Series vs parallel, without wrecking them

Connect them wrong and you either get nothing useful or you damage them.

Parallel (positive to positive, negative to negative): capacity adds, voltage stays the same. More runtime at the same voltage.

Series (positive of one to negative of the next): voltage adds, capacity stays. Same total energy at higher voltage — which means lower current for the same power, and cheaper wiring.

The rules that keep you out of trouble:

Match everything. Same chemistry, capacity, age, ideally brand. A mismatched bank is governed by its weakest member.

Never mix old and new. Adding a fresh battery to a tired bank drags the new one down.

Wire it symmetrically. This is the one people miss. If you take your main positive and negative from the same battery in a parallel bank, that battery does more work and ages faster. Take positive from one end and negative from the opposite end. Costs nothing, makes the bank age evenly.

Series needs balanced cells. Any capacity mismatch means one hits full or empty first. Many lithium banks need batteries specifically rated for series — check before you commit.