How it's calculated
Average current = Maximum current × % used
Run time = 20 × (Ah ÷ (Current × 20)) ^ k × Usable fraction
This uses Peukert's law (k ≈ 1.2 for AGM, 1.05 for lithium) because trolling motors draw high currents that reduce lead-acid capacity. See the Peukert calculator for more.
Worked example
A 55 lb thrust 12 V motor drawing 50 A at full speed, used at an average of 50% current (25 A), on a 100 Ah LiFePO4 battery: about 3.3 hours. On a 100 Ah AGM battery, only about 1.5 hours before reaching 50% discharge.
Tips
- Run at lower speeds whenever you can. Doubling speed can quadruple the current.
- Lithium batteries are much lighter and deliver close to full capacity at high current, which is why they're popular for kayaks and small boats.
- Use thick, short cables and a breaker or fuse sized to the motor's maximum current.
Frequently asked questions
How long will a 100Ah battery run a trolling motor?
At a typical average of 20–25 A, about 3.3–4.2 hours on LiFePO4 or 1.5–2 hours on lead-acid (to 50%).
What size battery for a 55lb trolling motor?
For a full day (6 hours) at moderate speed, about 150–200 Ah of LiFePO4, or roughly twice that in AGM.