BMS, we first need to determine which of the sense wires (the many thin wires) is the first one (destined for the first parallel group). Look for the wires to be numbered on one side the board. Mine is on the backside of the board and I forgot to take a picture of it before installing it, but trust me that I took note of which end the sense wires start on. You don’t want to make a mistake and connect the sense wires starting in the wrong direction.
Make sure to consult the wiring diagram for your BMS, because some BMS’s have one more sense wire than cells (for example, 11 sense wires for a 10S pack). On these packs, the first wire will go on the negative terminal of the first parallel group, with all the rest of the wires going on the positive terminal of each successive parallel group. My BMS only has 10 sense wires though, so each will go on the positive terminal of the parallel groups.
The wiring diagram supplied with my BMS
Before actually wiring the BMS to the pack, I hot glued it to a piece of foam to insulate the contacts on the bottom of the board and then hot glued that foam to the end of the battery.
Then I took the sense wire labeled B1 and soldered it to the positive terminal of the first parallel group (which also happens to be the same as the negative terminal of the second parallel group, as they are connected together with nickel strip).
When soldering these wires to the nickel strip, try to solder between two cells and not directly on top of a cell. This keeps the heat source further from the actual cell ends and causes less heating of the battery cells.
I then took my second sense wire (or your third sense wire if you have one more sense wires than parallel groups) and soldered it to the positive terminal of the second parallel group. Again, note that I’m soldering this wire to the nickel in between cells to avoid heating any cell directly.
I continued with all 10 sense wires, placing the last one on the positive terminal of the 10th parallel group. If you aren’t sure about which groups are which, or you get confused, use your digital voltmeter to double check the voltages of each group so you know you are connecting each wire to the correct group.
The last step of wiring the BMS is to add the charge and discharge wires. The pack’s positive charge wire and discharge wire will both be soldered directly to the positive terminal of the 10th parallel group. The negative charge wire will be soldered to the C- pad on the BMS and the negative discharge wire will be soldered to the P- pad on the BMS. I also need to add one wire from the negative terminal of the first parallel group to the B- pad on the BMS.
You’ll notice that for my charge wires I used larger diameter wires than the sense wires that came with the BMS. That’s because charging will deliver more current than those sense wires will. Also, you’ll notice the discharge wires (including the B- pad to the negative terminal of the pack) are the thickest wires of all of them, as these will carry the entire power of the whole pack during discharging. I used 16 awg for the charge wires and 12 awg for the discharge wires.
You’ll also notice in the following pictures that my charge and discharge wires are taped off at the ends with electrical tape. This is to keep them from accidentally coming in contact with each other and short circuiting the pack. A friend of mine recently tipped me off to another (and probably better) option to prevent shorts: add your connectors to the wires first, then solder them onto the pack and BMS. Doh!
Below is a video I made showing how to add a BMS to a lithium battery.
Sealing your DIY ebike battery with heat shrink
This step is somewhat optional. You should seal your battery somehow to prevent it from shorting on all of that exposed nickel, but it doesn’t necessarily have to be with heat shrink wrap. Some people use duct tape, plastic wrap, fabric, etc. In my opinion though, shrink wrap is the best method because it not only provides a largely water resistant (though not water-proof) seal, but also provides constant and even pressure on all of your connections and wires, reducing the risk of vibration damage.
Before I seal my batteries in heat shrink, I like to wrap them in a thin layer of foam for added protection. This helps keep the ends of your cells from getting dinged if the battery receives any rough treatment, which can happen accidentally in the form of a dropped battery or ebike accident. The foam also helps to dampen the vibrations that the battery will experience on the bike.
Cutting foam to size before wrapping
I use white 2mm thick craft foam and cut out a shape slightly larger than my pack. I wrap it up and seal it with electrical tape. It doesn’t have to be pretty, it just has to cover the pack. Your next step will hide the foam from view.
Next comes the heat shrink tube. Large diameter heat shrink tube is hard to find, and I got lucky with a big score of different sizes from a Chinese vendor before his supply dried up. Your best bet is to check sites like eBay for short lengths of heat shrink in the size you need.
