How driving time is calculated
First work out the charge missing from the house bank: nominal capacity in amp-hours times the state-of-charge gap, divided by a charge efficiency to cover what the battery does not store. A DC-DC charger delivers its rated current for essentially the whole bulk phase, so dividing missing amp-hours by the charger rating gives the driving time.
For example, a 100 Ah bank at 50% needs 50 Ah; at 95% charge efficiency the charger must deliver about 53 Ah, which takes a 30 A charger about 1 h 45 min of engine running.
What it asks of the alternator
The charger is a power converter, so its input side draws more current than it outputs: output current times the house-bank charge voltage, divided by the alternator-side voltage and the converter efficiency. A 30 A charger on a 12 V system draws roughly 35 A; the same 30 A output into a 24 V house bank draws about 70 A from a 12 V alternator. Keeping that continuous draw at or below about half the alternator's rating leaves room for the vehicle's own loads and for the fact that alternator ratings assume good airflow, not idling in traffic.
Why real charge time can differ
The v1 model assumes the charger runs at full output for the whole drive. In reality the last stretch to 100% tapers: lead-acid absorption can add hours, and lithium tapers in the final few percent. Some chargers also derate at high engine-bay temperatures, and short stop-start driving keeps interrupting the charge. Treat the result as a best-case bulk-phase figure — the planning range adds 25% for these effects.
Methodology
The calculator UI, the charger comparison table and the displayed formulas all call the same calculation function. To size the bank this charger refills, see the Battery Bank Sizing calculator; solar's side of the same job is covered by Solar Charge Time.
- DC-DC charger input current exceeds rated output (converter power balance) — Victron Orion-Tr Smart DC-DC charger manual
- Smart/variable-voltage alternators need DC-DC charging — REDARC technical guidance on smart alternators
- Charge (coulombic) efficiency ~95–99% for LiFePO4, ~85% for lead-acid — Battery University BU-808c and manufacturer datasheets
- Alternator continuous-load guidance (~50% of rating) — Balmar alternator installation and temperature guidance
Assumptions last reviewed on 2 September 2026. If a source disagrees with your battery or equipment datasheet, trust the datasheet.
Frequently asked questions
Why do I need a DC-DC charger instead of a split-charge relay?
Modern smart alternators vary their output voltage for fuel economy, so a plain relay can leave the house bank chronically undercharged or overcharge lithium. A DC-DC charger converts whatever the alternator supplies into a proper charge profile for the house bank chemistry.
Why does the charger draw more current than its rated output?
Power in equals power out divided by converter efficiency, and the house-bank charge voltage is higher than the alternator-side voltage. Together that makes the input current roughly 15–20% higher than the rated output for a 12 V to 12 V charger, and about double for a 24 V house bank on a 12 V alternator.
How big a DC-DC charger can my alternator support?
A common guideline is to keep the charger's continuous input at or below about half the alternator's rating. Alternators are rated for peak output with good airflow, not for hours of full load at idle, and the vehicle's own electronics need their share too.