18650, 20700 & Other Batteries, Conversions, Configurations

Conversions

Convert mAh to Watt Hours

The formula is (mAh)*(V)/1000 = (Wh).

Example: 2200 (mAh) * 3.7 (V) = 8140 / 1000 = 8.14Wh

Convert Watt Hours to mAh

The formula is (Wh)*1000/(V) = (mAh).

Example: 7.92 (Wh) * 1000 = 7920 / 3.6 (V) = 2200

Series and Parallel Battery Configurations

Series

  • Voltage combines.
  • The Positive side of the battery is connected to the negative of the next battery and so on.
  • The battery voltage is checked basically from touching the positive terminal on the first battery and the negative on the very last battery.

Parallel

  • Voltage stays the same.
  • The positive side of the battery is connected to the positive side of the next battery and so on.
  • The battery voltage is checked essentially be checking all negative and positive terminals at the same time as they are connected together on each end.

Links & References

Series & Parallel Examples

Example, acronyms

S stands for series which indicates the amount of cells connected in series and P is for parallel, and indicates the amount of cells connected in parallel.

6SP3 example

6S indicates we have 6 cells connected in series, meaning battery #1 positive is connected to the negative end of battery #2 and so on till all 6 cells are connected and we have a positive unconnected end and an unconnected negative end.

3P then means we have 3 sets of the 6S above, which are then connected in parallel. Meaning now the positive ends are all connected and so are all the negative ends connected together.

Batteries in series have their voltage combined, batteries connected in parallel have their capacity combined. So if each battery cell is rated at 4.2v then that’s 4.2v * 6 = 25.2v and if each battery capacity is 2000mAh then that’s 2000mAh * 3 = 6000 mAh total capacity of the pack.

In this pack most of the cells are going towards increasing the voltage, from 4.2v to 25.2v. To get the most capacity out of all the cells you would do 1S18P and every cell has its capacity multiplied, which would be 2000mAh * 18 = 36,000mAh, but the voltage stays the same at 4.2v even though we have 18 cells in total.

28S3P example

28S indicates we have 28 cells in series, meaning battery #1 positive is connected to the negative end of battery #2 and so on till all 28 cells are connected. If the voltage of the individual cells are 4.2 then we’ve got 117.6v now as voltage combines in series connections. 3P means we have 3 of the 28S, which are connected in parallel. Meaning now the positive ends of the 3 sets of 28S are all connected and so are all the negative ends connected together.

Scenarioes

Scenario 1

We would like to create a 12v 18650 battery back. We have 84 spare cells. First we’ll need to find out how many cells required to reach 12 volts. You could do 12/4.2 to get 2.857… we obviously can’t split cells so we round up to the nearest number so that would give us 3 cells. 4.2*3=12.6 which is fine.

So now we know we need 3 in Series that’s S3, now to increase the capacities. We’ve got 84 cells and we need to divide them equally. We should also keep the series consistent so we can’t have 1 with an extra battery.

84/3=28, so now we’ve concluded our pack is 3S28P. That’s 28 cells in 3 groups, connected together to then give us 1 positive and 1 negative connections.

Scenario 2

How to know what kind of capacity we’ll have?

Each cell, for instance has a capacity mAh of 2000.

Links & References

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