Reusing the existing cable because the old bank was the same amp-hours.
Amp-hours are not the issue. Lithium can deliver far higher current into a fault or an inverter.
Sizing
How to Size Boat Battery Cable: Current & Voltage Drop
Size every run for the highest sustained current that run can actually pass, then check it again for voltage drop over the round-trip length. The cable on an older boat was sized for a bank that could never deliver lithium's currents.
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- 4 September 2026
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Equipment and information required
- The continuous rating of every device on the run
- Round-trip cable length, positive and negative
How to Size Boat Battery Cable: Current & Voltage Drop: steps
Find the highest sustained current on each run
For the inverter feed that is its continuous rating, not its surge. For the DC-DC charger it is the charger's output. For the windlass it is the loaded draw, not the free-running figure.
Size for current first
Choose a conductor rated to carry that current continuously at the temperature it will actually sit at. Cable bundled in a hot engine bay carries less than the same cable in free air, and the rating tables assume free air.
Then check voltage drop over the round trip
Measure positive and negative together, since the current goes both ways. A charging run that drops too much voltage will never finish the charge, and the monitor will show a bank that never quite fills.
Take the larger of the two results
Current capacity and voltage drop give two different cable sizes. On long runs, voltage drop almost always wins, and it is the one people skip.
Use tinned marine cable and proper terminations
Crimped, heat-shrunk, tinned lugs, torqued to specification, with no more than four connections on a stud. A loose terminal on a lithium bank is a heat source with a very large power supply behind it.
Re-check the old runs you are keeping
Cable that was adequate for a lead-acid bank and no inverter may be badly undersized for what the new system can pass. Any run you reuse needs the same two checks as a new one.
Common installation and calculation errors
Related equipment and battery guides
More on sizing
How to Calculate Boat Electrical Loads in Amp-HoursMultiply each load's current in amps by the hours it genuinely runs in a day, then add them up. A 5 A fridge cycling at 45% duty is 5 × 10.8 = 54 Ah, not 5 × 24.How to Size a Boat Lithium House Battery BankTake your daily amp-hour draw, subtract half of what your solar returns on an average day, multiply by the nights you spend away from shore power, then divide by 0.8 because you cycle lithium to 80%. Round up to a bank you can buy.How to Size a Marine DC-DC Battery ChargerTake the smaller of a quarter of your bank's capacity and half your alternator's rating, then round down to a charger you can buy. A 300 Ah bank behind a 90 A alternator gives min(75, 45) = 45, so a 40 A charger.How to Size an MPPT Solar Controller for a BoatDivide your array's watts by the bank's nominal voltage to get charge current, add headroom for a cold bright day, and buy a controller at or above that figure. 400 W on a 12.8 V bank is about 31 A, so a 30 A controller is marginal and a 40 A is not.