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DC-DC Charger Size Calculator

Alternator-safe B2B charger sizing

Quick answer

Size the DC-DC charger to the smaller of ~50% of alternator output and your battery charge acceptance. Required for smart alternators and lithium; an older alternator with lead/AGM can often use a cheaper VSR instead.

120A
Leisure battery type
200Ah

Recommended charger

60A

Alternator limit (~50%)

60A

Battery acceptance

100A

Shop 60A DC-DC chargers
  • Kept to ~50% of alternator output so the charger does not overload the alternator at idle/low revs.
  • A DC-DC charger is required for smart (Euro 5/6) alternators and lithium batteries. For an older alternator with lead/AGM, a cheaper VSR may be enough.

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Frequently asked questions

What size DC-DC charger do I need?

Size it to the smaller of about half your alternator output (so it does not overload at idle) and your battery charge acceptance (roughly 0.5× capacity for lithium, 0.2–0.3× for lead/AGM). 20A, 30A and 40A are the common choices.

Do I need a DC-DC charger or a VSR?

A DC-DC (B2B) charger is required for smart (Euro 5/6) alternators and lithium batteries. For an older alternator with lead/AGM, a cheaper VSR may be enough — use our split-charge relay calculator to check.

Can I fit a 60A charger to charge faster?

Only if your alternator can supply it without overheating, especially at low revs. A charger that pulls more than the alternator can spare at idle risks damage. Sizing to ~50% of alternator output is the safe rule.

Will a bigger charger charge lithium faster?

Up to the battery acceptance limit, yes. Lithium accepts high current, so the alternator is usually the limit. Lead-acid accepts far less, so a big charger is wasted on it.