How to Wire a 12, 24 or 36-Volt Trolling Motor and Charge Its Batteries

Write down the motor's voltage, its maximum amp draw and the breaker and wire size from its chart. Measure the cable run from the batteries to the motor plug: the longer the run, the heavier the wire the chart will call for. Minn Kota says its motors ship with 5-foot, 10-gauge leads, recommends a qualified marine technician for extensions over 25 feet, and says not to alter the leads on its 30-pound Endura portable motor.

Choose deep-cycle batteries (flooded, AGM or lithium) that all match. Put them in a dry, ventilated spot close to the motor if you can, to keep the high-current cables short. Secure each one in a box or tray with a strap: a loose battery in rough water can short against metal or tear its cables off. Fit a non-conductive cover on every positive terminal.

Use a manual-reset circuit breaker (or a marine fuse) on the positive lead, sized to the maker's chart. Minn Kota recommends mounting it on the positive side as close as possible to the battery bank, and notes that its QUEST motors must not use a breaker larger than 60 amps. ABYC standards also set a maximum distance from the battery, so mount it close. A manual-reset breaker doubles as an off switch when you trailer or store the boat.

Never skip the breaker. It protects the wire, and an unprotected trolling motor cable is a real fire risk.

Use tinned marine battery cable in the gauge the chart gives for your total run. Keep red for positive and black for negative. Route the cables away from sharp edges, the fuel system and moving parts, use grommets through bulkheads, and support them with clamps so they can't chafe. Crimp lugs with a proper crimper and seal them with adhesive heat-shrink. Do not connect anything to the batteries yet.

Following Minn Kota's series diagram, with the motor off:

1. Connect a jumper cable from the positive (+) of battery 1 to the negative (−) of battery 2.
2. Connect the motor's negative (black) lead to the negative of battery 1.
3. Connect the motor's positive (red) lead, through the breaker, to the positive of battery 2.

The voltage only adds up across the two "outside" terminals: battery 1 negative and battery 2 positive. Use a jumper at least as heavy as the motor's power cables, or the size the maker specifies.

Same idea, one more battery:

1. Jumper from battery 1 positive to battery 2 negative.
2. Jumper from battery 2 positive to battery 3 negative.
3. Motor negative to battery 1 negative; motor positive (through the breaker) to battery 3 positive.

To get more run time on a 12-volt motor, two matched batteries can be wired in parallel: positive to positive and negative to negative. Minn Kota's diagram then takes the motor's positive from one battery and its negative from the other, so both batteries share the load evenly. Parallel adds run time, not voltage. Never wire batteries in series for a 12-volt motor.

Set the multimeter to DC volts and measure across the motor's positive and negative cable ends (or the receptacle). A healthy bank reads a little above 12, 24 or 36 volts, depending on the setup, and the reading must be positive with the red probe on the red cable. If you see about half the voltage you expect, a jumper is missing or on the wrong terminal. If the reading is negative, the leads are swapped. Fix it before connecting the motor.

A marine trolling motor plug and receptacle lets you remove the motor or disconnect it quickly. Use one rated for your motor's current and voltage, wire it to its instructions, and keep the polarity consistent. Connect and disconnect only with the motor turned off, so you don't arc the contacts.

BoatUS used a two-bank onboard charger on its 24-volt project boat. The usual way to charge a series bank is a marine onboard charger with one output per battery, so each 12-volt battery is charged on its own, even while the series jumpers stay in place. Check that your charger's outputs are isolated and that it is approved for series banks. A charger built for the full bank voltage is another option. Do not charge a 24- or 36-volt bank with a 12-volt charger clipped across just one or two of the batteries; the bank ends up unbalanced. Minn Kota states plainly that batteries connected in series cannot be charged from one charger bank, that its chargers are built for 12-volt batteries only, and that the jumpers can stay in place when each battery has its own bank. It also recommends not mounting an onboard charger below the waterline.

Want to top up the trolling bank while you run? Some makers sell an alternator (or "on-the-go") charger that feeds the trolling batteries from the outboard's spare charging output, but only after the starting battery is charged. Check that your engine's output and your battery type are on the charger's supported list.

• Set each charger output for its battery type (flooded, AGM or lithium).
• BoatUS stresses a marine-grade, ignition-protected charger and good ventilation while charging.
• Lithium batteries need a charger and settings made for lithium, and often a different approach. Follow the battery maker's instructions.

Do not tap 12 volts from one battery of a 24- or 36-volt bank to run a fish finder or lights: that battery drains faster than the others and the bank gets out of balance. Power electronics from the boat's house or starting system instead, unless the electronics maker says otherwise.

Run the motor through all speeds and check that no cable, lug or the breaker gets warm. After a few trips, recheck that all terminals are tight. At least once a season, clean terminals, look for chafed cables and load test the batteries; replace them as a matched set.