Guide
Boat house battery upgrade
Updated October 22, 2026. Prices checked on the makers' own sites that day.
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.
The layout
- Starting battery stays as it is, wired to the engine.
- DC-DC charger from the starting battery to the house battery, fused at both ends.
- House battery (lithium) with a fuse within 7 inches of its positive terminal and a battery switch.
- House loads (electronics, lights, pumps, fridge) through a fused panel. The bilge pump's automatic float switch usually stays wired so it works with the switch off; follow your boat builder's and ABYC's guidance on that circuit.
- Charging at the dock through a charger; keep a plug-in charger like the Redodo 14.6V 20A LiFePO4 Charger out of the weather, or use a marine-rated onboard charger.
The starter-to-charger cable for a 20 A DC-DC charger over 10 ft is 8 AWG on a 30 A fuse by our method. The full chart.
Size it
The planner has a boat preset. Set engine hours for the alternator, and set solar to what fits on a T-top or bimini. Our preset comes to 758 Wh a day and picks a Redodo 12V 165Ah Group 31 Bluetooth.
Day boats
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)Overnights and a fridge
Redodo 12V 165Ah Group 31 Bluetooth
2,112 Wh in a Group 31 case with a 165 A BMS: enough for a 1,500 W inverter on its own.
$384.99$559.99
See at Redodo (opens in a new tab)Charging from the engine
Redodo 12V 40A DC-DC Charger with MPPT
Charges the house battery from the alternator while you drive, and has a solar input too.
$175.99$189.99
See at Redodo (opens in a new tab)Know what's left
Redodo 500A Battery Monitor with Shunt
Counts every amp in and out so the percentage you see is real, not a voltage guess.
$92.99$99.99
See at Redodo (opens in a new tab)Running an electric reel?
Big-game electric reels pull hard from the boat's 12 V. Piscifun says its Piscifun AtlantiX 12000 Electric Reel (opens in a new tab) ($1,499.99) needs an external battery, so give it its own fused outlet sized to the reel maker's current rating, on the house bank rather than the starting battery.
More boat guides
Spark Powered covers the rest of the boat: boating hub, charging lithium on a boat and trolling motor batteries.
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. Shore power, inverter-chargers and transfer switches belong to a qualified electrician or marine installer, or at least their inspection.
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.
Questions people ask
Can I replace my boat's starting battery with lithium?
Not with a deep-cycle house battery like the ones in this guide; cranking needs a battery rated for it (Redodo sells separate cranking and dual-purpose models). The simplest upgrade keeps your starting battery and adds lithium as a separate house battery.
How do I charge a lithium house battery from my outboard?
Through a DC-DC charger from the starting battery. It protects the engine's charging system and charges lithium at the right voltage. Small outboards often have modest charging output; a 20 A unit like the Renogy 12V 20A DC-DC Charger ($140.99) is the gentle choice there.
What size house battery does a small boat need?
Our planner's boat preset (fridge, fish finder, nav lights, bilge pump, stereo, phones) uses about 758 Wh a day. With two days of reserve that's a 165 Ah lithium battery. Without the fridge, one 100 Ah battery is plenty for a day on the water.
Does ABYC require a fuse on the house battery?
ABYC E-11 calls for overcurrent protection on positive conductors within 7 inches of the battery connection, with limited exceptions, and a battery switch that can isolate the bank. Marine-grade tinned wire is the norm.
Sources
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.
DC-DC charger sizing
20 A or 40 A, smart alternators, and what an hour of driving puts back.
Lithium conversion
Swapping lead-acid for lithium: what it costs and the three things that must change with it.
Best lithium battery for a van
Redodo and LiTime batteries by price per usable watt-hour, BMS current and cold-weather charging.