B2B Charger

The reason such a device exists at all is worth understanding, because it decides whether you need one.

A split-charge relay simply connects the two batteries once the engine is running, and relies on the alternator holding a steady voltage high enough to charge the second one. A smart alternator does not hold that voltage: it varies its output to suit the engine management strategy, and can drop it low enough that the leisure battery never reaches a full charge — or stop charging altogether once the starter battery is topped up. The relay is still doing exactly what it was built to do; the assumption underneath it has gone.

A DC-to-DC unit removes that assumption by regulating its own output regardless of what the alternator is doing, which is also why it can charge a battery chemistry the alternator was never designed for.

What this page does not give you is a size. The current rating you need depends on your leisure battery's chemistry and capacity, on your alternator's rated output, and on the cable run between them — and getting it wrong in either direction is expensive, so it is a calculation rather than a rule of thumb. No source cited here publishes a sizing table, and inventing one would serve you worse than sending you to the manufacturer's.