A quick note: when you get into large sizes of heat shrink, the method of quoting the size often changes from referring to the diameter of the tube to referring to the flat width (or half the circumference when in a circle). This is because at these large sizes, it’s not so much a tube anymore as two flat sheets fused together, sort of like an envelope. Keep that in mind and know what size is being quoted when you buy your large diameter heat shrink tube.
There are formulas out there for calculating the exact size of heat shrink you need but I often find them overly complicated. Here’s how I figure out what size I need: take the height and width of the pack and add them together, and remember that number. The size of heat shrink you need when measured by the flat width (half the circumference) is between that number you found and twice that number (or ideally between slightly more than that number to slightly less than twice that number).
Why does this formula work? Think about it: heat shrink (unless stated otherwise) usually has a 2:1 shrink ratio, so if I need something with less than twice the circumference (or perimeter rather, since my pack isn’t really a circle) of my pack. Since large diameter heat shrink is quoted in half circumference (flat width) sizes, and I want heat shrink with a circumference of a bit more than the perimeter of my pack, then I know I need the half circumference size to be a bit more than half of my pack’s perimeter, which is equal to the height plus the width of my pack.
That might of sounded confusing, so let’s talk in real numbers. My pack is about 70 mm high and about 65 mm wide. That means that half of the perimeter of my pack is 70 65 = 135 mm. So I need some heat shrink tubing that has a flat width (or half circumference) of between 135 to 270 mm, or to be safer, more like between 150-250mm. And if possible, I want to be on the smaller end of that range so the heat shrink will be tighter and hold more firmly. Luckily, I have some 170mm heat shrink tube which will work great.
One more thing to note about large diameter heat shrink: unless otherwise stated, this stuff usually shrinks about 10% in the long direction, so you’ll want to add a bit extra to the length to account for both overlap and longitudinal shrinkage.
But there’s still another issue: now if I just slip my pack inside some shrink wrap tube, I’ll still have exposed ends. This is more or less ok structurally, though it won’t be very water resistant and it will look a bit less professional.
So I’m going to first use a wider (285 mm to be exact) but shorter piece of shrink wrap to go around the long direction of the pack. That will seal the ends first, and then I can go back with my long and skinny piece of heat shrink to do the length of the pack.
If you don’t have an actual heat gun, you can use a strong hair dryer. Not all hair dryers will work, but my wife’s 2000 watt model is great. I own a real heat gun but actually prefer to use her hair dryer because it has finer controls and a wider output. Just don’t go mess up your wife’s hair dryer!
Sliding on and shrinking the second layer
Now I’ve got all of my pack sealed in heat shrink with my wires exiting the seam between the two layers of shrink wrap. I could have stopped here, but I didn’t particularly like the way the shrink fell on the wire exit there, from a purely aesthetic standpoint. So I actually took a third piece of shrink wrap, the same size (285 mm) as that first piece and went around the long axis of the pack one more time to pull the wires down tight to the end of the pack.
That resulted in a total of three layers of shrink wrap which makes for one very protected battery!
Below is a video I made showing how to heat shrink a lithium battery.
The only thing left to do at this point is to add the connectors, unless you did that before you soldered the wires on, which I actually recommend doing. But of course I didn’t do that, so I added them at this step, being careful not to short them by connecting only one wire at a time.
You can use any connectors you like. I’m a big fan of Anderson PowerPole connectors for the discharge leads. I used this other connector that I had in my parts bin for the discharge wires. I’m not sure what that type of connector is called, but if someone wants to let me know in the Комментарии и мнения владельцев section then that’d be great!
You can also add a label or other information to the outside of your pack for that professional look. If nothing else, it’s a good idea to at least write on the pack what the voltage and capacity is. Especially if you make multiple custom batteries, that will ensure you never forget what the correct charge voltage for the pack is.
You’ll also want to test out the battery with a fairly light load in the beginning. Try to go for an easy ride on the first few charges, or even better, use a discharger if you have one. I built a custom discharger out of halogen light bulbs. It allows me to fully discharge my batteries at different power levels and measure the output. This specific battery gave 8.54 Ah on its first discharge cycle at a discharge rate of 0.5c, or about 4.4 A. That result is actually pretty good, and equates to an individual average cell capacity of about 2.85 Ah, or 98% of the rated capacity.
