How to Manage Your Boat’s Power All Day

Know your three batteries

Most fishing boats with an outboard and a trolling motor end up with three jobs for their batteries. Keeping those jobs apart is the single biggest step toward never running out of power.

Small boats often combine the first two, or run everything from one battery. That works when the day's use is small, but you lose the safety of a protected starting battery.

Minn Kota's guidance is to keep a gas outboard's cranking battery out of the trolling motor bank. The reason is simple: a trolling motor can empty a battery in a few hours, and if that battery also starts the engine, you're paddling.

Check your batteries before you launch

A lead-acid battery's resting voltage tells you roughly how full it is. "Resting" matters. The reading is only accurate after the battery has sat with no charging and no load, ideally for a full day, so the best time to check is the morning after the charger finishes.

Resting voltage for a 12 V battery, per Discover Battery. Readings taken while the motor runs or right after charging will read high.

Lithium (LiFePO4) batteries hold nearly the same voltage until they are almost empty, so a voltmeter tells you little. Use the battery's Bluetooth app if it has one, or a battery monitor with a shunt, which counts the amp-hours going in and out.

Plan the day's power

Add up what each item uses: amps times the hours it runs. Our Whole-Boat Power Budget Calculator does this for you and checks it against your batteries. These typical figures, from manufacturer specs, show where the power usually goes.

The trolling motor line assumes 40% speed with the motor running 35% of the day, the same assumption our calculators use.

Two patterns show up on almost every boat. The trolling motor is the biggest single user, and big-screen electronics plus forward-facing sonar can quietly use as much as a small battery holds.

Get more from the trolling motor

A trolling motor's draw falls off much faster than its speed. In one posted clamp-meter test of a 45 lb variable-speed motor, it drew 31 A at full speed, 10 A at half speed and 4 A at 30%. Half speed used about a third of the current.

• Use the lowest speed that does the job. Bumping from 50% to 75% doubled the draw in that test, from 10 A to 20 A.
• Run the outboard for long moves. Crossing the lake on the trolling motor at full speed is the fastest way to empty its batteries.
• Use a drift sock in wind. Slowing your drift lets the motor work at a lower setting.
• Watch spot-lock in wind and current. Holding position keeps the motor working most of the time, often at high speed.
• Choose variable speed when you buy. Variable-speed motors use far less power at low settings than 5-speed motors.
• Keep the prop clean. Weeds and fishing line on the prop make the motor work harder for the same thrust.

Our Trolling Motor Power Calculator shows run time at every speed for your motor and battery.

Trim what your electronics use

• Run the livewell on a timer or recirculate setting. An 800 GPH pump uses 4.1 A. Running it a third of the day instead of all day saves about 22 Ah.
• Turn off forward-facing sonar when you aren't using it. At about 1.75 A, it uses more than a typical 7-inch fish finder.
• Dim big screens on cloudy days. Large units list their maximum draw with the backlight at full.
• Switch to LED lights. An LED navigation light is rated about 2.4 W. A 10 W bulb uses roughly four times as much.
• Turn off the stereo and radio when nobody is listening.

Charging while you run

Your outboard's alternator recharges the battery it's connected to while the engine runs. Yamaha lists 35 A at wide-open throttle for its F75, F90 and F115, and output is lower at idle and trolling speeds. A run across the lake puts back the charge used to start the engine and some of what your electronics used. It rarely makes up for a day of trolling.

Battery switches

Many boats have an Off/1/2/Both battery switch. A common routine is to start on the starting battery, run long enough to recharge it, then switch to the house battery for the rest of the day. Never turn the switch through Off while the engine is running. Interrupting the alternator's output can damage its diodes and voltage regulator.

Automatic charging relays

An automatic charging relay (ACR) connects two batteries when one is being charged and separates them when charging stops. Blue Sea Systems' version connects when either battery reaches 13.0 V for 2 minutes and separates when both fall to 12.75 V for 30 seconds. The result is that the alternator charges both batteries, but your electronics can never drain the starting battery.

Lithium and alternators

A lithium battery can pull far more current than a lead-acid battery, enough to overheat an alternator. Victron Energy recommends charging lithium from an alternator through a DC-to-DC charger, not by connecting it directly.

Trolling motor batteries

A 24 V or 36 V trolling motor bank usually isn't charged by the outboard at all. An onboard charger with one output per battery is the simplest way to keep it topped up. You plug the boat in when you get home, and every battery charges without unhooking anything.

A day on the water, start to finish

1. The night before. Confirm every battery finished charging. Check the resting voltage on lead-acid batteries, or the app on lithium ones.
2. At the ramp. Start the engine on the starting battery. Leave the trolling motor and electronics off until you need them.
3. The first run. Run the outboard long enough to replace the starting charge before you shut it down to fish.
4. Fishing. Use the lowest trolling motor speed that works. Run the livewell on its timer, and turn off forward-facing sonar between spots.
5. Midday check. If your fish finder shows battery voltage, glance at it. A lead-acid battery reading noticeably lower under the same load than it did in the morning is telling you to slow down.
6. Moving spots. Use the outboard for long moves. Every run also puts some charge back.
7. Heading in. Switch off electronics before you trailer the boat. Leave the bilge pump's automatic switch powered if the boat stays in the water.
8. At home. Plug in the charger right away.

After the trip

• Recharge within 12 to 24 hours. Minn Kota names leaving lead-acid batteries discharged as the leading cause of early failure.
• Charge every battery fully. A multi-bank charger handles each battery on its own, so a 24 V or 36 V set stays balanced.
• Top up flooded batteries. Check the water level and add distilled water if the plates are showing.
• Turn the battery switch off for storage. Small drains like stereo memory and radios can empty a battery over a few weeks. Keep the bilge pump's float switch powered if the boat is stored in the water.
• Replace batteries as a set. Mixing a new battery with old ones drags the new one down.

Cold weather

Cold cuts what a lead-acid battery can deliver. Battery Tender puts capacity at about 80% of rated at 32°F and 50 to 60% at 0°F. Plan on fewer hours from the same battery on early-spring and late-fall trips, and on the ice.

Lithium batteries handle cold discharge well but should not be charged below 32°F. Charging a frozen lithium battery causes lithium plating, which permanently reduces its capacity. Many lithium batteries have built-in protection that blocks charging in the cold. Let the battery warm up first, or buy a model with a built-in heater.

Warning signs

• The trolling motor feels weaker at the same setting. Voltage is dropping. Slow down or head in.
• The fish finder reboots or shows a low-voltage alarm. Its battery is nearly empty, or the wiring is too thin.
• The engine cranks slowly. The starting battery is low. Don't keep cranking. If you have a battery switch, try the other battery.
• A battery is hot or smells like rotten eggs while charging. Stop charging and have the battery and charger checked.

Questions people ask

Can I run my electronics off the trolling motor battery?

On a 12 V motor, yes, and plenty of small boats do. The catch is that the trolling motor can drain it, and on some boats electrical noise from the motor shows up on the sonar. On a 24 V or 36 V bank, don't tap one battery for 12 V. It unbalances the set and shortens its life.

Will my outboard charge my trolling motor batteries?

Usually not, unless the boat is wired with a relay or charger made for it. Most anglers charge the trolling motor bank with an onboard charger at home.

Should I leave the battery switch on Both?

Not while you fish. On Both, your electronics drain the starting battery along with the house battery. Use Both only to borrow power when one battery is weak.

How do I know how much power I use?

Add up amps times hours for each item, or enter your gear into the Whole-Boat Power Budget Calculator. For real numbers on your own boat, a battery monitor with a shunt measures it directly.