Buying the biggest DC-DC charger in the budget.
Oversizing is the failure mode. The number is a ceiling, so round down.
Sizing
How to Size a Marine DC-DC Battery Charger
Take 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.
- Steps
- 6
- Last reviewed
- 4 September 2026
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Equipment and information required
- Your bank capacity in amp-hours
- Your alternator's rated output in amps
How to Size a Marine DC-DC Battery Charger: steps
Take a quarter of the bank's capacity
This is the ceiling the cells should be asked to accept from an engine. A 300 Ah bank gives 75 A. Charging harder than this shortens cell life without shortening the charge much.
Take half the alternator's rating
This is what the alternator can sustain without cooking. A 90 A alternator gives 45 A. Alternators are rated for short bursts with airflow, not for hours at full output feeding a bank that keeps asking.
Use the smaller of the two
Whichever number is lower is your ceiling. In the example above, 45 A. This is a maximum, not a target — a bigger charger is the exact failure this protects against, not a safer choice.
Round down to a charger that exists
Common sizes are 18, 25, 30, 40, 50 and 60 A. Round down, never up. From a 45 A ceiling you buy a 40 A charger.
Check the alternator's real rating, not the badge
A badge figure is often a cold, high-rpm bench number. If the alternator is old, small-case or poorly ventilated, treat its sustained output as lower and size down accordingly.
Consider external regulation above about 120 A
Past roughly 120 A of alternator output, DC-DC chargers get expensive and start needing to be paralleled. An externally regulated, temperature-sensed alternator specified for lithium becomes the better path.
Common installation and calculation errors
Sizing only from the bank and ignoring the alternator.
The alternator is usually the binding constraint on older boats.
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 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.How Many Solar Panels Does a Boat Need? Sizing StepsDivide the amp-hours you want solar to replace by 4.25 sun-hours and 0.75 for real-world losses, then multiply by the bank's nominal voltage to get watts. Replacing 100 Ah a day on a 12 V bank needs roughly 400 W.