A DC-DC (battery-to-battery) charger turns alternator power into a proper charge for your house battery while you drive.
Hours = capacity (Ah) × (target % − start %) ÷ 100 ÷ charger output amps. A 40 A charger refilling a 200 Ah LiFePO4 from 30% to 100% puts back 140 Ah: 140 ÷ 40 = 3.5 h of driving. Lithium accepts nearly full current until it is almost full; AGM slows down sharply above about 80%, so add 1–2 hours for the last 20%.
| Battery | 20 A | 40 A | 60 A |
|---|---|---|---|
| 100 Ah | 4.0 h | 2.0 h | 1.3 h |
| 200 Ah | 8.0 h | 4.0 h | 2.7 h |
| 300 Ah | 12.0 h | 6.0 h | 4.0 h |
| 400 Ah | 16.0 h | 8.0 h | 5.3 h |
The charger draws more than it outputs: output amps × charge voltage ÷ (alternator voltage × efficiency). A 40 A charger pulls about 46 A from the starter side, so check that your alternator has the spare capacity, especially stock smart alternators. Fuse both sides with the fuse calculator and size the cables with the wire size calculator.
Safety: These are general estimates, not a design. Follow your equipment manufacturer's instructions and local electrical code, and consult a qualified electrician for anything connected to shore power, a house, or high-current batteries.
From 20% to full, about 2 hours of driving for LiFePO4 (80 Ah ÷ 40 A). AGM takes longer because charging slows above about 80%.
No, quality chargers switch off when the engine stops or when starter voltage drops, so they only charge while the alternator is running.
Many lithium makers suggest charging at 0.2–0.5C. A 200 Ah bank is commonly paired with a 40–60 A charger if the alternator can supply it.
Yes. Many units include an MPPT solar input, and the times simply add: solar charge time is covered by the solar charge time calculator.
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