When you’re choosing the right e-bike for you, there are a lot of technical numbers out there, especially concerning the batteries. Below, you’ll find definitions of the key terms you need to know about e-bike battery specs. At the bottom, we put it all together so you know why those numbers matter so you know how to choose the right e-bike battery…

AMPS

An Amp is a measure of electrical current, or rate of flow. Basically, it’s how much power is moving through the system.

VOLTS or VOLTAGE

Think of a Volt as the “force” pushing Amps through the system. The higher the voltage, the more energy can be moved, or the faster it can be moved. So, a higher voltage system can send more energy through the circuits to the motor. 36V batteries are common, but some high performance bikes get 48V batteries. All else being equal, a higher Voltage system will deliver more torque for quicker starts, but it will drain your battery faster.

WATTS

Watts = Volts x Amps. Or, think of it as Force x Volume. Lets say you have a 10A 36V battery, then it’s able to produce 360 watts. This is the real measure of power that a system has.

AMP HOURS (Ah)

Amp-Hours (Ah) indicates how much capacity your battery has. Technically, it’s a measure of how much current the battery can deliver in one hour at a useable voltage. For example, a 14Ah 36V battery will provide 14 amps for one hour at approximately 36 volts. Or 7 amps for 2 hours, etc.

WATT HOURS (Wh)

This is one of the most commonly listed specs for batteries. Watt Hours refers to the number of watts that can be delivered in one hour. Using the 14Ah 36V example above, you’d have a 504Wh battery. If you paired that with a 500W motor and ran it non-stop at highest level of assist output, you’d get about an hour of use. If you were running it on a lower setting, say, using only about half (252W) of the motor’s capability, you’d get about 2 hours of use, and so on.

As such, Watt Hours provides the most useful number in determining an e-bike’s real, usable range or run time, especially if you know the motor’s Watt rating.

…BUT WHAT DOES IT ALL MEAN?

what does an e-bike battery watt hours rating mean
Panasonic’s e-bike battery spec charts show how, given a constant Voltage, Amp Hour increases correspond with Watt Hour ratings, and vice versa.

Really, it comes down to simple math. Bigger numbers mean more capacity and/or power. Since off-the-shelf e-Bikes have electronically limited top speeds and are designed around a specific voltage, number that you WANT to be bigger is the Watt Hour rating, but you typically get that with a larger Amp Hour rating. Here’s why:

Ah x Volts = Wh

So, increasing the Ah (battery capacity) for a given voltage (current flow rate) means more Wh (power delivered over time). Or, more total power available to you. Because the voltage is constant, it’s delivering those stored amps at a constant rate, so the more amps (as Amp Hours) that you have, the longer it’ll take to use them up. Meaning, this is how to extend an e-bike’s range.

 

SUMMARY

how to choose the best e-bike battery upgrade specs
Yamaha’s batteries come in different shapes and sizes, but it’s the numbers you need to pay attention to.

The Takeaway: Because most systems are standardized, what were really looking to maximize is total range, or time before you need to recharge the battery. To do that, look for an e-bike battery with a high Watt Hour rating.

Most modern major e-bike motor systems (think Bosch, Continental, Brose, PanasonicSR Suntour, and Yamaha) have 36V batteries. And most every brand’s system lists the battery’s Voltage and Watt Hour ratings. So, a little backwards math (Wh÷V=Ah) will give you both figures if you want to know the Amp Hour rating, too.

If you plan to upgrade your battery to one with higher capacity, just match your bike’s Voltage rating and get one with a higher Amp Hour rating, which will have more capacity and let you ride farther per charge.

WHY IT PAYS TO PAY FOR QUALITY

In a future article, we’ll explain how to translate these numbers into anticipated range, but a good rule of thumb we’ve come to live by is this: Get the best you can afford. As with all things in life, you get what you pay for. Battery technology is developing fast, and prices are coming down, but there’s usually a good reason a high quality, name brand system costs more than generic, non-branded systems. A common issue with cheap bike lights is that they boast super high lumens, but the electronics and battery can’t keep up. So while they start off ultra bright, they quickly fade as the battery discharges. High quality lights (think Light & Motion, Niterider, etc.) cost more, but they maintain steady light output throughout the entire discharge cycle. The concept is the same with e-bikes…look for bikes using high quality battery/electronic brands for best performance and longest life.