Manufacturers usually rate their cells’ capacity at very low discharge rates, sometimes just 0.1c, where the cells perform at their maximum. So don’t be surprised if you’re only getting 95% or so of the advertised capacity of your cells during real world discharges. That’s to be expected. Also, your capacity is likely to go up a bit after the first few charge and discharge cycles as the cells get broken in and balance to one another.
I didn’t include a charging a section in this article, as this was just about how to build a lithium battery. But here’s a video I made showing you how to choose the appropriate charger for your lithium battery.
Now it’s your turn!
Now you’ve got all the info you should need to make your own electric bicycle lithium battery pack. You might still need a few tools, but at least you’ve got the knowledge. Remember to take it slow, plan everything out in advance and enjoy the project. And don’t forget your safety gear!
A video version of my how-to:
If you’re like me, then you like hearing and seeing how things are done, not just reading about them. That’s why I also made a video showing all the steps I took here in one single video. The battery I build in this video is not the same exact battery, but it’s similar. It’s a 24V 5.8AH battery for a small, low power ebike. But you can simply add more cells to make a higher voltage or higher capacity pack to fit your own needs. Check out the video below:
I’ll leave you with a little more inspiration
Now I’m sure you’re all jazzed about building your own battery pack. But just in case, I’m going to leave you with an awesome video featuring battery builder Damian Rene of Madrid, Spain building a very large, very professionally constructed 48V 42AH battery pack from 18650 cells. You can read about how he built this battery here. (Also, note in the video his good use of safety equipment!)
Micah is a mechanical engineer, tinkerer and husband. He’s spent the better part of a decade working in the electric bicycle industry, and is the author of The Ultimate DIY Ebike Guide. Micah can usually be found riding his electric bicycles around Florida, Tel Aviv, and anywhere else his ebikes wind up.
In the future we are going to show you how Micah simplified the process of making custom packs
Check out Ebike School for more informative posts
At EbikeSchool.com you can learn everything you want to know about electric bicycles to help you find the right ebike for you, or better yet, convert your own bicycle into an ebike!
Introduction: How to Make a Lithium Battery for an Electric Bicycle
Electric bicycles use batteries made from lithium ion cells. One of the most common types is a cylindrical cell called an 18650 cell, named so because it is 18 mm in diameter and 65 mm long. I’ll show you how you can create your own DIY electric bicycle battery from these cells for much less than the cost of a retail ebike battery.
It’s actually quite easy, and because the for these cells get even better when you buy more of them, I often buy an extra large pile of cells and just make extra batteries to sell locally. That way the batteries I make for myself end up being free!
To make your own electric bicycle lithium battery you’ll need the following, and more details about each are included below:
- Lithium cells
- Battery management system (BMS)
- Spot welder
- Nickel strip
- Volt meter
- Soldering iron and solder
- Heat shrink tube
- Foam sheet
- Hot glue
- Miscellaneous wires and connectors
Lithium 18650 cells
All lithium-ion cells are 3.7V, and you’ll need to wire them in series to get the correct total voltage for your ebike battery, and in parallel to increase the capacity. There are a bunch of different cells on the market, each with their own advantages and disadvantages. I used Panasonic 18650pf cells in this battery, which are 2.9AH each and can deliver a maximum of 10A continuously. If you want a little more capacity, you can go with Sanyo 18650GA cells that are 3.5AH each and also provide 10A continuously. If you don’t need as much power though, the most economical cell is the Samsung 26F cell, which is 2.6AH and can provide about 5A continuous. The Samsung cell is better for ebikes that don’t need as much of a high power battery. Most of my ebikes use Samsung 26F cells because I like to build packs with larger capacities and use them on medium power ebikes. Just remember that because they can only provide 5A continuous per cell, you might need to use more of them in parallel. For example, in a 30A continuous pack, you’d need at least 6 cells in parallel.
I get my cells from Aliexpress, where payment to a vendor is held in escrow until you receive your goods and confirm that the goods match the description. I prefer it better than ebay, because this way I know my money is safe and I can get it back if I have an issue with a seller. But I only use reputable sellers of battery cells, like those linked above, so I haven’t had an issue with cells yet.
