or, “4p24s? Huh?”
How you connect your batteries together is crucial in determining your final voltage and how well your cells will stay even on charge and discharge. The first simple step is to understand the difference between a series and a parallel connection.
For all the times I get confused on this, it’s really pretty simple. Series connections are just that- putting your batteries in a line, connecting them end-to-end, positive to negative. Series connections add voltage, but don’t add your aH rating. Take 6 12V 20 aH batteries and put them together in series and you get 72V, at 20 aH.
Parallel batteries are, well, in parallel. They sit side-by-side, if you’ll allow me my visuals, they connect positive to positive, negative to negative, and they don’t add the voltage. They do, however, add up the capacity, the aH rating. Four 3.2V 10 aH lithium cells – like Headways – in parallel give you 40 aH but still only 3.2V.
To get to the voltage and capacity you’re looking for, you can connect the cells up in several different ways but the best is to set up your cells to hit your aH target within your base cell voltage. For example, if I have 48 3.2V 10 aH Headway cells, the best way to connect them is to make groups of two cells in parallel, then connect all of them together in series. I get 24 “clusters” of 3.2V 20 aH, then together get 76V or so. The reason behind this strategy is just the fact that the individual cells charge and discharge more evenly when they’re in parallel. The more cells you can buddy up with each other, the more even the BMS and charger can keep the pack overall.
The description you’ll see for this is a “_P_S” term- for the pack above it would be 2P24S, or 2 parallel, 24 series. A little baffling when you first see it, but it makes perfect sense.
As you’d expect, the final word on this, with some slick illustrations, is at the Battery University site: http://www.batteryuniversity.com/partone-24.htm