Series is how 24 and 36 volts happen
Two 12-volt batteries in series make 24 volts; three make 36. Amp-hours stay those of one battery. Battery University BU-302 is the rule: same type, same capacity, same age. The bow motor’s voltage sticker decides the series count. A 36-volt motor on two batteries is not a setting; it is a mismatch.
Parallel is more time at 12 volts
Two 12-volt deep-cycles in parallel stay at 12 volts and roughly add amp-hours, if the cables are honest and the batteries match. That is how some 12-volt bow motors get a longer morning. Parallel does not make 24 volts. People order “another battery for more power” and mean voltage when they needed series, or mean time when they needed parallel. We ask which.
Do not tap 12 volts off a 24-volt series string
BU-302 warns that taking an accessory load from one battery in a series string unbalances the pack. The used battery ages faster; the other is along for the ride. Lights and a shallow-water anchor belong on a separate 12-volt battery or a proper DC-DC tap, not on the middle of the trolling string.
Lithium series is a maker permission
Some 12-volt LiFePO4 modules may be series-connected; some may not. The BMS and the sheet decide. We do not copy the old flooded series diagram onto lithium because the trays lined up.
On a bass-boat visit
Motor voltage, number of batteries, and whether anything else is wired off one cell. Orlando and Leesburg Rangers are this layout. We replace the string as a string when age is shared.
Labels on the jumpers
We like to see series jumpers marked so the next person does not tap 12 volts off the wrong post. BU-302’s imbalance warning is how those taps kill a string. If you already have a 12-volt accessory on the trolling bank, we will move it or add a dedicated battery. That is not a lecture; it is how the new cells last.
Sources
If this is the job on your boat, see how we replace batteries at the boat. We come to you — Florida towns and counties.