Battery Management System (BMS)You’ll need a BMS to monitor your cells during charging and discharging. Basically it protects the cells from getting drained too far or getting overcharged. When choosing a BMS you need to match two main factors: voltage and current rate (more important for discharge than charge current). If you are building a 36V battery, you’ll need a 36V BMS (or usually called 10s, meaning 10 cells in series) to match your battery. A 48V battery uses a 13s BMS and a 52V battery uses a 14s BMS. Just make sure you choose a BMS configured for the same amount of cells as the battery you are building. Also remember to check the discharge current. If you want your battery to be able to handle 20A continuously, choose a BMS that is rated at least 20A, and higher is better to give you a safety buffer.
You really need to use a spot welder to make a lithium battery out of 18650 cells. It is technically possible to solder the cells together, but it creates a lot of heat on the end of the cells that can damage them and prevent them giving their full capacity. I have a few different inexpensive spot welders. Even with the price of the spot welder, your DIY lithium battery is likely to end up costing less than a retail ebike battery. Plus you can make a few more batteries and sell them! I got all of my spot welders from Aliexpress for the same reason as my cells. because I know I’ll get a good product or my money back! I like to use a fairly simple spot welder without hand probes which I got on sale for about 150, but I’ve also had good experiences with the 709 series welders and others that have extra hand probes, though they cost a bit more.
A friend of mine wanted to build his own battery but didn’t want to invest in a spot welder. He ended up buying one, building his battery and then selling the spot welder for more than he paid for it on ebay, since they are pretty rare in the US. Whatever works for you!
You’ll use nickel strip to join your cells together. Make sure you get 100% nickel strip and not nickel plated steel, which is cheaper but has much higher resistance. Be careful, some vendors try to sell nickel plated steel strip as real 100% nickel strip since it is nearly impossible to tell. To ensure you received genuine 100% nickel strip you can either use the spark test or the salt water test as described here.
Heat shrink tube
You’ll need some large diameter heat shrink tube to seal your battery. I picked up some 10 meter rolls of many different sizes of heat shrink tubing from 110mm all the way up to 300mm. You can get it by the 1 meter length though if you aren’t building as many batteries as I am.
The rest of the parts and tools you need are smaller and I’ve covered them in the following steps.
Step 1: Determine the Size and Shape of Your Battery
I wanted to make my battery fit inside this under-seat bag. I also wanted the battery to be 36V, meaning I’d need 10 cells in series. This limits me to multiples of 10 cells.
To see how many cells I could fit in that bag, I laid it on a piece of paper and traced the outline. Then I just started putting cells on the paper within the drawing until I couldn’t fit anymore.
It turned out that I could get 30 in there, but 40 was going to be too much. So I settled on a 30 cell battery, meaning 10 cells in series and 3 in parallel for a 36V 8.7AH pack (2.9AH per cell x 3 cells = 8.7AH). This was going to be a nice small pack for a lightweight folding bicycle and should be good for about 20 mph and a little under 20 miles of range.
To mark approximately where the cells would go in the battery, I removed each cell from the paper one at a time and drew a circle in its place. This would help me with the wiring diagram in the next step.
Step 2: Plan Out the Order and Wiring of Your Cells
Next I colored every other group of 3 cells darker to differentiate the parallel groups. The dark circles represented positive ends of the cells and the white circles represented negative ends of the cells. Each group would have 3 cells wired in parallel (positive ends together and negative ends together).
To decide the order of the cells, I simply started at the small end of the bag and named the first set of 3 cells “group 1”. Then I drew a line connecting the top (positive) of those cells to the negative of the cell group sitting next to them. That’s why half the cells are upside down, so that the positives and negatives of adjacent groups can be connected in series.
I then continued, making sure each successive group of parallel cells was connected to the next, positive to negative and negative to positive.
It is important to keep track of your connections as you draw them. On the opposite side of the paper I traced the circles and colored them opposite to front side of the paper. that way it was like a real-life model with positive ends of cells on one side of the paper and negative ends on the other. If the positive and negative of two parallel groups was connected on one side of the paper, I made sure not to connect them on the other. Otherwise that would have resulted in a short. You really want to avoid that. Shorted battery cells will heat up quickly and can catch fire or explode.
