What Is a DC-DC Charger (B2B)?
An intermediary charger that takes power from the starter battery line and feeds the service bank with the correct charging profile — a peace deal for mixed battery chemistries.
A DC-DC charger (B2B) is a unit that is powered by the alternator or starter battery and charges the service bank using a profile matched to its chemistry. It also electrically isolates the two banks: the starter battery is always protected.
It is effectively essential in two cases: when the service bank has been converted to lithium (lithium’s low internal resistance can overload and damage the alternator if connected directly, while the B2B limits current) and on modern engines with smart/variable-voltage alternators (charging voltage can be too low for the service bank).
Decision and installation
Decision tree: if you have two banks, the same lead-acid chemistry, and a simple system, a VSR may be enough; if you have a lithium service bank or a smart alternator, a B2B is standard.
Heat and cabling: the unit will run continuously at its rated current, and will derate in an unventilated locker; the input cable must be sized to suit the unit’s current draw.
Profile selection: the battery type setting on the unit must match the bank.
Unlike a conventional split-charge relay (VSR), a B2B does more than just connect the banks — it converts the charge. Any boat owner planning a move to lithium should factor in the B2B and cabling cost from the outset; unavoidable extras always cost more when they turn up later.