Voltage first
The motor is 12, 24, or 36 volts. That sets one, two, or three 12-volt batteries in series, or a purpose-built 24/36 lithium pack the maker allows. Everything else is secondary. A high-Ah 12-volt battery on a 24-volt motor is the wrong object.
Compare Ah and RC inside one chemistry
BU-105: reserve capacity is minutes at 25 A; amp-hours are a stated hour rate. Those numbers compare two flooded deep-cycles, or two AGM deep-cycles. They do not convert a starting battery into a trolling battery, and they do not make lithium and flooded equal without a DoD conversation.
Flooded, AGM, or lithium
Flooded group 31 is the lake-country default: heavy, cheap, needs water, sulfates if it sits (BU-802). AGM costs more, handles a pounding deck, hates extra heat. Lithium cuts weight if series is allowed and the charger can stop at full (BU-409b). Pick from the boat you have, not from a forum chart.
The charger is part of the choice
A three-bank flooded charger will float lithium to death. A portable trickle charger will not refill three 31s after a long Saturday. If you will not plug in, you will buy batteries more often. That is the honest trade.
On the visit
We read the motor, the trays, and the charger. Polk and Lake County driveway quotes follow those three facts. We do not pick a battery from a hull length.
Used boats and mystery strings
A used TRACKER with three different brands in the bow is already unbalanced. We quote a matched string rather than one replacement that will be dragged. If the motor was upgraded and the batteries were not, voltage is the first correction. Amp-hours come after the motor is actually 24 or 36 volts.
Sources
- Battery University BU-105 — Battery Definitions (RC at 25 A)
- Battery University BU-802 — What Causes Capacity Loss
- Battery University BU-409b — Charging Lithium Iron Phosphate
If this is the job on your boat, see how we replace batteries at the boat. We come to you — Florida towns and counties.