Trolling Motor Battery FAQs

Trolling motor batteries generate more specific questions than almost any other marine battery topic. Anglers want to know how long their batteries will last on the water, which chemistry is best, how to size their bank, and how to get the most out of every charge. These are the most common trolling motor battery questions answered in plain language.

Choosing the Right Battery

What is the best battery for a trolling motor?

A dedicated deep cycle battery is always the correct choice for a trolling motor — either AGM deep cycle or lithium LiFePO4. Deep cycle batteries are built for the slow, sustained current draw that trolling motors demand over many hours of use. AGM deep cycle batteries are the most popular choice for their sealed construction, maintenance-free operation, and compatibility with all standard chargers. Lithium batteries are increasingly preferred by serious and tournament anglers for their dramatically lighter weight, higher usable capacity, and consistent voltage output throughout the day.

Can I use a starting battery for my trolling motor?

No. Starting batteries use thin plates designed for short bursts of high current — the exact opposite of what a trolling motor needs. Using a starting battery for trolling motor duty causes rapid plate degradation, significantly shortened battery life (often failing within a single season), and poor runtime. Always use a dedicated deep cycle battery for trolling motor applications.

Can I use a dual-purpose battery for my trolling motor?

A dual-purpose battery is better than a starting battery but will not match the runtime or longevity of a dedicated deep cycle battery for serious trolling motor use. Dual-purpose batteries are a reasonable choice for casual anglers on smaller boats who only run their trolling motor for short periods. For anglers fishing all day with a large motor, a dedicated deep cycle battery is the right tool.

What is the difference between AGM and lithium for a trolling motor?

AGM deep cycle batteries are sealed, maintenance-free, vibration-resistant, and compatible with all standard chargers. They must not be regularly discharged below 50% of rated capacity, giving a 100Ah AGM battery approximately 50Ah of practical daily use. Lithium LiFePO4 batteries are 50 to 70% lighter than equivalent AGM, can be safely discharged to 80 to 100% of rated capacity, maintain consistent motor speed throughout the discharge cycle due to their flat voltage curve, and last four to six times longer. Lithium costs significantly more upfront but is often the more economical choice for anglers who fish frequently.

What group size battery is best for a trolling motor?

Group 31 is the preferred choice for serious trolling motor use — it offers the highest capacity in the standard case size. Group 27 is a practical mid-range option for most fishing boats balancing size, weight, and amp hours. Group 24 suits smaller motors on boats with limited battery space. For 24V and 36V multi-battery systems, Group 31 batteries in each position maximize the bank's total capacity.

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Sizing and Runtime

How long will my trolling motor battery last on the water?

Runtime depends on your battery's usable capacity and your motor's average amp draw. Divide your battery's usable capacity by the motor's average draw. A 100Ah AGM battery (50Ah usable) powering a 55 lb thrust motor drawing an average of 25 amps gives approximately 2 hours of runtime. A 100Ah lithium battery (80Ah usable) on the same motor gives approximately 3.2 hours. At lower speeds, amp draw drops significantly and runtime increases proportionally — most anglers run at less than full throttle most of the time.

How do I calculate how much battery capacity I need?

Find your motor's average amp draw from the owner's manual and multiply by the hours you want to fish. For an AGM battery, divide the result by 0.50 to account for the 50% usable capacity limit. For lithium, divide by 0.80. For example, a 55 lb thrust motor averaging 25 amps over 8 hours needs 200Ah of energy. That requires a 400Ah rated AGM bank or a 250Ah rated lithium bank. When in doubt, size up — running out of power on the water is more costly than carrying slightly more battery capacity than needed.

How many batteries do I need for a 24V trolling motor?

A 24V trolling motor requires two 12V deep cycle batteries wired in series. The batteries must be identical — same chemistry, same amp hour capacity, same age, and ideally same brand and model. Each battery still needs to be charged individually, so a two-bank charger with one output per battery is the correct charging solution. Never charge a series bank with a single charger output connected across the full 24V — this charges batteries unevenly.

How many batteries do I need for a 36V trolling motor?

A 36V trolling motor requires three 12V deep cycle batteries wired in series. All three must be identical. A three-bank charger with one output per battery is required. As with 24V systems, replace all three batteries as a matched set — mixing ages or capacities in a series bank causes uneven charging and accelerated degradation of the weakest battery.

Does a higher thrust trolling motor drain the battery faster?

