18650 Lithium Battery Calculator
D.I.Y. guys want a fast calculator to help with their custom 18650 battery design; however, since things are complicated with different voltages and capacities of each cell, we think those of you who are designing the battery packs should know some basics of lithium 18650 battery design.
18650 Battery packs achieve the desired operating voltage (i.e., Total Battery Pack 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) to help you further understand how it works; pls read the explanation below:
#1 For a series connection, batteries with almost the same voltage and capacity are connected to raise the voltage of the final battery packs. The positive terminal of the first battery is connected to the negative terminal of a second battery, and so on, until the desired voltage is reached. The final voltage is the amount of all the battery voltages added together, while the final capacity (Ah) remains unchanged.
#2 As for the parallel connection, batteries with almost the same voltages and capacities are connected together to increase the capacity of the overall battery pack. The positive terminals of all batteries are connected together or to a common conductor, and all the negative terminals are connected in the same way. The final voltage remains unchanged, whilst the capacity of the assembly is the sum of all the individual cells together for the parallel design.
Let's use a Lithium-Ion 18650 3.7V 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.
Laptop batteries with 11.1V 4.8Ah battery pack commonly have three 3.7V 18650 battery cells in series (3S) to achieve a nominal 11.1 V and two in parallel (2P) to boost the capacity from 2.4Ah to 4.8Ah. As you can see, it will be a configuration called 3S2P, meaning three cells in Series and two in parallel.