Battery Capacity and How it is Measured

As a consumer, the new world of electric vehicles continues to require us to learn a new vocabulary.

Last issue, we learned about how volts and amps are like water flow. In this issue we will learn how to determine the size of an  Electric vehicle “gas tank”.  The capacity of the gas tank in our cars is measured in gallons. In an electric vehicle, the capacity is measured in amp-hrs or watt-hrs. After the article, you will understand that amp-hr capacity of a battery does not tell the whole story, you must know the watt-hrs. More watt-hrs means longer distance.

Battery Capacity is measured in Amp-hrs or watt-hrs. The higher the capacity, the longer a battery will run

Example:  A battery is rated with  a 10 amp-hr capacity.  This means the battery will deliver 10 amps for one hour. (assuming the battery has a 1C rating, which we will discuss later).

The rate of discharge affects the battery capacity. When the discharge current  is increased the capacity of the battery goes down.  Battery companies will list the battery  capacity based on the “C” rating.  A 1C rating means the battery was discharged at the rated capacity.

Example: A battery with a 10 amp-hr rating at 1C was tested at 10 amps for one hour. A battery with a 10 amp-hr rating at  C/10 was tested at 1 amp for one  hour. The first battery, with the 1C rating will run longer than the battery with the C/10 rating.

In electric bikes, batteries  should be rated at a 1C rating. Electric bikes require high current during acceleration. A battery with a C/10 rating will offer poor acceleration and the capacity will be much lower when discharged at higher currents. Lead acid batteries tend to be rated with C/10 ratings while NiMH are rated at 1C ratings.

The other way to describe battery capacity is the Watt-hrs. Watt-hrs combine  the amp-hrs with the voltage  and are more accurate when comparing one battery pack to another.

Example: Bike A  has a NiMH 24 volt 12 amp-hr pack and Bike B has a NiMH 36 volt 12 amp hour pack. Even though the amp-hrs are the same, Bike B has 50% more capacity.

Bike B has a 36 volt battery versus the 24 volt battery on Bike A.  Bike A has a capacity of  286-watt-hr while Bike B has a 432 watt-hr capacity, which means Bike B will runs 50% longer.

Next Tech blog we will discuss why higher battery voltages make electric bikes more efficient.