The final connection went like this:
1- connected only to itself
10 connected only to itself
Step 3: Check That All Cells Are Equal Voltage
This step is very important! You’ll need to ensure that all the cells you plan to use are the same voltage. They can be /- a couple hundredths of a volt, but more than that and you’ll have a pretty good amount of current flowing through them trying to equalize them when you connect them in parallel.
If you have brand new cells straight from the factory, they should all be basically identical. All of my cells read 3.63V except for one cell which read 3.59V. That’s probably still a decent cell, but the fact that it has self discharged somewhat meant that it wasn’t quite up to the standards as the others, so I replaced it with another cell that was identical to the rest. I can still use that cell in other projects in the future, especially lower power ones. I just don’t want to risk putting a cell that might have an issue into a larger pack with a bunch of perfect cells.
Step 4: Start Hot Gluing and Spot Welding Your Cells
Now you are ready to start putting your pack together. Depending on the size and shape of your pack, you’ll either start by welding or hot gluing. The first parallel group in my pack was arranged in a triangle, so I started by gluing the cells together, then spot welded them.
I put about 6 or 8 spot weld points on each cell for each layer of nickel strip. The nickel strip I used is 7mm wide and 0.15mm thick. I had at least 5 pieces of nickel strip connecting each group in series so that there was a lot of material for the current to flow through. Some people connect all of their cells in parallel and then just use a single strip of nickel to make the series connection but this is a bad idea. It results in all of the current trying to cram its way through a single, thin piece of nickel. It’s better to put many strips of nickel stacked on top of each other for the series connections. Think of it like a road. A single strip of nickel is like a one lane street, and 5 pieces of nickel stacked on top of each other are like a 5 lane highway. the highway can handle a lot more traffic zipping along it!
The welding arms on my spot welder can only reach about 2-3 cells deep in a pack, so I only glue a couple parallel groups onto the pack at a time, do the welds, then glue more on. If you have a welder with handheld probes then you can actually glue the whole pack together from the beginning and then weld it all at once.
Step 5: Continue Welding Your Cells
Continue gluing and welding your cells until you reach the final group. In my case, I would put the entire pack onto my paper template after each parallel group was added just to confirm that I was maintaining the shape that I needed.
If you use a square shape, this will be much easier as the cells will line up naturally and you won’t have to keep checking to make sure your pack stays within its planned shape.
After I finished all of my welding, I found that my cells did fit in the bag, but it was really tight, and I still had to add some foam and heat shrink before it would be finished. To account for this, I decided to rearrange my pack just slightly. I removed two of the cells from group 9, which was the battery’s widest part near the rear of the battery, and I moved them to the absolute rear of the pack where I had more room left. On one side of the pack I could still weld these directly to last group (group 10), but on the other side I didn’t have a straight shot, so I used a short length of thick wire soldered to the nickel already welded to the cells.
Step 6: Prepare Your Connectors for Charging and Discharging
I like to prepare my connectors before I add them to the battery. This way I have less chance of shorting the pack on accident. For the charging connector I chose RCA connectors. I use a female on the battery and a male on the charger.
For making the female end on the battery I’ve developed this neat trick of actually using mono to RCA adapters because it gives me a female RCA connector with a lot of soldering surface and makes a rigid, strong connector.
I used 16 awg silicone wire for the charger connectors and held the wire and connector in a helping hands device, which just makes the soldering easier. I started by soldering the positive wire to the end of the mono adapter, then covered it with heat shrink. Next I soldered the negative wire to the long barrel of the mono adapter and covered the whole connector with heat shrink.
I apologize that I forgot to take pictures of adding the discharge connector, but I just used Anderson PowerPole connectors crimped onto the end of 12 awg wires.
Step 7: Connect Wires to the BMS
I like to add my wires to the BMS before I connect the BMS. There are 3 wires that need to be soldered onto the board: the C- (charging negative), P- (the pack’s negative, i.e. the negative wire that will exit the pack and plug into your controller) and B- (the battery’s negative, i.e. the negative end of the first parallel group of cells).
I soldered all three of these wires to the board after checking to make sure that I had cut the wires long enough. I used 14 awg silicone wire for the B- and P- connections.
