How to Replace a BMS in an e-Bike Battery
How to replace the BMS of an eBike battery — how to test, buy a replacement, and replace it.
Dana Hooshmand
I’m an entrepreneur and adventure traveller. I build software, immerse myself in other cultures and languages @ Discover Discomfort and make fun of the business world @ The Vanity Metric. posts by Dana Hooshmand.
Dana Hooshmand
Recently (as in a few months ago) I had a problem with my e-Bike where my battery wouldn’t hold a charge, and wouldn’t accept a charge from the charger.
Normally, when I plug my eBike‘s battery in, the charger’s cooling fan whirls to life and one of the lights changes colour, indicating it’s charging. This stopped happening!
I did a suite of tests, tried a few techniques recommended on forums and by the lead tech at Luna Cycle, and eventually confirmed my BMS was broken and needed to be replaced.
So in this guide I want to outline
- A bit about my bike (as background)
- How to test your BMS to confirm it needs replacing (or whether it’s something else)
- Tests to do on a BMS and battery before you replace it
- Why a BMS might fail
- Where to get a BMS to replace it with — what to look for, and what to pay; and
- How to change the BMS over
About my electric bike
My electric bike is semi-custom. It is built like a custom, but by Luna Cycle in the USA. It has a Bafang (八方电气, “Eight Sides Electrical Appliances”) motor with a 50A controller that Luna Cycle calls their “Ludicrous” controller, though I have it set to operate in 25A mode.
Because this was a crazy year, it ended up being stored fully charged, and allowed to fully deplete, while in storage. This isn’t good for an eBike battery, it turns out, and so a few things needed to be done to get it back on the road.
The BMS — or Battery Management System — of a battery pack is the part that regulates both input (charging) and output (discharge) voltage and current from the cells.
Even though this might appear to be a guide for one kind of electric bike battery, it’s generally applicable to all e-Bike batteries, and even batteries on other devices like scooters and mobility devices.
Why? Because many of these batteries are built with the same underlying technology — a pack of lithium 18650 cells.
How to test an electric bike’s BMS/charging system
When the charging system of an electric bike fails, it means one of a few things:
- Supply failure: The charger might be broken (not delivering voltage or current necessary to charge)
- Mechanical failure: A connector wiring inside the battery might be broken,
- Cell failure: Some of the cells inside the battery may have failed, or
- BMS failure: The battery management system may not be operating
To get to the core of the problem, you have to test everything one by one.
The first (and easiest) thing to test is the charger. You measure the output voltage. For a 52V battery like mine, it should be supplying about 58V. For a 48V battery it should supply around 54V.
After you measure the output voltage, you do what’s called the “light bulb test” — where you use an incandescent bulb hooked up to the outlet. This is easier in America (or Japan I guess) where the voltage supply is 110V, but it still works with 220V bulbs.
You could also test it with an automotive bulb if you have one. But it might blow!
Second, test for mechanical failures. Probe around with a multimeter and make sure you read operating voltage in the places where you should.
Also, open up your e-bike battery and check all the wires are intact, and that none of the solder joints have broken. Bikes get beaten up and it’s possible — likely — that a joint will fail at some point, especially if your battery has gone flying across the road because you forget to lock it (guilty! Actually I lost the key for a while. )
Finally, you have to test the internals of the battery.
I did a suite of tests that Luna Cycle said I should do
- Opened it up and tested all the wires and connections
- Did a BMS battery reset
- Tested voltages across the pins — making sure every individual cell was operating correctly
One trick for testing voltage across the pins of the BMS is that they’re often coated with silicon. You should scrape it away gently before checking the voltage.
How does the BMS of an e-Bike battery fail?
A BMS is a delicate issue. There are actually people who believe in charging batteries without a BMS, like this guy on YouTube:
If “jumping” your BMS is unsucessful, you can do more extensive testing on your battery pack and on your BMS.
Watch the video below. The core of it is to check individual cell voltage (confirming they’re in the 3.6-3.8V range), making sure no cell is dead. If it’s dead, you can replace it, probably for about 15-20 of parts (and. a spot welder and some nickel strips).
Look at the number of pins and the style of connector at the top of the BMS.
It seems a lot of BMS manufacturers have an informal agreement as to what the connector should look like. This is good news!
The second rating to look for is the current rating. My bike is rated for 50A peak, so I found a controller that promised to get to that spec.
I would treat current ratings on eBay with a grain of salt. It’s possible they might be truthful, but it’s possible they’re wildly exaggerating. Given they’re so cheap, get the biggest spec one you can reasonably afford, assuming it’ll be a weak point.
Installing the new BMS
There are three steps to installing the new BMS.
Firstly, remove the connector at the top. Mine is a 14-pin connector; you might have 10 or 12 pins or some other number. This should be a plug-and-play replacement for your current BMS.
Secondly, use a low-power soldering iron to de-solder the three connectors at the bottom.
Finally, use the soldering iron to connect the wires to your new BMS.
You should now be ready to power up and give your repaired battery a go. If you’re lucky, like I was, then your charger will whirr to life and your battery will take a full charge.
