E-Bike Batteries: Volts, Amps, Watt Hours Explained
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What Are These Volts, Amps, and Watt-Hours? How Battery Specifications and Capacity Equate to Capability and Cost
Understanding e-bike batteries can be challenging, even for those of us in the know; the nitty-gritty details are figured out by electrical engineers with years of education and experience under their collective belts – and for good reason, it’s all chemistry and math over there!
You’ll encounter a host of terms when reading about e-bikes or looking at electric bike battery specifications: things like battery size, capacity, voltage, amp hours and watt hours. Some of these words are more-or-less interchangeable, others are related but distinct. All of them can be confusing, but they are also hugely important in understanding electric bikes and their capabilities – most notably when trying to interpret how far they can take you before needing to be recharged.
In this guide to e-bike batteries, the helpful writers at Electric Bike Report will help you to understand the meaning of common battery terms and their relation to the performance of the electric bikes they power.
E-Bike Batteries Explained
Batteries are one of the core elements of electric bikes. They are needed to supply power to the motor, which in turn provides assistance to the rider, and reduces the amount of human effort needed to move the bike.
E-bike batteries come in various sizes, and can be mounted to the frame in different ways. Some are fully internal, and are sealed inside the bike’s frame. As such, they are not removable, except by using special methods and tools available to professional technicians. Others are removable for easier charging and replacement, whether mounted completely externally (outside the frame), partially recessed (sunken into the frame to some degree), or completely recessed (sunken entirely and nearly invisible on the bike).
Regardless of their type, all e-bike batteries are actually battery packs, and are made up of groups of cells, similar to the standard AA or AAA batteries used in everyday applications. The number of cells and the method used to cluster them together determines how quickly they can provide power and how long they can continue to supply it.
In contrast to standard AA or AAA batteries, however, those used in e-bikes are most commonly rechargeable lithium-ion batteries similar to those used inside smartphones and in conjunction with cordless power tools. Lithium-ion batteries are efficient and can be recharged hundreds or even thousands of times if cared for properly. The Light Electric Vehicle Association, or LEVA, has a great article that they allowed us to re-publish regarding proper battery care and safety to ensure maximum life span.
Fully integrated batteries such as the one on the Velotric Nomad 1 can match the bike’s color and disappear into the frame.
Electric Bike Battery Terms and Definitions
Before we dive deeper into the details, let’s consider a couple of examples of e-bike battery specifications in relation to how they usually appear:
V = Volts and Ah = Amp-hours
V = Volts and Wh = Watt-hours
Both examples convey two basic measurements, albeit a little differently. In both examples, we see volts first; this measurement relates to the availability of the electrical energy the battery can deliver. Next, either amp-hours or watt-hours are shown; these represent a battery’s capacity, or the amount of power it can store.
Let’s define these words (and a few helpful additional terms) a bit more clearly:
Current: the flow of electricity, or transfer of electrons, through a circuit.
Circuit: a closed system of wires and electrical components through which current can travel.
Volts (V): the amount of electrical force or pressure the battery can produce; the speed of the battery’s output of current. This is also sometimes referred to as the electromotive force, and is more specifically the speed at which electrons move through the system.
Note that this is a nominal rating that is used for classification purposes. In reality, a battery’s voltage varies based on the amount of power being drawn from it at a given moment, as well as the battery’s present level of charge. As current is drawn from the battery, its voltage decreases. This can be seen in an e-bike battery voltage chart.
Voltage is determined by the number of battery cells arranged “in series”.
Amps or amperes (A): a measurement of the strength of the battery’s output, or current. specifically, the volume of electrons passing through the system. This is limited by the size of the wires making up the system. Larger wires allow more current, smaller wires allow less. Generally, systems with higher voltage should use smaller wires (that limit amperage) to prevent overheating.
Amps can also be thought of as the amount of energy being drawn from the battery by what it is powering, and can fluctuate from moment to moment. In the case of e-bike batteries and their motors, a greater number of amps are drawn as the motor works harder (i.e. going uphill or using only the throttle).
Amp-hours (Ah): a measurement of charge; the amount of energy that can be delivered through an electrical system over the course of an hour.
In the case of a 10 Ah battery, it can deliver 10 amps of power in one hour, or 1 amp of power for 10 hours, etc, depending on the needs of the component that is delivering power to.
Amp-hours are determined by the number of clusters of battery cells arranged “in parallel”.
Watts (W): a unit of power, determined by volts and amps; the amount of work that can be done by one amp of current delivered at 1 volt. The amount of work is determined by the rate at which the energy is used.