Yes, at the same speed setting. Higher thrust motors are physically larger and draw more current at each throttle position. However, higher thrust motors are often run at lower speed settings to achieve the same boat speed as a smaller motor at higher settings — which can partially offset the current draw difference. The key variable is average amps drawn over a fishing day, which depends more on how you use the motor than on its maximum thrust rating alone.

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Charging Trolling Motor Batteries

How should I charge my trolling motor battery?

Charge after every outing — never let a depleted trolling motor battery sit overnight or between trips. Use a multi-bank onboard charger or portable smart charger matched to your battery chemistry. For AGM batteries, use AGM mode. For lithium, use a lithium-compatible charger. Size the charger output at no more than 20% of the battery's amp hour rating per bank — a 10-amp output for a 100Ah battery, 20-amp for a 200Ah battery. Charging promptly after use prevents sulfation and keeps the battery in peak condition.

How long does it take to fully charge a trolling motor battery?

Charging time depends on the battery's amp hour capacity, how depleted it is, and the charger's output. A rough formula: divide the amp hours that need replacing by the charger's amp output, then add 20% for the absorption stage. A 100Ah AGM battery depleted to 50% (needing 50Ah) on a 10-amp charger takes approximately 6 hours to fully charge. A 20-amp charger halves that time. Lithium batteries charge faster than AGM at the same charger output due to their higher charge acceptance rate.

Can I charge my trolling motor batteries while the boat is on the trailer?

Yes. Many anglers use a portable multi-bank smart charger connected to all trolling motor batteries while the boat sits on the trailer in the garage between trips. This is one of the most effective ways to keep trolling motor batteries in optimal condition. Alternatively, a permanently mounted onboard charger with a shore power inlet allows charging by plugging into a garage outlet without opening any hatches or making any connections.

Should I leave my trolling motor batteries on the charger between fishing trips?

Yes — a smart charger with a float maintenance mode is safe to leave connected indefinitely. Once the batteries reach full charge, the charger drops to a low maintenance voltage that keeps them topped up without overcharging. This is the ideal setup for anglers who fish frequently and want their batteries always ready. Never leave a basic single-stage charger connected without supervision — only smart chargers with float mode are safe for long-term connection.

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Performance and Troubleshooting

Why is my trolling motor running slower than it used to?

Reduced trolling motor speed is caused by one of three things: a battery that no longer holds adequate charge, voltage drop in the wiring, or a problem with the motor itself. Start by testing the battery — a battery that reads full voltage at rest but collapses under load has degraded plates and needs replacement. Next check the wiring connections, particularly the quick-connect plug at the motor, for corrosion. Measure voltage at the motor terminals while running at full speed — significantly less than battery voltage indicates a wiring voltage drop problem.

Why does my trolling motor battery die faster than it used to?

Reduced runtime is the most common sign of battery aging. As lead-acid batteries cycle, plate degradation reduces their effective capacity. A battery that once lasted a full day may only last half a day after two or three seasons of hard use. Have the battery load-tested at West Marine — a battery that passes a voltage test but fails a load test has lost significant internal capacity and should be replaced before the season starts.

Can I run my trolling motor while the battery is being charged?

Not recommended for onboard chargers — running the motor while the charger is connected creates a conflicting load that can interfere with the charger's ability to deliver the correct charge profile. The engine alternator, however, does charge the trolling motor bank through a combiner relay while the engine is running — this is normal and acceptable. For shore power charging, disconnect the motor and charge the bank fully before your next outing rather than trying to run and charge simultaneously.

What voltage should my trolling motor battery read after a full day of fishing?

After a full day of fishing and with the motor off, let the battery rest for at least 30 minutes before measuring. For AGM batteries, a reading above 12.2 volts indicates the battery was not excessively discharged. A reading between 12.0 and 12.2 volts means it is at or near the recommended 50% discharge limit for AGM — acceptable but the battery should be charged promptly. Below 12.0 volts means the battery was discharged below the recommended limit — charge immediately and consider increasing your battery bank size.

How do I know when my trolling motor battery needs to be replaced?

Replace your trolling motor battery when it fails a load test, when it no longer lasts a full fishing day despite starting the trip fully charged, when it takes significantly longer than usual to charge, when resting voltage does not reach 12.65 volts after a full charge, when the case is swollen or cracked, or when the battery is more than 4 to 5 years old with any performance decline. West Marine offers free battery load testing at all store locations.

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