How to Charge a Trolling Motor Battery

How to charge a trolling motor battery comes down to four habits: use a charger set for the battery’s chemistry, recharge after every trip, give each battery in a 24- or 36-volt bank its own charger output, and stay inside the temperature limits your battery maker publishes. Lead-acid, AGM, and lithium batteries each have their own rules, and the makers do not always agree, so this guide shows where they differ.

This guide is general product information, not repair or electrical safety advice. Follow the installation, wiring, and charging instructions in your product’s manual, stop using any battery, charger, or electronics that overheat, swell, or smell of burning, and contact the manufacturer.

The same steps cover how to charge a marine battery that runs a trolling motor, and every instruction below comes from a battery, charger, or motor maker’s manual or support page, linked where it is cited. Your own battery’s and charger’s manuals are the final word, and wiring belongs to those manuals or a qualified marine technician.

Step 1: Set the Charger for the Battery Chemistry

LiTime, a lithium battery maker, says in its marine battery maintenance guide that lead-acid, AGM, gel, and LiFePO4 batteries require chargers that match their battery type, and that the wrong charger or settings can keep a battery from reaching a full charge or cause overcharging or undercharging.

• Flooded lead-acid and AGM: the ProMariner charger FAQ says to use the AGM setting if your charger has one, adds that most AGM batteries also charge well on the lead-acid setting, and recommends checking with the battery maker. The Odyssey battery manual recommends an automatic charger with an AGM setting.
• Gel: ProMariner says gel batteries have quite different charging and float voltages and cannot be charged with lead-acid or AGM batteries.
• Lithium (LiFePO4): LiTime recommends a dedicated LiFePO4 charger. Battle Born Batteries says a lead-acid charger can work sometimes, but only if it can be set to the correct voltage and has no equalization mode, and that you should verify compatibility with your battery maker’s specifications.

LiTime’s is the more cautious reading: unless your charger’s manual lists a LiFePO4 profile and your battery maker approves it, plan on a different charger. That matters most when you replace a lead-acid battery with lithium and keep the boat’s old charger. Our comparison of lithium and AGM batteries covers the rest of that choice.

Step 2: Recharge After Every Trip

LiTime’s maintenance guide calls prompt recharging after each trip an important part of battery care, and the Newport Kayak Series owner’s manual says to recharge batteries after each use and follow the battery maker’s recommendations. The same manual says to disconnect power from the motor when charging batteries.

How full counts as full depends on chemistry. Odyssey says its AGM2 batteries reach their rated cycle life only when 103 to 105% of the discharged amp-hours are returned after each discharge, and that over- or undercharging costs cycle life and capacity. Lithium is more forgiving: Battle Born says partial recharges are fine for long-term life. On lead-acid, LiTime’s lifespan guide warns that batteries are particularly vulnerable to sulfation when left undercharged, so leaving a half-charged battery until next weekend is the habit to break. How long a full charge lasts on the water is a separate calculation: see how long a 12-volt motor runs on one charge.

Onboard or Portable Charger

A portable charger stays in the garage and the battery comes to it; an onboard charger is mounted in the boat and plugged into AC power at home or the dock. ProMariner’s FAQ says its waterproof onboard chargers charge one battery on each output, suit small fishing boats such as bass boats and center consoles, and are meant for batteries that are not in use while they charge. LiTime’s trolling motor battery FAQ gives the battery maker’s version: a standard charger works if you charge off the boat in a dry area, but a waterproof charger is a must if the battery stays onboard.

• Kayak or canoe: portable, since the battery comes home after every trip. Skip an onboard charger if the hull has no dry, solid mounting spot.
• Jon boat with one battery: either works; onboard saves lifting a heavy lead-acid battery in and out.
• Bass boat with a 24- or 36-volt bank: an onboard charger with one output per battery, so two or three batteries stay in the compartment.

ProMariner recommends sizing a waterproof charger to about 10% of the total amp-hour capacity it charges, and estimates 8 to 14 hours for a full charge, depending on battery type, condition, age, and depletion. Odyssey asks for more current on its AGM2 batteries: at least 40% of the 10-hour amp-hour rating for optimum charging, and never less than 10%. When a charger maker’s rule of thumb and your battery manual differ, follow the battery manual. Mounting and wiring follow the charger’s installation manual.

How to Charge Trolling Motor Battery Banks (24V and 36V)

How to charge trolling motor batteries in a bank starts with the motor: a 24-volt motor usually runs on two 12-volt batteries in series and a 36-volt motor on three; our guide on how 12-volt and 24-volt systems differ explains why. The charging concept is simple: each 12-volt battery gets its own charger output. ProMariner says its waterproof chargers put out 12 volts on each set of leads, that a two-bank charger charges a 24-volt system with one set of leads on each 12-volt battery, and that you need one output for each battery you want to charge.

