Lithium Battery Packs by the Numbers

What do these numbers mean?

Looking at the label of any lithium based battery you will see a set of numbers that tell you what is inside. The first number you will see is the Voltage expressed as a V.  Typical voltages are 12v, 24v, 36v, 48v and 52v. This number represents the potential that is stored between the positive terminal and negative terminal (Red and Black). Voltage is the force that pushes electrons through a conductor.  I like to think of Voltage as water pressure. The higher the number the higher the pressure.

The second number is the Amp Hour expressed as Ah. Examples 8ah, 10ah, 13ah…  This is the amount of stored energy in the battery pack. A 10ah battery can put out 10 amps (A) for 1 hour or 1a for 10 hours.  Provided you have a good quality battery pack you could use 20a for 30 minutes. The larger the number the more energy is stored. This is also known as the batteries capacity.

The third number that most packs should have labeled is the Watt Hours (Wh).  This number is derived by multiplying the Voltage x Amp Hour = Watt Hours. This is the same number used to calculate your electric utility bill.  Using Wh is a great way to compare different voltage batteries to each other. A 36v 10ah battery will have 360wh of stored energy. A 48v 10ah battery will have 480wh.  The 48v battery will give you better range because more energy is stored, provided you go the same speed. I use Wh to calculate range per charge. The typical 500w eBike will use around 12-24wh per mile.  A 36v 10ah 360wh battery will give you between 15-30 miles range per charge using 100% of the capacity. The consumption rate depends on many factors such as the amount that you pedal, speed, wind, rolling resistance, weight, hills and how much throttle you use.  You will get the best range if you use your pedal assist (PAS) level 1.

Tip:  These numbers are only valid when a battery is brand new.  As the battery ages the energy storage capacity will slowly diminish.  The battery will lose capacity whether it is used or not.

Other numbers that you should know

People confuse Amps (A) and Amp Hours (Ah) all the time.  They are not the same. The Ah rating is the amount of stored energy in a battery.  An amp is how much current is flowing in a circuit. The amp rating of a battery is how much current can be drawn at once.  Some suppliers will give you this information. I would want to know the continuous amp rating and the peak amp rating. This is very important if you are building your own ebike system.  Drawing too much current from the battery will shorten it’s life and could potentially cause a fire.

The continuous amp rating would be how much current you can draw for extended periods of time.  The peak amp rating would be how much you can draw for 10 seconds.

Another number you may see is the C-Rate expressed as a C.  This number is the ratio of amps that can be drawn per amp hour.  Say you have a 10ah battery rated at 1C and 2C max. This would mean that the battery is rated for 10a continuous and 20a max or peak discharge.  Going over these limits will shorten the life of your battery from excessive heat generation. Some lithium polymer (LiPo) battery packs are rated for 60C and above.  This means you could draw 600a from a 10ah battery, WOW. At this rate a 10ah battery would last less than a minute.  

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