Can I use a car alternator to charge leisure batteries?

The elephant in the room nobody's mentioned yet is the charging curve. Car alternators are designed to maintain a float voltage (~13.8V) once the engine's running, which is fine for starter batteries but absolutely terrible for properly charging leisure batteries. You'll get maybe 80% charge and then it plateaus—your batteries never fully cycle, sulphation sets in, capacity drops.

If you're serious about this, you need a smart regulator (Victron MPPT Lithium charger, or if you're budget-conscious, a Redarc SBI12 can work). But at that point you're looking at £300-500 before you've even fitted the alternator properly to your narrowboat engine.

Honestly? For a boat, I'd stick with a decent split-charge relay or VSR (voltage-sensitive relay) paired with a proper alternator upgrade if the original engine alternator is undersized. Simpler, fewer moving parts, fewer headaches when you're miles from shore.

What's your actual power draw like? Might be worth calculating whether a larger solar array would give you better returns. That's what most

The alternator's the cheap bit—managing it's the nightmare. I learned this the hard way when I first fitted mine to the narrowboat engine.

Car alternators dump charge at whatever RPM you're running. So you're either crawling along the canal getting trickle charge, or hammering the engine to push 80A through your leisure batteries in ten minutes flat. Neither's ideal. You'll knacker the alternator or cook your batteries.

What actually works: stick a decent split charge relay (Redarc or similar) between your engine and leisure bank, and accept you're only getting meaningful charge when you're actually using the engine—cruising, moving lock to lock, that sort of thing. It's supplementary at best.

If you're serious about charging off the engine, a proper DC-DC charger like the Victron Orion is money better spent. Handles the voltage regulation properly and won't destroy your batteries.

That said, if it's just topping up between solar sessions on a narrowboat, the free alternator beats doing nothing. Just don't expect it to be your primary source.

Car alternators are brilliant until they're not—mine spent three months cooking my leisure batteries like a slow roast before I fitted a proper DC-DC converter. The alternator's a tenner, the regulator that stops it murdering your bank is where your budget actually lives.

@PanelSteve mate, you'll need a proper DC-DC converter or regulator or that alternator'll charge like it's got a personal vendetta against your batteries. It's not the alternator that's free—it's the complications. Been there. Unless you fancy expensive MPPT-style charging kit, stick with solar. At least panels don't sulk.

Mate, I tried this on the narrowboat—alternator cost £40, the Victron DC-DC converter that actually keeps your batteries alive cost £300. But yeah, free power whilst cruising beats the solar panels sulking all winter, so horses for courses innit.