What's a good battery for a kayak trolling motor, and how do I know how much is left?

Answered by RV Parts Giant · Upland, CA · October 3, 2026

Short answer: A LiFePO4 battery in the 50 to 100 amp-hour range suits most kayak and small-boat trolling motors. Do not trust a voltage gauge to tell you what is left: lithium holds nearly the same voltage until it is almost empty. A shunt monitor such as the Victron SmartShunt counts amps in and out and gives a real percentage.

Lithium iron phosphate is the usual answer now: a fraction of the weight of lead-acid for the same usable capacity, which matters when you carry it to the launch. The catch is that it changes how you read the battery.

How big

A 50 to 100 amp-hour battery covers most kayak and small-boat trolling motors. Where you land in that range depends on the motor, the wind and how many hours you fish, which is why the monitor below matters: within one outing it tells you whether you chose right. The Victron 12.8 volt Smart range runs from 50 to 200 amp-hours.

Why the gauge lies

Lead-acid voltage falls steadily as it empties, so a voltage bar graph is a rough fuel gauge. LiFePO4 holds almost the same voltage from about 90 percent down to 15 percent, then drops off a cliff. The bars read full all day and empty a minute before the battery's BMS cuts the motor.

The only honest measurement is counting current. A SmartShunt sits in the battery negative, subtracts every amp the motor draws and adds every amp the charger puts back. Your phone shows a real percentage and the time remaining at the current draw. It works on 12, 24 and 36 volt banks, so one 500 amp unit on the bank's main negative covers a series trolling bank too.

Fitting it in a kayak box

One wiring rule: the battery negative goes to the shunt and nothing else. The motor, charger and fish finder negatives all connect to the shunt's other side, or that current is invisible.
Keep it dry. The standard SmartShunt is not sealed. Inside a closed battery box is fine; on an open boat, put it in a dry compartment.
Protect the positive with a waterproof manual-reset breaker, which doubles as a disconnect, and add an Anderson-style plug on the load side so the battery comes out without tools.

Charge it to full

The shunt resets to 100 percent when it sees the charger hold full voltage with current tapered off, so the battery has to actually reach full now and then. A lithium charger with a proper profile, such as the sealed Blue Smart IP65, gets it there. Set the shunt's charged-voltage to the battery maker's figure, typically 14.2 to 14.4 volts on 12 volt lithium.