Battery Terms that Every E-bike Owner Should Know
Batteries are easily the most expensive and precarious component of any electric vehicle. This is especially true for e-bikes and other Light Electric Vehicles (LEV), where the battery cost can be as high as 50% of the overall price tag. Considering how much of the cost and performance of e-bikes is determined by the battery itself, it is a good idea for riders to familiarize themselves with some of the most common terms and metrics used in the industry to describe these devices.
In this segment, we will cover Voltage, Capacity, and Energy.
This is the big one. Well, it’s generally one of the first metrics you hear about, at least. The voltage of a battery is a way of describing the amount of energy that each individual electron comes with, right out the gate. The higher the voltage, the higher the electron’s potential energy.
An easy way to visualize this is to compare voltage to gravity; imagine you live in a 2nd floor apartment and you forgot your keys. No worries; your roommate is home and will gladly toss them to you from the balcony. You stand outside and catch the keys as they're dropped down to you. At the moment the keys impact the palm of your cupped hands, there will be a very slight sting, but it will likely be barely noticeable after only a few seconds.
Now imagine catching the keys from a 4th floor apartment. A 10th floor apartment. A 100th floor apartment! As the distance between the balcony and the ground increases, so does the sting in your hands. By the 10th floor, trying to catch the keys will become outright painful. Also, I’m pretty sure throwing keys from a 100th floor apartment is illegal.
This is exactly the way to think about voltage! Just replace balcony height with voltage and keys with electrons. Just like the analogy, the higher the voltage that you “drop” the electrons from, the sharper the “sting” each electron will have.
Putting this into a practical context, you should expect the electrons coming out of a 36V battery to have 75% of the energy (AKA sting) as electrons coming out of a 48V battery. Likewise, the electrons coming out of a 96V battery will have twice as much energy as the ones coming out of a 48V battery. E-bike battery voltages typical run between 36-48V.
When we talk about a battery’s capacity, what we really mean is how many total electrons we can get out of it. That’s it. The units we use for this are Amp-hours [Ah] or milli-Amp-hours [mAh], the latter just being one-one-thousandth of an Amp-hour. You may come across someone who insists on calling an Amp-hour an Ampere-hour, but they are probably just trying to sound smart.
The quantity of electrons that make up a single Ah is equal to the number of electrons you would collect if you drew one Amp of current – which is really just a bunch of electrons flowing in the same direction – for one hour straight. Hence the name "Amp-hour."
So how many electrons is that? Let’s find out:
Since one Ah = one Amp-hour = one Amp x one hour
And one Amp = one Coulomb-per-second*
And one Coulomb = 6.24 x 10^18 electrons
And also one hour = 3,600 seconds
We have one Ah = [6.24 x 10^18 electrons-per-second] x [3,600 seconds] = 2.25 x 10^22 electrons!
So every Amp-hour contains 22,500,000,000,000,000,000,000 electrons!!! As you can see, it’s a lot easier to just say write Ah.
Practically speaking, the higher the capacity, the better. The Ah value of your battery has a direct effect on range and an indirect effect on the amount of torque your e-bike will produce. Typical capacity values for e-bikes range between 8-15Ah. The additional weight and cost make higher capacities than this impractical for most commercial e-bike batteries.
The Watt-hour [Wh] is a measure of the total energy of the battery. “Whoa, whoa, whoa…” I hear you protest. “Didn’t you say that Voltage was the measure of energy!?”
Well, you’re right. We’re both right. As it turns out, there are almost as many different ways to describe energy as there are electrons in one Amp-hour! For now, let’s focus on just the two: voltage, and Watt-hours.
Whereas voltage is a way of describing the energy we get from each individual electron, Watt-hours are a way of describing the energy we get from all the electrons. When people refer to the Watt-hours of a battery, they are referring to a number that describes the total amount of useful energy available from the entire battery.
To obtain the energy of the battery in Watt-hours, all we need to do is multiply the battery voltage [V] by the battery capacity [Ah] (which just tells us the number of electrons in our battery) -- and that’s our answer. So why isn’t energy written in terms of Volt-Amp-hours [VAh]? It is – we just need to look a little more closely:
Since one Wh = one Watt-hour = one Watt x one hour
And one Watt** = one Volt x one Amp
We have one Wh = [one Volt x one Amp] x [one hour] = [one Volt] x [one Amp-hour]
Which is just Voltage [V] x Capacity [Ah]!
As an example, a 36V 10Ah battery would have an energy of 360 Wh. If you wanted a battery with twice as much energy as this (720 Wh), you could either double the capacity (36V 20Ah) or double the voltage (72V 10Ah), or do something in the middle like 48V 15Ah.
Two final notes about e-bike battery energy. First, you will sometimes see this written in kilo-Watt-hours [kWh], which is just the regular Wh value divided by 1,000 (720 Wh = 0.72 kWh).
Secondly, people sometimes refer to the Watt-hours of a battery as the “capacity.” Strictly speaking, this is incorrect …. What they mean is the energy capacity. “Capacity” alone is generally taken to mean Amp-hours, as described above.
Speaking of OEMs, of the three battery metrics discussed so far (V, Ah, & Wh), e-bike sellers tend to only list (2) of them. But now you know the secret; as long as you have at least two of these metrics, you can always calculate the third!
There's a lot more to say about e-bike batteries, but these three terms should get you a long way to help decide which e-bike you should buy! In case you need some help, head to this post. Please feel free to contact us with any questions!
* The coulomb is a measure of electrical charge. And that’s about as much as you will ever need to know about a coulomb.
** The Watt [W] is a unit of power. More on this later.