Trolling Motor Battery Runtime Calculator

A trolling motor draws current proportional to the thrust it produces. At full throttle, a 55 lb thrust motor on 12V pulls roughly 50 amps. Drop to 50% speed and current falls to around 20 amps, not half, because thrust is non-linear with power.

Runtime = (battery capacity × 0.8) / amp draw

The 0.8 factor is the safe discharge limit for lead-acid batteries. Running below 20% state of charge (below about 11.8V on a 12V battery) degrades the battery noticeably. Lithium batteries tolerate deeper discharge but cost more upfront.

Typical amp draw by thrust and speed setting:

The system voltage is why the 80 lb row looks wrong. An 80 lb motor makes about half again as much thrust as a 55, yet the table says it pulls only 6 more amps. That is because it runs on 24V, so the same amps buy twice the watts. The amp figure is what drains each battery in the bank, and in a 24V bank the batteries sit in series: both see the same current, so runtime is set by the capacity of one battery, not two. Enter the rating of a single battery here, not the bank total.

Most anglers spend the majority of time at 30 to 50% throttle while positioning, with brief bursts to move spots. For trip planning, 50% average throttle gives a reasonable estimate.

Battery capacity drops in cold weather. A 100 Ah battery rated at 77°F loses roughly 20% at 32°F. If you fish in winter, build in that margin.

Two batteries in parallel double your amp-hours at the same voltage, which doubles runtime. Two in series give 24V at the same amp-hours, which is a different motor, not a longer day. Wiring them the wrong way is the most expensive mistake in this whole subject.