Calculator
Marine DC-DC Charger Size Calculator
Alternator rating and bank capacity, returning the charger to fit and the time to refill from your engine.
The alternator's rating, not the engine's horsepower. Check the unit itself if you are unsure.
We take a quarter of the bank as the ceiling the cells should accept from an engine, and half the alternator’s rating as what it can sustain. The smaller of the two, rounded down to a charger you can buy.
Your numbers
40 A
DC-DC charger to fit
| Ceiling from the bank (25%) | 100 A |
| Ceiling from the alternator (50%) | 45 A |
| Charger to fit | 40 A |
| Refill from flat, engine only | 8.0 h |
| Sizes available | 18, 25, 30, 40, 50, 60 A |
Calculator assumptions and limits
- Charge current ceiling: the lesser of 25% of bank capacity and 50% of alternator rating
- Rounded down to a commercially available charger size
- Refill time assumes the charger runs at its rated output throughout
Every assumption here is versioned and dated on the methodology page. If you disagree with one, the working is there to argue with.
Battery sizing calculation guides
The same arithmetic this calculator runs, written out step by step.
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 Charge Boat Lithium Batteries from an AlternatorPut a DC-DC charger between the alternator and the bank, sized to the smaller of a quarter of the bank's capacity and half the alternator's rating. Connecting a lithium bank straight to an alternator is what destroys alternators.
The other calculators
House bank calculatorYour loads, your nights away from shore power, your solar and your alternator, returning the bank, the DC-DC charger, the AGM equivalent and the weight off the boat.Trolling motor runtimeThrust rating, the speed you actually troll at and your bank size, returning hours on the water and the bank you need for a full day.Solar top-upPanel watts against your daily draw, returning the average daily return and whether the array covers your consumption.AGM to lithium swapYour existing AGM capacity, returning the lithium bank that delivers the same usable energy and the weight you take off the boat.