Thread: Batteries
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Old 11-10-19, 08:24 PM
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Battery energy capacity which is the aH rating x Voltage is not referencing the power but the energy.
The 18650 cell format size comes in many different internal design variations that can give different performance for both the maximum power (the maximum discharge current). The energy is expressed in aH. The power in amps or C

One 18650 cell typically has a chemistry delivering it's aH at 'nominal' 3.75V going from 4.2V at full charge to 2.75V at full discharge or empty. This is the energy capacity. It is generally listed on the spec in maH Milliamphour for example 2600mah. If you put 4 of these in a row you get 10.4aH, many eBikes use this. If you put 5 of the same cell ina row you get 13aH and 25% more range.

If you take the row of 4 batteries and stick 13 of them end to end you get the 48V nominal battery you often see. Different chemistry will deliver different nominal Voltage. This type of pack would be 54V full charge and around 35V when discharged.
Putting 14 in a row gives 53V nominal.
The real energy in wH is the aH x the voltage but as the voltage varies a lot as a function of charge level and also discharge power the ah is what manufacturers count and quote.

So for the same Voltage you can get more energy by buying a battery with more aH.
You can get more aH by buying more batteries in a row example buying 13sx4p going to 13sx5p with same chemistry gives 25% more energy or by buying better chemistry 3000mah cell vs 2600mah.

For an eBike get the range from the aH but not the power
The power is measured by a different cell specification. The discharge rate often quoted in amps or in 'C' which is multiples of the nominal ah rating of the cell.
If you drain a battery at less than 1C 2.6amp for 2600mah cell it will last many cycles. If you drain it at 4C, 10amp it's life will be shorter. Each cell design had it's maximum recommended discharge rate.

The designer selects the cell to give enough amps and power for the application. For a high powered mountain climbing beast you want lots of amps and have a motor or controller to suit. For this you need a cell with very high discharge current capability which often don't last as long. Radio control devices also like lots of current For a long range commuting eBike range is more critical and designer will focus on ah not discharge rate to give you best range You need enough amp to meet the thirst of your motor and controller. There are literally thousands of different varieties of ah and discharge rate capability out there and the battery used in your device is selected for the application needs.

So to review a battery pack, learn it's construction (# in series x # in parallel) eg 13sx4p
Compare to the quoted voltage and aH or wH rating and you can guess at the cell spec.
Check the maximum discharge rate.

If possible get the cell specific info
Learn its cell and chemistry if you can (example Samsung 18650 2600mah)

The are many online independent test reports for individual cells where you can then check your devices characteristics during charge discharge and life time cycles.
You can also estimate the amount of sag or voltage drop you would expect under load.
It is well worth doing your research as there are many marketing ploys out there to trip up the uninformed buyer.

If you change designers selection be ready for surprises. Your controller and motor may not be capable to eat what you throw at it. For example adding extra aH at same voltage generally will also increase Max amps.
Adding extra voltage (series) may give you more power and range but your controller may drain it too far reducing life and increasing chance of destroying your pack.
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