Glossary

DC-DC charger

Also B2B charger, Battery-to-battery charger

What is DC-DC charger?

A DC-DC converter between starter and house battery that turns the alternator voltage into a regulated charge with its own profile — regardless of what the alternator delivers.

What matters about DC-DC charger in practice?

It is the answer to two problems: a standard alternator whose voltage is too low for the house battery bank, and a lithium battery bank that should not be charged directly by the alternator.

Its size is limited by the alternator, not by the battery bank: a DC-DC charger draws its current via the starter side, and the alternator must be able to deliver it continuously.

When do you need a DC-DC charger?

  • The service battery is a LiFePO4 battery and should be charged with its own charging profile.
  • The vehicle has a smart alternator that lowers the voltage while driving.
  • The charging cable is long and the voltage at the service battery is too low.

How large should the DC-DC charger be?

The charger draws more current on the input side than it delivers, because it raises the voltage and has losses. A 30 A unit needs roughly 35 to 40 A on the input. The alternator must be able to supply this reserve continuously on top of the vehicle loads.

How is it connected?

Between starter and service battery, with both cables fused close to the source. It switches on via the D+ signal or by voltage sensing. More on charging while driving on the page lithium battery for motorhomes and on charging from the engine under boat alternator.

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