Do I need a DC-DC charger or just a split charge relay?

Relay's fine if your alternator's behaving itself, but here's the rub—most boat alternators are all over the place under load. I ditched mine for a Victron Orion after my leisure bank kept sitting at 80% charged. The DC-DC regulates properly, protects both banks, and actually fills the tank. Worth the outlay if you're serious about off-grid time.

The relay works but you're leaving efficiency on the table, especially if you're doing shorter runs. A proper DC-DC like a Victron Orion will manage the charge curve better and protect your leisure bank from voltage spikes—boats are notorious for dodgy alternator regulation. Worth it if you've got decent solar already, otherwise the relay's a cheaper starting point.

I ran a relay on my narrowboat for two seasons and honestly, it cost me. Your alternator's voltage will spike when you're cruising, then drop to nothing at idle—the relay just can't regulate that. Switched to a Victron Orion 48/16, sorted the charging curve properly, and added maybe 15% usable capacity to my leisure bank. Worth every penny if you're living aboard.