People want a fast calculator to help on their custom 18650 battery design, however, since things are complicated with different voltage and capacity of each cell, we think people designing the battery packs should know some basics of lithium 18650 battery design.
18650 Battery packs achieve the desired operating voltage by connecting several 18650 cells in series( S in short ); each 18650 cell adds its voltage. Parallel( P in short) connection attains higher capacity by adding up the total ampere-hour (Ah).
let’s use Samsung 18650 2.6Ah( 2600mAh) for example.
1S1P 18650 pack that’s 3.7V 2.6Ah battery pack
2S1P 18650 pack that’s 7.4V 2.6Ah battery pack
1S2P 18650 pack that’s 3.7V 5.2Ah battery pack
2S2P 18650 pack that’s 7.4V 5.2Ah battery pack
Many 18650 battery packs may consist of a combination of series(S) and parallel(P) connections.
For Laptop batteries with 14.8V 4.8Ah battery pack, it commonly has four 3.7V Li-ion cells in series (4S) to achieve a nominal 14.8 V and two in parallel(2P) to boost the capacity from 2.4Ah to 4.8Ah. As you can find it will be a configuration is called 4S2P, meaning 4 cells in Series and two in Parallel.
Let’s now calculate another 11.1V 100Ah 18650 battery pack, let’s see how many cells would be needed: 11.1V/3.7V=3, so that’s 3S 100Ah/2.6Ah=38.5 so we can use 38P(98.8Ah) or 39P(101.4Ah)
people mostly will use 3S38P so that’s 114 cells in total(3*38=114)
is all clear to you now?
What If I drop the mAh rating from 2.6Ah to 2.2Ah(which is more common for 18650 type batteries), I now need 135 cells in total with 3S45P configuration to get the same total capacity of 11.1V 100Ah(actual capacity is 11.1V 99Ah).
PS: 11.1V/3.7V=3 100Ah/2.2Ah=45.5
The calculate is still on—
What if we use Lifepo4 18650 cells(which are rated 3.2V 1.5Ah)? we would need 264 cells in total with 4S66P configuration to get the same total capacity of 11.1V 100Ah(actual capacity is 12.8V 99Ah).
PS: 11.1V/3.2V=3.5 people normally use 4S to boost voltage,
Want More Details: Download our battery design ebook.
Lithium Battery Design Design ebook (2M, 20 pages, PDF)
Le’ts roll the ball on.
what will happen if Watt/Hour(Wh in short) are involved?
If you have, for example, 2Ah 18650s then each of those stores 7.4Wh (3.7V*2Ah=7.4Wh) of energy and you need 136 of them (1000/7.4 ~ 136) for a 1kWh battery. 136 in parallel will give you a 1kWh battery with a nominal voltage of 3.7V.
If you want higher voltage, and you probably will, you have to put them in series as well. 7s is a typical minimum for a Home UPS battery. 136 cells can’t be evenly distributed over 7 packs in series, you then need 140 cells for a 7s20p setup.
All of above are just examples, there are plenty other possibilities. It would be very hard to put this into an automatic calculator which then provides meaningful results because almost all of this depends on variables the calculator doesn’t know or that have to be put in in the first place. It is easier to calculate this yourself.