• One chemistry per setting. ProMariner says its chargers output voltages for one type of battery in each setting, so lead-acid and gel batteries cannot be charged together.
• Matched batteries. LiTime’s FAQ says every battery in a bank must be the same model, with the same capacity and BMS current rating, ideally bought within a month of each other. Odyssey says multiple-battery setups should be matched in voltage, and replacement batteries should be at a similar state of charge.

A single 24- or 36-volt lithium battery is one battery, not a bank: use the charger its manual specifies. For the connections, follow the diagrams in your charger’s and motor’s manuals, or have a qualified marine technician do the work. If you are still sizing the bank, see what size battery your motor needs.

Charging While on the Water

ProMariner’s waterproof chargers run on AC power and, it says, are for batteries not in use while charging, so they stop helping once you leave the dock. LiTime’s maintenance guide lists engine alternators, DC-DC chargers, and solar as other ways to recharge marine batteries. Battle Born says alternators are designed for starting batteries, not deep-cycle lithium systems, so connecting them directly can cause high current surges or voltage mismatches; a DC-to-DC charger, it says, prevents alternator overload and ensures proper charge profiles. For solar, it says MPPT controllers are preferred for lithium systems. These are installation projects for the manuals or a qualified marine technician. On most days the simpler plan is Newport’s: start charged and keep enough power in reserve to get back with the motor.

Cold and Heat: Charging Temperature Limits

LiTime says LiFePO4 batteries without low-temperature protection or a heating function should not be charged below 32°F, and that cutoffs vary by model. Battle Born gives 32 to 113°F as the ideal charging range. Its battery management system blocks charging below 24°F per its winterizing FAQ, or below 25°F per its article on leaving marine batteries in the boat over winter, which also recommends bringing batteries above freezing before charging. The winterizing FAQ warns that charging in extreme cold can cause lithium plating.

Treat that cutoff as a backstop, not a target: the two pages differ by a degree, and both makers point to charging above freezing. Before charging in an unheated garage in March or after ice-out, check whether your battery has low-temperature protection or self-heating.

For AGM, the Odyssey manual we read lists no minimum charging temperature. It calls for temperature compensation instead, raising charge voltage as temperatures fall, and ProMariner lists automatic temperature compensation on its ProNautic Professional chargers. Check your battery’s manual for any low-temperature limit.

Heat limits differ too. Odyssey says not to charge its batteries above 104°F, Battle Born’s ideal range tops out at 113°F, and LiTime says to keep charging ventilated to avoid excessive heat. A closed compartment in July sun runs hot, so charge in the shade or after the boat cools.

Charging Safety by the Manual

The Newport manual warns that a battery produces hydrogen and oxygen gases while charging, that this potentially explosive mixture may form an explosive atmosphere around the battery for several hours afterward, and that arcing or flames can ignite it. It also says to make sure all switches are off before connecting a battery, and warns that the sulfuric acid in batteries can cause severe burns. Odyssey says its sealed AGM2 batteries must not be stored, operated, or charged in a sealed or gastight enclosure.

• Ventilate. Charge where air moves, away from sparks and flames.
• Check flooded batteries. ProMariner says stored batteries need checking at least once a month, including fluid levels in wet-cell flooded batteries.
• Look before you connect. LiTime says to check the case for cracks, leaks, or swelling and to stop using the battery immediately if anything abnormal turns up.

Stop charging and contact the battery maker if a battery swells, gets unusually hot, or smells of burning. Odyssey says any battery producing an odor or visibly venting should be removed from service, and disconnected only after the area has been ventilated. This guide can’t diagnose a specific battery; the maker can, with the model and symptoms.

Off-season charging follows your battery manual’s storage schedule, and the makers differ there as well; our guide to how long a trolling motor battery lasts covers winter storage.

Frequently Asked Questions

Can you charge a trolling motor battery while on the water?

Not with a typical AC onboard charger. Charging underway needs a DC source, such as an alternator feeding a DC-to-DC charger, or solar, set up following the manuals or by a qualified marine technician.

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

ProMariner’s rough estimate is 8 to 14 hours, depending on battery type, condition, age, and how deeply the battery was discharged.

Can I use a car battery charger on a trolling motor battery?

Check it first. Odyssey says standard automotive chargers normally charge to 80 to 95%, and that some portable chargers exceed 15.0 volts, the level above which it says its AGM2 batteries suffer irreversible damage. For lithium, use a charger with a LiFePO4 profile your battery maker approves.

Can I leave a trolling motor battery on the charger?

ProMariner says yes if the charger is set for the right battery type, using its Auto-Maintain mode, though stored batteries still need a monthly check. Odyssey says float or trickle chargers suit keeping a full AGM2 battery topped up but not recharging a deeply discharged one. Battle Born advises against prolonged high-voltage float charging on lithium.

More on batteries and power for small boats is in the Batteries & Power section, motors are in the Trolling Motors section, and how we evaluate products is on How We Review.

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