Lithium chemistry cells are not a good match for 12-volt systems, because almost exactly 3.5 cells in series would be required. To get around this, and to reduce the size of copper wire needed to carry high currents, most people run 24-volt systems, or even 48-volts. Since this project needs to start small and bootstrap up to larger sizes, I’ve decided on 24 V.
This means that I need 7 “cells” in series, although of course these will not be single cells but rather large packs of cells in parallel. Many other home battery builders have opted for large packs of 80-120 cells in parallel, and this has the advantage of getting large storage capacity fairly neatly. However, it means that 7 x 80 = 560 cells are required before the 24 V battery can first be “turned on”. I want to quantise the battery into smaller packs so that I can go “live” well before reaching 560 cells.
With cell holders that allow a triangular-close-packing of the cells, I can make packs of 2×10 cells that are attractive and slim-line. Interestingly, a battery of 7 20-cell packs would have about 1 kWh of storage for realistic reclaimed cell capacities.
There’s also a maintenance advantage of using smaller packs strung together. When the battery is larger, it will be possible to remove one of the 20-cell packs from a set of parallel packs and leave the battery operating (although it will become unbalanced if left like this for long).