Lastly, I wrapped the entire BMS in polyimide high temperature, non conductive tape and then hot glued it to the pack with a thin piece of foam underneath. The foam gives a small amount of shock protection and the tape and foam together ensure there won’t be a short between the bottom of the board and cells if the heat shrink were to ever break on the cells.
Step 8: Add BMS Connections
Next I soldered all the little cell wires (10 in all) for the BMS. Each one is marked 1 through 10 so you know where to solder each one. Note that I soldered them to the nickel plate in between cells and not right over a cell. This helped keep as much heat out of the cells as possible. you don’t want to heat the cells themselves if you can avoid it.
Anywhere I had wires running over cells, especially the ends of the cells with exposed nickel strip, I used the non conductive tape to create a barrier, just in case.
After the cell connections were complete, I then added the main charge and discharge wires. The P- from the BMS goes out to the discharge connector for the pack while the B- gets wired to the negative end of the first cell group. The thick red wire is soldered to the positive end of the 10th cell group and exits the pack along with the P- wire to the discharge connector.
Again, I tried to do all of my soldering in between cells on the nickel strip to avoid heating the cells themselves.
Step 9: Wrap Your Pack in Foam
Some battery builders skip this step but I think it is important. I use a thin foam layer to surround my cells and give them a bit more padding and protection. I usually use a 2mm thin sheet of foam but on this pack I decided to use an even thinner 1mm sheet of foam because it was already going to be a tight fit. I cut the foam to the approximate shape of the pack, leaving the foam a bit long on all sides so that it will end up being two layers thick on the corners. the areas most likely to receive jostling and impacts.
I used my same heat resistant tape to seal the foam. it doesn’t have to be pretty.
Step 10: Add Heat Shrink
Now it’s time to finish off your battery with some professional looking heat shrink. Most heat shrink will shrink to about 50% of its normal diameter and about 10% of its length, so keep this in mind when sizing the right size heat shrink for your battery. The method I use to calculate the right size is to measure the perimeter of the pack in whichever direction I will be surrounding it, then use that number to calculate the size heat shrink tube that I need, which will end up being anything between that number and twice that number, with the sweet spot being somewhere in between.
It’s fairly simple in practice. For example, my first piece of heat shrink tubing I used went around the pack I made in the long direction. I measured the pack and found the perimeter of that shape to be about 42cm. Heat shrink tubing is normally measured by the diameter, but really big heat shrink tubing like this is often measured by the half circumference instead because it comes flat, not round like small heat shrink tubing for wires. So if the perimeter of my pack is 42 cm, and the heat shrink will shrink to half of its size, that means I need a heat shrink section with a half perimeter of between 21 and 42 cm (though it’s better to stay away from the extreme ends of that range so the heat shrink doesn’t end up being too tight or too loose. I ended up using 26 cm heat shrink for this piece.
For any piece of heat shrink that is slipped around the sides of the pack, meaning it runs 90 degrees to the direction of the cells, I cut it 11 cm wide. This 11 cm has proven to be the magic number that gives it enough overhang at the tops and bottoms of the cells to wrap around them but not too much that you get extra floppy material that has to be cut off.
You should use a heat gun on the heat shrink tube, but make sure you don’t turn it up too high or you can actually burn or melt the heat shrink. My heat gun is quite powerful and so I often use my wife’s hair dryer on high which works great for heat shrink tubing!
After your first piece of heat shrink tubing, you’ll likely want to add a second piece going in another direction to cover the ends of the pack. For my pack, the circumference at the widest part was about 35 cm, and so I used 190 mm heat shrink for this section.
Step 11: Optional.- Add a Handle
I wanted to make sure it was easy to remove the battery from the bag even with a tight fit, so I added this handle to the pack. I laid some 1″ nylon webbing around the pack to form a circle and added a little extra to allow me to fit a few fingers into the handle.
I marked the overlap and took it over to my sewing machine. I picked up this cool beginner sewing machine that has proven to be super handy. I’m using it for all sorts of projects that I wouldn’t have expected. like in battery building! I’m still a beginner on my sewing machine but I think the stitching came out pretty well and it felt plenty secure.
I placed the loop around the battery and hot glued it in place on three sides, leaving a handle formed at the back end of the battery.