This measurement is generally applied only to an e-bike’s motor, but its battery must support the motor’s needs.
Watt-hours (Wh): another measurement of capacity. In this case, the amount of work that can be done, or the amount of power that is spent, over the course of an hour. This is a direct result of a battery’s voltage multiplied by its amp-hours.
As such, a 24V, 20 Ah battery and a 48V, 10 Ah battery might look different on paper, but they have about the same amount of energy. This makes watt-hours a more reliable indicator of capacity when comparing different batteries.
Controller: A device that limits the flow of electricity through a circuit, and prevents a battery from discharging its energy all at once. In terms of an electric bike, this is the “brain” that adjusts the pedal assist system, the amount of input the motor contributes, and the e-bike’s speed.
Everything you need to know about e-bike batteries [from a battery engineer]
Would you be the person taking the stairs or the escalator?
I’ll be honest. barring the one-off day that I’m feeling particularly sprightly, I would just hop on the escalator with those 30 people on the right. And I’m willing to guess that most of you would too.
What we can gauge from this picture is that most people would rather do as little work as possible to get from point A to point B. This is especially true when it comes to commuting on a bike. The picture above is analogous to the difference between a regular bike and an e-bike.
Even if we address all the concerns when it comes to biking in a city (like safe biking infrastructure), we can’t expect to change fundamental human behavior. when given the option between less work or more work to achieve the same outcome, people will more likely choose to do less work.
Since getting my e-bike, I can comfortably bike from my home in Somerville to the Seaport district in Boston. a roughly 5-mile trip. in just about 20-minutes. All of a sudden, biking 5-miles is a piece of cake. I also don’t have to spend time sitting in traffic, waiting for public transit, or worry about showing up to a meeting looking like I swam across the Charles river to get there.
The beauty of an e-bike is that it makes cycling an inclusive mode of transportation because it doesn’t discriminate by age or physical ability.
When it comes to purchasing an e-bike though, there are a plethora of options for both the bike and battery. So how do you decide which one is best for your needs? As a battery engineer who has built hundreds of batteries and logged way too many hours soldering battery packs, here are my thoughts on the most commonly asked questions when it comes to e-bike batteries.
If you’re new to battery terminology, you might want to start here: Battery terms that every e-bike owner should know.
In this post, we’ll cover the following questions:
What is the best e-bike battery?
This is one of the hardest questions to answer. There are so many variables that go into what makes a good battery and what’s best for you, may not be the best for me. Even then, a good battery can perform poorly if it’s not cared for properly.
Battery packs are made up of individual battery “cells”. Cells are classified into cylindrical cells (like your AA and AAA) and prismatic cells (like the one in your phone). Each class of battery is manufactured in a variety of form-factors (in the battery world we use this term to mean size). The most commonly used form-factor of cells in an e-bike battery pack is the 18650.
A battery pack is only as good as it’s weakest cell.
When it comes to batteries, in my experience, there is a strong correlation between price and quality. I don’t follow this rule when it comes to most things like for example, box wine (I’m just saying, there are plenty of really good box wine options these days!). When it comes to batteries though, you really don’t want to be compromising on quality because you’ll eventually end up having to pay the price.
Here are some things to keep in mind when purchasing an e-bike:
Cell Manufacturers: Panasonic, LG, and Samsung have a good reputation in the battery industry for their high quality cells, so paying a premium for these cells is certainly worth it. If the e-bike you’re trying to buy doesn’t have or provide cell manufacturer information, they’re likely not going to be a reliable source anyway.
Cell Chemistry: Lithium-ion (li-ion) batteries are the best option for e-bikes. Although lead-acid batteries are significantly cheaper, they’re three times as heavy as their li-ion equivalents.
Li-ion has several variants of cell chemistry. The most popular ones for e-bikes are Nickel Manganese Cobalt (NMC), Lithium Cobalt Oxide (LCO), and Lithium Iron Phosphate (LFP). The metrics to look for when selecting a cell chemistry are:
- Specific Energy: has an impact on the range of your battery.
- Specific Power: how the battery handles high load scenarios like going up
- a hill.
- Safety: does the chemistry have a history of high in-field failures.
There are trade-offs when choosing one chemistry over another, but as we’ve shown in the image below, NMC and LFP are both great options that both offer the best value in terms of performance, price, and safety.
Picking the right battery chemistry has to do with figuring out what matters most to you. Do you want a battery that has a longer range (higher specific energy) but doesn’t have as much power? Or do you want a battery that has a more power (higher specific power) but may not last as long?