Optional — I realised, as I was writing this, that there was a chance I could have destroyed the BMS again! The battery was discharged, and a surge might have fried it.
Luckily, this didn’t happen. But you should consider perhaps directly recharging each cell of the battery pack if you have a 3.6V battery charger available.
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Range Anxiety? Be Prepared (and Stop Worrying)
Want to take a long trip with an ebike? Just want to proof yourself against running out of juice on your commute? Here are a variety of solutions.
I’ve put rather a lot of effort into proofing myself against running out of battery juice. In all the years I have been using an ebike as a daily driver – almost always for utility rather than for recreation – I have never run out of battery power. Even when I’ve forgotten to charge before a ride (more on that below).
There Are Solutions
Lets explore some range-extension options. Hopefully you’ll come across something here you hadn’t thought of and can take advantage of.
Use a Big Battery
This is the most obvious one. If you don’t want to run out of gas, put in a big gas tank. This is not a new idea. Nowadays when a gearhead hears about a Corvette Z06, a super fast, light and powerful version of that car comes to mind… but back in 1963, if your option code was RPO Z06, that meant you had the “big tank” Corvette… with a freaking 36 gallon gas tank to minimize refueling stops during races. Or Cannonball runs.
If you are doing a DIY ebike conversion, unless you have specific weight goals, you typically want to fit the biggest battery you can afford. Same goes for a manufactured ebike. If it has a larger battery option… you want that. Whether you can take advantage of an option will boil down to the size of your wallet. An XL-sized battery will also let you preserve your battery by charging it to 80% or 90%, but thanks to it being oversized you still have enough in the tank to go wherever you please.
I am all about big batteries on the bikes I build. The Great Pumpkin has a 31 amp-hour, 52 volt custom triangle pack. The Lizzard King has a 32ah/52v brick hiding under its floor. That ties for biggest pack in the fleet with 2Fat – now a recreational bike, it needs big power to run through remote stretches of beach without inland access. That bike has two parallel’d 16ah/52v packs joined together to make a single 32ah battery.
Bigger is better only up to a point. Big batteries equal big weight. So there’s a limit to what you can and should get away with. You can’t go this big on normal neighborhood ebikes, nor should you.
With all that said, going big on a battery can also save your bacon when you do something like forget to charge your battery… there’s enough extra capacity to eke out a ride home rather than having to figure out a way to sleep over at the office.
Bring Along a Spare Battery
This is my least favorite solution, but it may work for you. If you have a battery, buy another one just like it and toss it into a backpack or pannier. Swap it in when needed. This is probably most likely going to appeal to folks with a manufactured ebike and thus no other options. Unfortunately with a solution like this, you can’t get anywhere near as much out of two batteries as you would be able to for a big single one, or for two joined together in parallel (you can to only partially drain each of your packs, hence the loss in capacity). But you suffer the same weight penalty.
Sidebar: Don’t parallel batteries together unless you know EXACTLY what you are doing. Running packs in parallel increases the potential for danger dramatically, and should only be messed with by folks with the experience to know how to mitigate those increased risks.
Onboard Charging (Permanent Mount)
I have written up my experiences with using Mean Well power supplies as CCCV ‘Smart chargers’, and mentioned they are fanless and weatherproof. This and the fact they have mounting tabs means they can be mounted permanently. Assuming the bike is large enough to have a brick bolted on without anyone really noticing. That can mean cargo bikes and any bike with a front rack – the charger works great as a rack deck. And on the front, you don’t really miss the fact you can’t put a rack trunk on.
Pictured above on the left: The Big Fat Dummy and its 185w/3a charger gassing up at the park. The charger is bolted onto the lower deck, up front on the rack. On the right: The Great Pumpkin‘s 320w charger on the front rack is good for 5 amps.
The 480w monster now on the front rack of 2Fat is good for a whopping 8 amps. Its supersized, as when I need a recharge on that bike I am in the middle of nowhere and facing darkness, fog … and may need to negotiate with an unpleasantly high tide if I dawdle.
Onboard Charging (Carried in a Bag)
You don’t always want to be lugging a charger around; nor do you always have a place to bolt one on. I have both 185w and 320w portables that I bring along occasionally on bikes that don’t have a permanent charger mount. For instance, I didn’t want to add a heat-generating charger to the largely enclosed basement battery box on The Lizzard King. So I carry the 320w unit you see below when circumstances warrant (not the shoe. Thats just there for size comparison). Being able to pump in 5a into any battery is going to add a whole lot of range if you plug in while having lunch.
Speaking of open outlets, where are they best found? Here in the USA I have really good luck with public parks. Oftentimes a picnic canopy will have a working power outlet. You can also stop at a roadside cafe, shop or gas station and ask the owner if you can plug in while you are there visiting. This works best if you are stopping somewhere for lunch and will be there for awhile. I’ve also found plugs attached to the outside of restroom buildings at state parks.
Obviously, this approach works best on regular routes where you can determine in advance what is available. Keep your eyes open, scope out your options and file that information away for the time when you need to use it.