I should have incorporated a piece of heat shrink tubing into the loop before I sewed it, but I forgot, so I had to think of a good way to cover the small end of the pack. I first tried to use a small piece of heat shrink tubing but it wouldn’t stay in place since it was on a wedge shape and just slid down the pack as it shrunk, ultimately falling off the tip.
Instead, I had to cheat a bit. I developed this method for when I want to get heat shrink onto a wedge shape but just can’t get it to stay by itself. First I cut a piece of heat shrink tubing sized to cover the small end of the pack and extend almost all the way to the large end of the pack. Then I glued it in place at the far ends so it wouldn’t slip back down. Then I slide a piece of heat shrink tube over it and heated that in place. When that piece shrunk down, it locked the piece of heat shrink under it, keeping it firmly in place. Lastly I applied heat to the tip of the battery and that piece of heat shrink I had originally cut to place there sealed the end of the battery, covering the nylon strap at the tip and stayed in place due to the heat shrink above it holding it tightly.
Step 12:. And That’s It!
That’s everything! I test fit the battery and it slid in the bag nice and snug. The little door at the back covers the connectors and allows me to access them without removing the battery each time.
I hope you found this helpful and feel free to ask any questions in the Комментарии и мнения владельцев below!
If you’d like an even more detailed writeup, I created a how-to article here, and I also made a video on YouTube that shows this whole process on a different shaped battery here.
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Lithium battery enjoys a good reputation in the electric vehicle industry. After years of improvement, it has developed a couple of variations that have its own strength.
18650 lithium battery originally refers to NI-MH and Lithium-ion battery. Now it mostly represents Lithium-ion battery since NI-MH battery has been used in a less frequency.
The 18650 gets its name from its cell size: 18.6 mm in diameter and 65.2 mm in length with weight around 47g.
LiFePO4(LFP) and LiNiaCobMncO2(NCM) are the most popular types. Tesla has performed a larger number of battery tests in order to find out the best battery for its electric vehicle and 18650 is the winner. The 18650 became the base of battery technology development in Tesla‘s production line ever since.
Before being adopted into the electric vehicle industry, the 18650 has long been used in electronics like laptops, cameras, which makes it the earliest, most stable type of lithium battery. After years of development, the manufacturers use the technical improvement that gains from consumer electronics products on vehicle batteries.
Compared to other inferior forms of battery, the 18650 is highly compatible with various applications that have different voltage, current, and size requirements.
Safer in structure
The 18650 was tested to be nontoxic, inflammable, nonexplosive, and free of contamination, under the certification of RoHS.
Thermal stability under high temperature also outperforms other Li-ion systems. It sees 100% discharge rate at 65℃.
Further more, the battery cell is sealed in a steel cylinder, which allows possibility of smaller size. Thanks to its tiny shape, containing a small amount of energy, the failure of single battery unit posts minimum impact to overall battery performance.
To those outdoor electric bike company like Super73, higher safty level is important for the battery to resist shocking when ride on rough terrain. Tesla has been cautious on battery selection as the fraction between battery cells that generated by different road conditions can result in serious hazards like explosure.
Excellent Energy Density
A single 18650 lithium battery has capacitance of 1200mAh – 3600mAh while others have around 800mAh. This feature enable the battery set to go beyond 5000mAh if they are put together.
At the same weight, the capacity of 18650 battery can be 1.5-2 times of that in NI-MH battery. The self-discharge rate is also low.
According to data provided from Tesla, the specific energy level can go as high as 250Wh/kg to meet the range requirement of Tesla electric vehicles.
High Price-Performance Ratio
The 18650 battery has fairly long expectancy in comparison with regular batteries. The cycle life can be 1000-2000 under proper maintenance. After years of development, the structural design, manufacturing technology and the equipment are competent, allowing lower cost of operation and maintenance.
For its outstangding performance, the 18650 lithium battery can also be found in Super73‘s production lines, the most popular vintage electric bikes brand in the U.S.
However, the 18650 lithium battery is still facing some drawbacks like high self-heating, complicated grouping and slow charging, which are the exact reason that Tesla collaborating with battery giant Panasonic to develop the 21700 lithium battery.