In my opinion, the best e-bike batteries are likely going to be made from cells manufactured by Panasonic, LG, or Samsung with either LFP or NMC cell chemistry.
What is the range of an e-bike battery?
The range of a battery pack depends on the amount of energy packed inside of it and is measured in Watt-Hours (Wh). Watt?
Watt-hours are calculated by multiplying the battery capacity, in Amp-hours, by the battery Voltage, in Volts.
Let’s assume that, on average, 1-mile requires about 25Wh of energy. So a 14Ah, 36V battery should get you about 25-miles per charge.
Keep in mind that the weight of the rider, outside temperature conditions, and the amount of pedaling will make a significant difference in range.
A word of caution: the range that e-bike manufacturers provide should be taken with a grain of salt. That number is generated from tests that are run in perfectly tailored lab conditions. Do you charge any of your electronics in an incubation chamber set at 28° C with a lab-grade charger that applies the perfect current while charging? Yeah, I don’t either. And so, We should assume that the manufacture-specified range is delivered only if the battery is charged and discharged under ideal conditions i.e. not real world conditions.
For a more realistic estimate, shave off 15% of the manufacturer specified range and assume this padded number to be your real range.
If you’re looking for a longer range, choose a battery that has higher capacity (Ah). If you’re looking for more power, choose a battery that has higher voltage (V). Learn more why voltage and capacity matter.
What is the lifespan of an e-bike battery?
There are several factors that affect the lifetime of a battery such as:
- environmental conditions: temperature during charging discharging
- charging rate: how fast or slow your battery is charged
- charging voltage: what voltage the battery is charged to
- depth of discharge (DoD): what voltage the battery is discharged to
The list above isn’t exhaustive but, in general, batteries decay as a function of time in the charged state. Period.
Day 1: You get your new e-bike and charge it up to 100% and go on a bike ride. When you come home, you charge the bike back up to 100% and you’re excited to ride it again soon.
Day 2. 364: Life get’s in the way and you still haven’t been out on your bike since that first ride.
Day 365: One year later, it’s the perfect day for a bike ride and you finally have some time on your hands. You head to your basement, unlock your bike, and excitedly turn it on. 80% charge. What? You clearly remember charging your bike to 100% last year before moving it to the basement!
The truth is, we can’t beat thermodynamics. I’ll say it again: batteries decay as a function of time in the charged state.
Now, because you left your battery at 100% for a whole year in a basement with no temperature control, you inadvertently caused your battery to lose a certain amount of irreversible capacity. Your range will be ~20% lower and you’ll likely have to replace your battery sooner than you expected. The table below shows you how much recoverable capacity exists in a battery after storing it at different temperatures and different charge states for 1-year.
This is why a lot of electronics come with batteries that are only partially charged. to help slow down this decay. That being said, it’s hard to track how long e-bikes and their batteries have been sitting in warehouses before being delivered to your door so you could get a battery that has been decaying for a year or two.
Manufacturers also tend to overrate their batteries and will make claims about certain batteries having a lifetime of at least 1,000 cycles. Show.me.the.data.
The lifetime of a lithium-ion battery is described as the number of cycles until the capacity (Ah) drops below 80% of it’s initial capacity. In general, this is roughly 250-400 cycles (depending on battery chemistry and other factors) which amounts to roughly 1.5 to 2 years if you charge discharge daily and care for your battery properly.
How to charge your e-bike battery to make it last longer
- The thing that will kill your battery faster than anything else is leaving it charged at elevated temperatures. If it’s 80 degrees outside and you have your e-bike fully charged, move it indoors where it’s cooler and try to drain the battery as soon as possible.
- Charge your battery at room temperature as often as possible.
- When sourcing an e-bike battery charger, the slower the charge rate the better. For example, if you have a 2-Amp charger, and your battery is a 14 Ah battery pack, you are charging at 14 Ah / 2-Amps = 7-hours. This is a nice, slow charge which will certainly improve the longevity of your battery pack. Avoid charging at rates that are faster than 2-hours for a full charge.
There’s a lot that goes into choosing the best battery for you e-bike, and there certainly isn’t a one-size-fits-all approach. But if I were buying an e-bike battery today, here’s what I’d do: LFP or NMC, slow charge, avoid storing or charging in hotter temperatures, and leave the battery at around 30% charge if you don’t plan on using it for a while.
Have questions? We’d love to help. You can get in touch using the contact form or find us on @somerville_ev
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