How do you calculate trolling motor runtime from amp draw on a 24V lithium battery?

Direct answer: Runtime in hours is roughly the usable amp-hours divided by the motor’s average current at the speeds you use. Take the current from the motor maker’s amp-draw chart or measure it with a clamp meter, keep a reserve for the trip back, and remember that wind, current and hull weight change the draw. The battery’s continuous current rating must also exceed the motor’s maximum draw.

Worked example, with stated assumptions. Suppose the motor maker lists about 20A at the speed you fish most and you want to keep 20% in reserve. A 24V 100Ah battery then gives about 80Ah usable, or 80 ÷ 20 = 4 hours. If you spend half the day at a setting that draws 40A, average current is 30A and runtime falls to about 2.7 hours. These numbers are only an illustration; use your motor’s figures.

Battery figures. Redway’s 24V 100Ah lists 25.6V, 2.56 kWh, 100A continuous, 200A peak for 3 seconds, 501 × 185 × 242 mm and 19 kg. The 24V 200Ah lists 5.12 kWh, continuous current options of 100A, 150A or 200A and 40 kg. The 24V 100Ah EU thin size lists 552 × 110 × 288 mm and 21 kg, for narrow spaces. All list IP65 and a 0°C to 60°C charge range.

Check the motor’s maximum. Most trolling motors draw their highest current at full thrust. That figure must be below the battery’s continuous rating. If the motor maker lists a maximum above 100A, the 100Ah pack is not suitable even if runtime looks fine.

Voltage behaviour. A lithium battery holds a fairly steady voltage, so the motor’s speed stays more even through the day than on lead-acid, but the battery management system will switch off at the low cut-off (20V on these pages). Watch a battery monitor rather than waiting for the motor to slow.

Boat installation. Secure the battery, protect terminals from spray, fit a fuse or breaker close to the battery sized for the motor cable, and follow the motor maker’s wiring guide. ABYC E-13 covers lithium-ion batteries on boats; ask your marine electrician how it applies. Our post on mounting a lithium battery in a boat covers restraint and location, and two 12V in series vs one 24V covers the wiring choice.