BONUS INFO YOU PROBABLY DON’T NEED TO KNOW

(Huge thanks to Trevor H. for helping us fill this section in – super interesting, but possibly overkill for the average e-bike commuter).

As we explained above, the Watt Hour rating is the final number that’ll tell you what your e-bike is really capable of. How the manufacturer arrives at that number is interesting in that it tells you a little about how the system is designed. To illustrate, let’s look at the math and compare two seemingly similar systems:

  • BIKE A: 10Ah x 36v = 360Wh
  • BIKE B: 5Ah x 72v = 360Wh

Which one would you want? Both have the same Watt Hour rating. But Bike A has more available energy stored in the battery. Bike B has half the capacity, but because its voltage is twice as high as Bike A, the Watt Hours rating the same. Technically, both could be designed to perform similarly, but higher voltage systems are usually used for faster power delivery for quicker acceleration rather than efficient cruising. Think of Bike A as a fuel efficient V6 in a minivan, and Bike B as a turbocharged, nitrous-equipped V6 fed by a high pressure fuel pump. You cooooould ride Bike B like a minivan, but you’d be wasting its potential. In reality, virtually every street legal e-bike’s power output is regulated by law, which is why most are 36V systems.

MULTIPLYING BATTERY POWER

Expanding that concept further: Batteries have a “C” (or Current) rating, which tells how quickly it can deliver its power. The rate is usually a multiple of its Ah rating, which is how a 360Wh battery can power a 500W motor. Let’s use Bike A from above to illustrate, assuming it has a C2 rated battery and a 500W motor. It’s rated at 10Ah but with a C2 rating, so it can discharge a max current of 20 amps (10Ah x 2), but it’d blow through all of its capacity in 30 minutes. However, because it’s pushing 20A x 36V, the resulting output would be 720 W. Which, if not moderated by the system, would provide a serious boost to your 500W motor. Some manufacturers intentionally downplay their specs on paper but allow such “turbo” settings to happen to make their bikes more enjoyable or more capable on steep hills and under load.

For what it’s worth, that same 2C 10A battery could also deliver 5A for 2 hours, which is how bikes deliver different levels of assist.

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3 COMMENTS

  1. A battery hot off the charger will read out a higher voltage than specified, example a 48v battery would read 54.4. This provides the most amount of “power” at the start of the ride but as you ride the voltage will drop until the BMS (battery management system) cuts out at around 40 or so depending. As the voltage drops so does the potential power output and the more the voltage drops the faster it does depending on the amount of watts being drawn from the battery.

    The meters on most bikes track voltage so that is why under hard use you can see the “bars” drop but return upon letting off. Whenever I see someone say “I went 40 miles and had two bars left so I can go 60 miles per charge” I call BS. The last two “bars” will not yield the same amount of energy as the first two, actually substantially less due to voltage drop. A much better way to track your battery consumption is by having an Ah reading that tells you exactly how much energy is on hand. Also a readout of wh/mi is good to use as a gauge. The important thing is to learn your batteries limits that are the result of YOUR riding terrain/technique as it will differ wildly depending on if you are going for speed or distance and atmospheric conditions.

    There is also a lot of chatter about the best way to care for your battery being to not fully charge or discharge it for longer life. Personally I try to come in to the barn with a fully depleted battery, which the aforementioned BMS has control of the low end, and fully charge for the next ride to get the maximum amount of Ah’s so I can go further or faster depending on my needs. A well constructed battery will still provide many miles of energy before it starts to degrade using this method.

  2. The bonus info is still not correct. Battery A and Battery B have identical Wh and, thus, identical stored energy. The different Ah do not matter. You would use battery A with a 36V motor and battery B with a 72V motor. For a given power output (like 500W) the 72V motor will draw half the current as a 36V motor. Thus, the range, etc. would be identical between the two setups. You would NOT want to connect a 36V motor to a 72V battery. You would overdrive the motor by at least a factor of 2 (i.e. 1000W instead of 500W) and would overheat and likely cook the motor and the associated electronics. Unless the motor was connected to a motor drive that could turn down the output to match the motor. In that case the battery voltage wouldn’t matter at all.

What do you think?