UNDERSTANDING ELECTRIC BICYCLE BATTERIES Part 2: What Does an Amp-Hr Tell Us?

How are Electric Bicycle Batteries Rated?

In Lesson 1 we learned how energy is measure in watt-hrs and this is the like the size of the gas tank.  You will often hear the term amp-hrs used to rate an  electric bicycle. This is similar to watt-hrs, but only half the story.   Amp-hours indicate how many amps a battery can discharge in one hour. A 20 amp-hour battery can discharge 20 amps for one hour before it’s drained. But that’s not the whole story; the entire amount of energy  in the battery is the amp-hours times the voltage of the pack, which is called watt-hrs. Knowing the battery capacity in watt-hrs, instead of amp-hrs, lets you easily compare batteries of different voltages.

Lets see how this works. An electric bike with a 12-volt pack with 10 amp-hours capacity has 120 watt-hours and is not the same as an electric bike with a  36-volt pack with 10 amp-hours capacity, which has 360 watt-hours.  The 36-volt pack at 10 amp-hours has three times the energy of the 12-volt battery at 10 amp-hours because the voltage is three times greater.  The electric bike with the 360 watt-hr battery can go further and is more capable of doing long range touring.

So the standardized way to look at batteries is in watt-hours, measured in volts times amp-hours, which indicates total energy.

We all know watt-hours because our electric bills charge us by the watt-hour. It’s not only how big your light bulb is, but also how long it is on. In other words, they’re charging you for the duration that your bulb is on, in addition to how big the bulb is to obtain the bill in watt-hrs. We’re used to paying the utility in watt-hours, which is energy.

Remember the difference: power is in watts and energy is power (watts) over time measured in watt-hrs.

Excerpted from  “The Electric Bike Book” by Jim Turner. Available FREE on I Pad and Kindle. Click here to see the technology behind our e-bikes.