Basic battery pack wiring to achieve specific voltage and amp-hour capacities
Batteries come with voltage values which depend on the battery chemistry, and on the arrangement of battery cells to form the battery pack. The amp-hour capacity is based on the battery chemistry, and essentially how heavy the battery is because the more material in the battery the more electrical charge it can store.
Typical per-battery-cell voltages are:-
- Lead acid: 12 volts (technically lead acid batteries are delivered as multiple cells packaged as one battery)
- NiMH: 1.2 volts
- Lithium ION: 3.7 volts
- LiFePO4: 3.2 volts
How do we get a 36 volt, 48 volt, 120 volt (or more) battery pack? It's simply done by wiring the batteries in parallel or serial fashion to get the desired numbers.
In a series-connected battery pack the measurements come out as follows:
voltage = volts-per-cell * number-of-cells
amp-hours = amps-hours-per-cell
In other words, for a series connected pack the voltages add based on the number of cells. However the amp-hour capacity does not increase.
A big assumption in this is that each battery in the pack has the same rating. It's best to have matched batteries in a battery pack. It's been observed that in a mismatched pack cells with less capacity will cause the rest of the pack to work harder, and the pack becomes damaged more quickly.
In a parallel-connected battery pack the measurements are the reverse of the series-connected ones:
voltage = volts-per-cell
amp-hours = amp-hours-per-cell * number-of-cells
You can also combine techniques if necessary, to create a series-parallel battery pack
You can wire a series-parallel pack as shown (wire the batteries in parallel "strings", and connect each string in series) or the other way around (wire them in series strings, connecting each string in parallel). The effect will be the same, namely:
voltage = volts-per-string * number-of-strings
amp-hours = amp-hours-per-string * number-of-strings