Don’t Be Such a Pig
This next one is obvious… or is it? Its a technique I have used and it gets the job done so here goes:
Use less power, as in dial back the assist. My Bullitt with its Great Big Battery was about 3 miles into a 16 mile Saturday morning Costco run when I realized I had forgotten to charge it after work on Friday. Its 52v/14S battery reads 58v when its full, and was already down to 52v when I realized my mistake. Not only would I be blowing my morning turning around and going back home, it would be hours before that battery was charged. I decided to just go for it. So I reduced my assist to the minimum and continued. When I returned home with a cartful of groceries stuffed into my cargo box and panniers, I was down into the mid 40’s, voltage-wise – and more than a little worn out.
But I made it. I wouldn’t have if I had not gone overboard with the size of the battery.
After this I made sure I carried a charger with me on these trips. There is a park midway on the journey with a publicly available power plug. I can plug in, sack out and catch a nap next to a water fountain and be on my way. Late… but I’ll have beaten the system.
Charge at Public (J1772) EV Charging Stations
Yes really. It may be difficult to find an open plain vanilla AC power outlet that you can use… but nowadays electric vehicle (as in automobile) charging stations are popping up all over the place.
If you do not live in the USA, you will want to find a different adapter than what I am describing below (from what I hear non-USA charging stations in the EU are much more likely to have an ordinary, separate outlet available for public use).
But in the Land of the Free, this may be the only obviously available power plug you can get hold of. I’m seeing them increasingly in parks and ordinary store parking lots. Likely they are also springing up at the more refined campsites and national parks.
This is an option that hasn’t been available until recently, and is still not widely known or even understood. Above is a picture of the adapter I have. It plugs into a USA-standard J1772 EV charger plug and terminates in a female NEMA 5-20 plug on the other side. NEMA 5-20 plugs are also compatible with NEMA 5-15 plugs. Folks in the USA know of the 5-15 as your garden variety 3-prong grounded electrical plug. Using this adapter, you now have a bridge directly from a 240v EV car charger to a plug that you can connect your charger into.
Fzzzzzzzz… BOOM!
Thats what could happen if you just plug in without making sure your charger can handle 240 volts of current versus the usual 120.
Here’s the thing: Many ebike chargers are manufactured to run on global power grid voltage. In the USA, we use 120v. Much of the rest of the world uses a lot more volts. 240v in particular. So if you are manufacturing chargers and want to sell them everywhere, you make one that can handle the various voltages right out of the box, so you only have to make one model. However, you can’t count on this feature being there. So check first.
How can you tell? Look at the fine print on the label. The really tiny print that you never read. In the case of the Mean Wells I use, its written clearly in big letters, since they are meant for commercial use and nobody cares if they look pretty.
Yup it’ll handle 240 volts, alright. Since I have also made chargers for relatives who use them on their ebikes in the EU, I know they work just fine on the higher EU voltages.
But thats me. YOU have to figure this out for yourself on your own charger. You won’t know until you go look.
So Much For The Good News…
Here comes the bad news: These adapters are expensive. I have seen them selling for as much as 200. Oddly enough, after some googling I found a seller only an hour or so down the road from me who seems to have the lowest sale price on the web. I paid 85 for mine. Thats still a lot. Lets hope the price is only going down as these types of units become more common.
Or better yet, lets hope that EV charging stations in the USA start commonly having normal AC plugs available.
Whether that happens or not, you should be able to do one or more of the things above, and turn range anxiety into something you used to have … but don’t anymore.
What You Get For 2,800 USD
WinderBike allows you to tweak max speed, torque, andpower modes in a phone app. Battery monitoring shows real time battery status, the charging progress, and has set notifications for fully charged battery or for the remaining 20%.
The eBike is designed with a phone as the main screen. It allows you to see the battery percentage, current speed,notifications, settings, use Google Maps, or play your favorite music. It (of course) comes with USB charging.
Within the Parts List I give customers the option of purchasing custom batteries, 3D printed parts, and accessories from me directly.
Order parts straight from suppliers to your address using the Parts List
Watch learn from the Video Guide, which brings you through the entire build
Enjoy the process of creating your own unique personalized eBike for only the price of parts
YOU DON’T NEED TO BUILD BATTERIES OR HAVE A 3D PRINTER
Depending on your knowledge and skill level WinderBike may vary in price. If you’re looking to buy a completed battery, the difference in battery cost may be 420. If you’re looking to purchase 3D printed parts instead of printing yourself, that will cost an extra 80. All additional package information is available within the Parts List.
VIDEO GUIDE
Affordability
The base model price of 3,100 includes estimated shipping and import fees. For this price your eBike will have the samepower, battery, and functionality as WinderBike. All information regarding extra accessories is included in the Parts List, allowing you to add them as you please.
Big parts like the motor, controller, frame, andbattery, I suggest different sources based on your location, so you can avoid expensive shipping and customs. Most packages come with free shipping. The availability of all parts is global! Building it yourself gives you the ability to customize andpersonalize where you see fit.
I love how light-weight WinderBike is, how easy it is to maneuver it and how stealth it feels in traffic. It can easily make 60km per charge and fully charges in just 3 hours. Nobody ever suspects to have this amount of power in such a small body.
I can’t get over this high tech options we DIY people have in the 21 century. You can seriously make this in your garage.