How to calculate the capacity of an eBike battery

How to calculate the capacity of an eBike battery

When shopping for an eBike, “how far can I go?” is one of the most common questions. The range of an eBike largely depends on battery capacity. Battery capacity is measured in Watt-hours, “Wh” - you can think of Watt-hours as the equivalent to the size of the fuel tank on a gas-powered automobile.

Where do Watt-hours come from?

Let’s use a real example from an actual eBike battery. If we look at the fine print on the label, we’ll see some ratings such as:

  • 48 V (volts) Just like the electrical current in your home, the voltage needs to match between the device and the power source. Our example uses a 48 volt battery, motor, and matching electrical components.
  • 12.8 Ah (Amp-hours) An amp-hour is the length of time a battery can provide one amp of power. Our example battery could power a device that draws one amp for 12.8 hours. That’s not terribly useful alone because eBikes don’t draw exactly one amp… but this number does tell us more broadly about battery life, in that higher numbers mean more capacity.
  • 614.4 Wh (Watt-hours) This is the number most useful to an eBike owner. Assuming the demand on the battery is the same (think hills, headwinds, and rider input) a 614.4 Wh battery can take an eBike twice the distance as one equipped with a 307.2 Wh battery.

Watt-hours can easily be calculated by multiplying voltage by Amp-hours, i.e. 48 x 12.8 = 614.4 Wh. Assuming our example eBike has a 500 Watt motor, we can expect this battery to power it for roughly 1.2 hours (614.4 Wh / 500 W = 1.2 Hours).

The same battery capacity doesn’t mean the same range

It’s worth noting that two different eBikes with the same battery capacity in Wh can have dramatically different ranges. Consider that a Honda Civic and a Ferrari might hold the same number of gallons of gas, but get wildly different fuel economy, and so too will eBikes. EBikes tuned differently for efficiency, power, or speed will yield different ranges, even if they have the same battery capacity in Watt-hours. Make sure you’re making an apples-to-apples comparison.

Are larger batteries better for eBikes?

Is more necessarily better? Sometimes. Massive battery capacity sounds compelling (it yields more range, so less downtime between charges) but does have some big downsides - first, larger capacity batteries require more cells, which are heavy, increasing the weight of your eBike, and second, the battery is one of the most expensive eBike components, and making it even larger adds additional expense.

Heavy bikes aren’t much fun to ride, and can be challenging to lift onto roof or hitch racks, so too much battery capacity can actually be a downside.

There’s also a physical limit as to the size of the battery, because it must fit into the appropriate space on an eBike frame.

The battery on your eBike should match the rider’s needs

If your rides are commonly 10-20 miles, there’s no need for an eBike with a 60 mile range. The battery required to reach that range is needlessly expensive, and also heavy, which can make such a bike less fun to ride.

The reverse is also true. Riding an eBike with a battery that’s too small is frustrating - it needs to be charged constantly and interrupts your plans. If you DO require an eBike with a 60 mile range, there are multiple strategies, and a single larger battery is only one of the options. You can also use a second battery and swap between them - this allows you the flexibility to have extended range when required by taking both batteries along, and only carry a single, lighter battery when it isn’t.

Finally, consider the option of charging mid-ride. This may be possible if you stop for lunch or coffee (don’t forget your charger). Commuters to work or school may find that charging during the day is possible, leaving their eBike fully charged for the ride home - someone with a 50 mile daily round trip doesn’t need an eBike with a 50 mile range, they only need one with a 25 mile range if charging is possible during the day.

When measuring eBike range, YMMV

As always, your mileage may vary! Rider weight, pedaling input, use of throttle / not, hills, headwinds, and bicycle weight all contribute to range. Smaller, lighter riders who contribute more with their legs can expect range at the higher end of estimates, while heavier riders in hilly terrain, and those who rely more on the motor than their legs, should expect range in the lower end of estimates. 

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