Charging from the engine
Marine DC-DC Battery Chargers
An empty lithium bank will accept almost every amp an alternator can produce, and an internally regulated alternator built for lead-acid will keep producing them until something inside it lets go. A DC-DC charger fixes the current the engine is asked for and keeps the BMS off your cranking circuit. On any boat that charges from an engine it is not an accessory.
How to size a DC-DC charger
- Size to the engine, not to the bank: half the alternator's rating is the ceiling it should sustain all day.
- A quarter of the bank's capacity is the ceiling the cells should be asked to accept from an alternator.
- Take the smaller of those two, rounded down to a charger you can buy.
- Confirm it has a genuine LiFePO4 profile, not a lead-acid profile with a lithium label.
- Alternator temperature sensing is worth paying for on anything above 40 A.
The full reasoning is in Marine DC-DC Chargers: Purpose and Alternator Protection.
Sized for your boat
40 A
Charge current for a 400 Ah bank on a 90 A alternator
This is a ceiling, not a target. A charger larger than your alternator can sustain is the failure this whole page exists to prevent, so we list what fits at or under it — biggest first.
5 options fit — the first is the one we would fit
The options
Compare equipment ratings and applications
| Product | Charge current | Price band | Best suited to |
|---|---|---|---|
| Victron 18 A | 18 A | $230–$340 | A 40 A alternator, or a 100 Ah bank where the cells set the ceiling. |
| Redarc 25 A | 25 A | $390–$550 | A 40–60 A alternator on a 100–200 Ah bank. |
| Victron 30 A | 30 A | $380–$540 | A 60 A alternator, or a 200 Ah bank where 30 A is the cell-side ceiling. |
| Redarc 40 A | 40 A | $520–$720 | A 90 A alternator feeding a 200–400 Ah bank — the commonest boat on this site. |
| Enerdrive 40 A | 40 A | $590–$800 | Certified installs where the paperwork matters as much as the amps. |
| Redarc 50 A | 50 A | $680–$920 | A 120 A alternator with a 400 Ah or larger bank. |
| Victron 50 A | 50 A | $720–$980 | A 120 A alternator on a boat already committed to a Victron system. |
| Victron 60 A | 60 A | $880–$1,200 | A 150 A alternator feeding a 600 Ah or larger bank. |
Head to head
Asked most
Do I really need a DC-DC charger on a boat?
Engine charging needs compatible current control and protection. A DC-DC charger is one option; an appropriately regulated alternator with temperature and BMS controls can be another. Confirm the design for the actual engine and battery.
What size DC-DC charger do I need?
The smaller of a quarter of your bank's capacity and half your alternator's rating. On a 400 Ah bank with a 90 A alternator that is 40 A. The calculator on this site works it out from your own numbers.
Can I fit a bigger one later?
You can, but the cable and fusing were sized for the original. Size the cable for the charger you might fit rather than the one you are fitting, and the upgrade is a swap rather than a rewire.