How long will a 100Ah battery run a trolling motor?
A 100Ah battery does not have one fixed runtime. This calculator begins with rated capacity, adjusts it for discharge rate, applies your usable-depth setting, then holds back the chosen reserve. The same rated AGM bank will generally produce a shorter high-current planning runtime than LiFePO₄ because the default usable depth is lower and lead acid is more affected by discharge rate.
Your motor’s average current matters more than its maximum rating. Wind, current, weeds, hull weight, steering corrections, and the time spent at each speed can all change the real number. A battery monitor or ammeter reading from a typical trip gives the best input.
Battery-sizing guide
How many amp-hours do I need for my trolling motor?
As a planning starting point, required usable amp-hours are approximately average current in amps multiplied by desired runtime in hours. The labeled battery capacity must be higher when usable-depth limits and a reserve margin are included.
Usable Ah needed ≈ average amps × hours. Labeled bank Ah must also account for usable depth, reserve, and this calculator's documented discharge-rate adjustment.
- Use measured average current when possible. A motor's maximum draw is not the same as its normal trip average.
- Account for usable capacity. The calculator starts at 90% usable depth for LiFePO₄ and 50% for AGM or flooded lead acid; your battery documentation takes precedence.
- Keep a reserve. The default 20% margin leaves energy for changing conditions and estimate error.
- Compare systems by energy as well as Ah. Series-connected batteries raise voltage and watt-hours without adding Ah.
- Expect real-world variation. Duty cycle, wind, current, weeds, hull load, wiring loss, temperature, battery age, and motor efficiency can change runtime.
Illustrative battery-runtime scenarios
ILLUSTRATIVE / ESTIMATED — not manufacturer-certified performance. Each example assumes measured average current and applies the calculator's usable-depth, reserve, and discharge-rate adjustments.
| System | Average draw | Battery bank | Chemistry | Planning assumptions | Estimated runtime | Try it |
|---|---|---|---|---|---|---|
| 12V | 20A | 100Ah | LiFePO₄ | 90% usable depth; 20% reserve | 3 hr 22 min | |
| 12V | 20A | 100Ah | AGM | 50% usable depth; 20% reserve | 1 hr 25 min | |
| 24V | 25A | 100Ah | LiFePO₄ | 90% usable depth; 20% reserve | 2 hr 39 min | |
| 36V | 30A | 100Ah | LiFePO₄ | 90% usable depth; 20% reserve | 2 hr 12 min |
These are planning estimates, not guaranteed runtime, manufacturer performance data, or an electrical-system safety determination. Consult the motor and battery documentation for your equipment.
How long will a lithium battery run a trolling motor?
LiFePO₄ batteries usually allow more of their rated capacity to be used than lead-acid batteries. This calculator defaults to 90% usable depth as a conservative planning choice—not a technology limit. Some LiFePO₄ products publish up to 100% rated depth of discharge. Use the Advanced setting when your battery documentation supports a different value.
Battery-management-system limits, temperature, age, wiring, and the manufacturer’s instructions still matter. The battery manufacturer’s specification always takes precedence, and its continuous discharge rating must support the motor.
How does trolling motor speed affect battery life?
Speed dial percentage is not the same as current percentage. Propeller power generally rises faster than speed, and motor controls differ. A motor at a 50% setting often draws considerably less than half its maximum current, but no single curve is exact for every motor and boat.
Instead of pretending the relationship is linear, this calculator uses a documented curved planning estimate when starting from maximum current. Manufacturer motor classes use published maximum-current values—not a universal thrust formula. If you enter measured average amps, the calculator skips the load curve entirely.
Is lithium better than AGM for trolling motors?
LiFePO₄ generally provides more usable energy for the same rated Ah and typically weighs less. AGM is often less expensive upfront, widely understood, and can suit occasional use. Lithium requires a compatible charger and careful attention to cold-temperature charging and battery-management limits.
The better choice depends on budget, trip length, weight sensitivity, charger compatibility, and how frequently the battery cycles. This calculator compares energy availability, not purchase price, lifespan, or suitability for a particular boat.
What size battery do I need for a trolling motor?
Start with the average amps the motor will use, desired hours, usable depth, and reserve. The sizing mode reverses the same C20 Peukert-based adjustment used by runtime mode, then rounds the minimum up to a practical bank Ah target.
For a series bank made from 12V batteries, Ah does not add: two matching 100Ah batteries in series make a 24V 100Ah bank, while three make a 36V 100Ah bank. Parallel strings add Ah. Always use matching batteries of the same chemistry, capacity, age, and state of charge when the manufacturer permits the configuration.
How do 12V, 24V, and 36V trolling motor systems differ?
Higher-voltage systems deliver the same electrical power with less current, which can reduce wiring losses and support larger motors. A 24V bank can use two 12V batteries in series or a native 24V battery; a 36V bank can use three 12V batteries or a native 36V battery. Series watt-hours add even though Ah stays the same.
Voltage does not create free energy. A 24V 100Ah bank stores twice the nominal energy of a 12V 100Ah bank because it normally contains twice as many equivalent batteries. Use the voltage required by the motor and follow its wiring, breaker, and battery guidance.
Calculation limits
This is a planning tool, not a manufacturer specification or safety calculator. The generic Peukert exponents are rough defaults; battery-specific discharge tables are better. Capacity can fall in cold weather and as batteries age. Connections, cable loss, waves, current, and vegetation also affect results. Keep a safe reserve and verify critical trips with real measurements.
Calculation methodology and sources
Motor-class references come from the Minn Kota Trolling Motor Wiring and Battery Guide. Minn Kota states that its listed maximum draws occur intermittently and should not be treated as continuous loads.
The discharge-rate adjustment follows the C20 form of Peukert’s law described in the Victron BMV battery-monitor documentation. When battery-specific data is unavailable, Victron documents 1.25 as a generic lead-acid exponent and 1.05 for lithium; it also cautions that Peukert is an approximation.
Depth of discharge varies by product. The 90% LiFePO₄ default is deliberately conservative: for comparison, the Battle Born BB10012 manufacturer datasheet publishes 100% usable depth for that model. The 50% AGM planning default aligns with conservative sizing guidance in the Lifeline AGM technical manual. These examples do not replace the documentation for your battery.