Yes, you can mix different capacity lithium batteries, whether a normal 12V 100Ah battery or a Lithium server rack battery. . Here's a useful battery pack calculator for calculating the parameters of battery packs, including lithium-ion batteries. 2V; we need three cells in series to make a 12. In the figure above, the connections are indicated. With a 12V battery pack with 10Ah capacity, the calculator would determine how many 18650 cells to connect in series for voltage and in parallel for. . It's still a process that can be daunting for the first-time pack-builder though, because the other thing that most of us know about lithium ion batteries is that getting things wrong can cause fires.
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Fortunately [Adam Bender] is on hand with an extremely comprehensive two-part guide to designing and building lithium-ion battery packs from cylindrical 18650 cells. (Edit 2025: re-linked through Internet Archive.) In one sense we think the two-parter is in the wrong order.
A typical 18650 battery pack for laptops has a nominal voltage of 11.1 V. This is achieved by connecting three 3.7V 18650 battery cells in series (3S).
To calculate the capacity of a lithium-ion battery pack, follow these steps: Determine the Capacity of Individual Cells: Each 18650 cell has a specific capacity, usually between 2,500mAh (2.5Ah) and 3,500mAh (3.5Ah). Identify the Parallel Configuration: Count the number of cells connected in parallel.
Let's calculate for a 11.1V 100Ah 18650 battery pack: 11.1V/3.7V=3 (S), 38.5 (P) cells. So, 3S38P would require 114 cells in total (3*38=114).