Your E-Bike Battery

Posted by larry deangelo on

Your E Bike battery is important.    Your battery determines your maximum possible speed, acceleration, and how far your can go on a full charge (range).  The life of the battery will be determined by how many charge-discharge cycles, and the quality of the battery construction.  You do not need to review your two years of high school physics to evaluate the performance characteristics of a battery.  You should understand a simple explanation of how your battery works and the meaning of certain battery specifications.

A battery in its simplest form is made up of one or more cells.    Each cell has two pieces of metal submerged in a chemical.  The two mental pieces are called electrodes, and each is made up of a different metal.   The top of the electrode is available on the outside of the case and is called a terminal.   One terminal is labeled positive the other is labeled negative.  The chemical is called an electrolyte.

A chemical reaction between the electrodes and electrolyte creates a potential difference between the electrodes (Voltage).  When the electrode terminals are connected through a device (load) electricity flows through the device and back to the opposite terminal on the battery.  The flow of electricity is called current and is measured in Amps.  If you place a light bulb connected between terminals the bulb will light.    If you connect an e bike electric motor between the battery terminals the motor will turn.  The flowing current is the force that lights the bulb or turns the electric motor.

When looking at battery specifications you can get bogged down in the definition of each of the battery specifications: Voltage (Volts-V), Amperes (Amps-A), Amp-Hours (Ah), Watts (W), Watt-hours (Wh).  Your car performance measurements such include horsepower, torque, compression, and RPM.   Do you really understand how these measurements are calculated and the details of each measurement? Probable not. So here is what you need to understand about battery specifications and how your battery impacts bike performance.


Voltage (Volts, V)   The chemical reaction between the battery electrodes and battery electrolyte creates a potential different between the battery terminals.   When a device (load)is connected across the battery terminals electricity flows.  Think of the battery’s voltage as the “horsepower” that propels the electricity out of the battery.


Amperes (Amps, A) Amps are the measurement of the amount of electricity flowing from the battery and across the load at a given moment in time.   


Amp-hours (Ah) Amp-hours is a measurement of electric current (Amps) that the battery can provide over time (Capacity).  One Amp-hour is equal to the amount of current that passes a given point in one hour.  After that hour the battery is discharged. Think of battery capacity as the size of a fuel tank in a car.


Watts (W) If we only consider how many Amps the Battery can delivery over at one time, we neglect another factor of the battery capacity Voltage.  Watts are a measurement of Voltage and Current at one point in the operation of the battery (Power).   P(Power) = to Volts times Current.  Watts are an indication of how much work the battery can do.


Watt-hours (Wh) Watt-hours are a measurement of how much power a battery can deliver over time (another way to measure battery capacity).  A 1 W-hour battery can delivery 1 Watt of power for 1 hour.  Wh is a more convenient way to determine the range of your bike, the cost of your bike, and the weight of your bike - three very important factors to consider when buying an electric bike.



Ah vs Wh: This can get confusing, but it is very important to understand the difference. Amp Hours (Ah) means nothing unless you factor in the voltage. Watt Hours (Wh) is far more important because it factors in the Voltage and the Amp Hours together and determines how far you might go on a full charge.


Watt hours is a way to measure the capacity of a battery pack and estimate the range of a fully charged battery.    A 36-volt 10-Ah battery pack has 360-watt hours (36 X 10 = 360). Each mile you travel could cost you about 20-watt hours. Therefore a 360-watt hour pack will get you about 18 miles. The range can vary depending on the weight of your bike, and where and how you ride.  Depending on how fast you go, the more Wh you have on the bike, the more range you can expect from it.


An e bike battery usually lists its voltage (V) and amp-hour (Ah) rating. To calculate the Wh of an e bike battery pack, we multiply its V and Ah to get the Wh.  As mentioned above, the average e bike battery will yield one mile of travel for every 20Wh.  Of course, the weight of the bike, the speed of travel, the condition of the road are all factors that would determine the actual range of the bike.

    • A battery rated at 36 V and 10.4 Ah will have a 374.4 Wh capacity (36 x 10.4 = 374.4).
    • A battery rated at 48 V and 21 Ah will have a 1,008 Wh capacity (48 x 21 = 1,008).


Larry DeAngelo


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