Marine Battery Runtime Calculator

Minn Kota publishes a maximum amp draw for every motor it sells, in the sizing table it uses to specify wire gauge and circuit breakers. Those figures are the starting point for any runtime estimate, and they are the only draw figures Minn Kota publishes.

The amp column is Minn Kota's own; the watt column is that figure multiplied by the row's voltage, which is all watts are. A 70lb motor on 24V draws fewer amps than a 55lb motor on 12V and still does more work, which is the whole reason bigger motors move to 24V and 36V.

There is no speed column here because Minn Kota will not publish one, and it states the reason in its own words: wind, waves, current and battery condition vary so much that it will not put a number on draw at any setting below full power. Minn Kota also warns that its maximum figures "only occur intermittently" and should not be treated as a continuous load. Enter the maximum if that is all you know, read the answer as the shortest run you will get, and clamp a meter on your own motor if you want a truer figure. For a bank that also runs a fridge or lights overnight at the dock, the deep cycle battery runtime calculator covers appliance-level loads.

Newport Vessels and Discover Battery both publish the same one-line method: amp-hours divided by amp draw equals run time. Newport works it through for its own motor: 624 watts on a 12V battery is 52 amps, and 50Ah divided by 52 amps gives 0.96 hours. It is a fine first pass and it is always optimistic, for two reasons this calculator makes explicit.

The first is depth of discharge. That method spends the whole battery. Concorde prints 50% as the deepest its Lifeline AGM batteries should routinely go, so half the printed answer is off the table before you start.

The second is discharge rate, and it is the one most calculators skip. A battery's amp-hour rating is measured over 20 hours. Pull harder and you get less: Trojan prints 225Ah at the 20-hour rate for its T-105, and 115 minutes at 75 amps, which is 144Ah, or 36% less capacity out of the same battery. A trolling motor is squarely in that territory, drawing a third to half the pack every hour. This page runs your numbers through a curve fitted to discharge tables Trojan, Lifeline and Victron print, so that loss is in the answer rather than in a footnote.

Put together on the default vector: 100Ah divided by 50 amps is 2 hours by the makers' method, 1 hour once you stop at 50% depth of discharge, and about 42 minutes once the rate correction is applied. If you want the deeper discharge without the penalty, set the chemistry above to LiFePO4 and the same load runs on lithium, where the rating already assumes a 1C discharge.