Questions answered
Boat battery and wiring questions, answered
Small-boat questions, answered from Blue Sea Systems' ABYC-based guides and Minn Kota's own battery advice.
Updated October 24, 2026. Every answer lists the sources we read it from. Prices are the makers' own, from the day we last checked.
Disclosure: We may earn a commission if you buy through these links. It never changes what we recommend or what the tools calculate. How we make money.
How do I set up a dual battery system on a boat?
Short answer
A simple way is a dual-circuit battery switch plus an automatic charging relay (ACR), which is what Blue Sea's Add-A-Battery kit contains. The relay joins the batteries while charging and separates them during engine cranking and whenever nothing is charging. The switch keeps the engine and house circuits apart, and can join the batteries for an emergency start.
Blue Sea says the ACR shares the charge between two batteries more efficiently than a battery isolator, with no regulator adjustment or rewiring.
With a lithium house battery, follow the battery maker's charging rules; Battle Born recommends an isolator or a DC-DC charger between lead-acid and lithium (see below).
For your own rig: Adding a lithium house battery to a boat
Does every wire on a boat need a fuse?
Short answer
Nearly. Blue Sea, citing U.S. Coast Guard regulations and ABYC E-11, says every ungrounded wire except those in the engine starting circuit needs circuit protection. Choose the fuse to protect the wire: at least 125% of the load's current, and no more than the wire is rated for.
Blue Sea's example: an 80 A load needs at least a 100 A fuse, a 4 AWG single wire outside the engine room allows up to 150 A, and 125 A sits comfortably between them.
For your own rig: Wire and fuse chart
What is the ABYC 7-inch rule for battery fuses?
Short answer
ABYC E-11 says each ungrounded wire connected to a battery charger, alternator or other charging source needs overcurrent protection within 7 inches (175 mm) of where it connects to the battery or the DC system. Fuses that bolt straight onto a battery post, such as Blue Sea's MRBF terminal fuse, are made to meet it.
Blue Sea's MRBF fits 3/8 inch (M10) studs on batteries, battery switches and bus bars, and is ignition protected for gasoline boats.
That clause is the one Blue Sea quotes. E-11 has further distance rules and exceptions for other wires leaving the battery, and the standard itself is sold by ABYC rather than published free, so for the full text read E-11 or ask an ABYC-certified installer.
What size battery do I need for a trolling motor?
Short answer
Minn Kota suggests a 12 V deep-cycle marine battery of at least 110Ah, and says more amp-hours means more run time. Redodo rates its 12V 100Ah lithium for 12 V motors up to 70 lb of thrust, with two in series for 24 V motors up to 100 lb. With lithium, Minn Kota says to make sure the battery can supply the motor's maximum amp draw continuously.
Minn Kota also warns that lithium holds a higher voltage for longer, which can damage motors built for flooded lead-acid, and says to stay at 85% (speed 8.5) or lower on those motors.
Running a 24 V motor on 12 V: Minn Kota says it is not recommended, and electric-steer models such as Terrova and Ulterra cannot run on lower voltage at all.
Redodo 12V 100Ah Group 31 Bluetooth
1,280 Wh with a 100 A BMS, phone app monitoring and a low-temperature charge cutoff. Cheaper than Redodo's basic 100Ah the day we checked.
$260.99$389.99
See at Redodo (opens in a new tab)Can I use a car battery in my boat?
Short answer
Not for a trolling motor or house loads. Minn Kota says a starting battery is not suitable for an electric trolling motor, and that cranking-amp ratings say nothing about deep-cycle performance. Deep-cycle batteries are built to be run down and recharged; car batteries are not.
The reverse also holds: deep-cycle lithium is not a starter battery. Redodo says its self-heating 100Ah is not suitable for use as a starter battery.
Can I charge a lithium house battery from my boat's engine?
Short answer
Yes, through a DC-DC charger or an isolator, not a direct connection. Battle Born recommends one of those two between a lead-acid battery and lithium, and Victron warns that charging lithium straight from an alternator can overload it. Keep the lead-acid battery for starting the engine.
Minn Kota also recommends a separate deep-cycle battery for the trolling motor when a crank battery starts a gas outboard.
For your own rig: Boat house battery upgrade guide
Renogy 12V 20A DC-DC Charger
A gentle 20 A alternator charger for small banks and small or smart alternators.
Safety first
Fuse every positive wire as close to the battery or bus bar as you can, and use the wire and fuse sizes from a standard table, not a guess. Boats should follow ABYC E-11; anything that touches 120/240 V AC should follow the NEC. Have a qualified installer check any 120/240 V AC or shore power work.
Run the power planner
Battery, solar, charger, inverter, every wire and fuse, and a priced parts list.
Draw your wiring
Drag parts onto a diagram and get gauge and fuse labels on every run.
More questions answered
Keep going
Wire gauge and fuse chart
12 V and 24 V wire sizes and fuses by amps and run length, from the ABYC tables.
Lithium conversion
Swapping lead-acid for lithium: what it costs and the three things that must change with it.
Boat house battery upgrade
Adding a lithium house bank beside your starting battery without cooking the alternator.