Can I use a car alternator to charge leisure batteries?

Been mulling this over for my narrowboat setup, and I'm wondering if it's actually worth the hassle. I've got an old car alternator knocking about from when I last did an engine rebuild, and I'm trying to figure out if I can repurpose it rather than splash out on a proper leisure battery charger.

The thing is, my current system's a bit cobbled together—400W of solar panels, a basic MPPT controller, and twin 200Ah AGMs. On rainy weeks (and there are plenty of those in England), I'm finding the batteries are dropping quicker than I'd like, particularly through winter when the solar output is absolutely dire.

I've read a few threads suggesting you can wire an alternator to a leisure battery, but I'm not entirely clear on whether it'd actually regulate the charge properly. Don't want to bollocks up my batteries with overcharging, especially not the AGMs. Plus, I'd need to figure out what's driving the alternator in the first place—presumably a petrol gen set? That seems to defeat the object of being off-grid, or at least off-diesel.

Is there a properly engineered way to do this, or am I better off saving up for a Victron smart charger to run off the generator instead? Has anyone actually done this successfully, or is it one of those things that sounds good in theory but causes more headaches than solutions?

Cheers for any insights.

I've tried this route myself on the motorhome—seemed like a no-brainer at first, right? Free alternator sat in the shed.

Reality check: standard car alternators are designed for maintaining a fully charged battery at high RPM, not bulk charging from low state of charge. You'll need a regulator (which adds cost), and the output drops dramatically at idle—useless when you're stationary, which is most of narrowboat life.

The real killer is that most car alternators need sustained high engine revs to produce decent amps. Your engine'll be screaming and drinking fuel just to trickle charge leisure batteries.

If you've got engine hours anyway (cruising, powering through locks), it's worth installing a proper split-charge system with a Victron or CTEK controller—that's different. But retrofitting an old alternator as your primary charging? You'll likely end up frustrated and out of pocket once you factor in cabling and regulators.

What's your cruising pattern like?

Been down this road meself on the narrowboat. The real issue isn't whether it works—it does—but the charging profile. Car alternators dump max output regardless of battery state, which hammers leisure batteries. You'll need a proper regulator to taper the charge, and at that point you're spending money anyway.

What's your boat's engine setup? If you're running the main engine regularly, a dedicated leisure battery isolator with a split-charge relay is far more sensible. Something like a Victron or Redarc unit gives you proper control.

The alternator itself is only half the problem—you're also looking at wiring it up safely, sorting the mechanical mounting, and managing heat. For the cost and aggro, a small solar panel or a quality battery charger tends to be the better shout.

What amperage is the alternator rated for?

The charging profile issue @BurnWalker mentions is spot on. Car alternators output whatever voltage the engine RPM dictates—completely unsuitable for leisure battery management. You'll overcharge at high revs and undercharge at idle, which absolutely trashes lithium cells and accelerates lead-acid sulphation.

What actually works is a proper DC-DC charger between alternator and battery. Victron's Orion-Tr or similar units regulate the output intelligently, but at that point you're looking at £300+ for the kit. Most people find a small solar array plus a decent MPPT controller (even a basic Fogstar unit) more reliable and requires zero engine running.

If your engine's already running regularly for other reasons, then a regulated DC-DC setup makes sense. Otherwise, honestly, solar + battery chemistry-appropriate charger beats the hassle every time. What's your actual usage pattern like?

The real killer for me was the regulation. Car alternators are designed to maintain around 14.4V for engine starting circuits—that's simply too aggressive for leisure batteries and will cook them over time, especially if you're running lithium.

You'd need a proper DC-DC converter or regulator between the alternator and your battery bank, which rather defeats the "free alternator" logic. By that point you're looking at £200-400 for a decent unit like a Victron Orion.

On narrowboats specifically, the engine run time also works against you. Unless you're cruising constantly, you're only getting brief charging windows. Far better to size your solar properly and accept that winter requires either a small generator or time on the grid.

That said, if your engine's already running (engine-in mode cruising), tapping into the alternator output through a regulator is sensible additional charging. Just don't rely on it as your primary system.

The lads are spot on about the charging curve being a nightmare. I tried this on my shepherd's hut setup a few years back and ended up binning it after a month.

The thing nobody mentions is heat dissipation. Once you're pulling serious amperage into leisure batteries, that alternator gets hot because it's not designed for sustained charging—it's made for quick top-ups while driving. Without proper cooling, you're looking at efficiency loss and shortened lifespan.

If you're dead set on it, at minimum you'd need a DC-DC controller to regulate the output properly. By that point, you're spending enough that a small solar array becomes more appealing. I've got 400W on my setup now and it's infinitely less bother.

The only scenario where it makes sense is if you're running a vehicle regularly anyway and have genuinely nowhere else to generate power. Even then, a dedicated leisure battery charger off the engine would be cleaner.

What's your actual power situation? Might be worth looking at the overall picture before committing to the alternator route.

Tried this exact thing for my garden office setup a couple years back. The voltage regulation is only half the problem — your leisure batteries really need a proper three-stage charge (bulk, absorption, float) and a car alternator just can't deliver that.

What actually killed it for me was the current draw. Alternators are designed to trickle-charge a starter battery that's already 90% full. Soon as you connect one to a discharged leisure battery, the voltage sags and you get minimal current. Defeats the purpose entirely.

If you're after emergency backup charging on a narrowboat, honestly better off with a small portable solar panel or a decent battery charger that runs off your engine when needed. Victron make some solid 12/24V chargers that handle the regulation properly.

The alternator's better left for what it was designed for, I reckon.

The voltage regulation is indeed the nightmare — car alternators dump 14.4V without mercy, which absolutely mangles lithium cells. Even lead-acid prefers a proper charge controller.

What nobody's mentioned yet: the alternator itself becomes a parasitic load when your engine's off. You'll need diodes to stop reverse current, which adds complexity and losses. Then there's the mechanical side — belts slip, bearings wear, and you're looking at regular maintenance just to keep it limping along.

If you've genuinely got a spare alternator and a bit of time, a Victron Orion charger input could work, but at that point you're spending money to solve the problem the alternator created. Just run a second alternator properly off your engine with a decent smart regulator, or grab a decent leisure battery charger from the mains.

For narrowboat duty, your engine's already there — use a proper Split Charge Kit or modern VSR relay. Far less grief than retrofitting car electrics designed for nothing but keeping a starter battery topped up.

The real issue nobody's mentioned yet is the charge curve itself. Car alternators are designed to top up a car battery that's already 80%+ charged — they're rubbish at low-state charging, which is exactly what you need for leisure batteries.

I tried something similar on my cabin setup about five years back with a Bosch unit and a basic regulator. Spent more time faffing about with it than it was worth. The alternator would either starve your batteries at idle or cook them at higher revs. That's before you factor in the mechanical coupling nightmare.

If you're committed to charging from an engine, honestly look at a proper leisure battery alternator or stick with a Victron Orion-Tr for charging from an existing automotive alternator. At least then you've got actual voltage regulation and MPPT-style logic.

The salvage value of that alternator probably isn't worth the aggro. Better off flogging it and putting the cash toward a second solar panel or a quality DC-DC charger, especially if you're doing narrowboat stints where engine hours are inconsistent anyway.

What's your current solar setup like?

The real killer for me was the current draw. Got one wired up years back on my van conversion and it'd spike to 80+ amps on cold starts, which absolutely cooked the leisure battery terminals and caused voltage sag all over the system.

If you're deadset on using it, you need a proper DC-DC charger in between — something like a Victron Orion or similar. Otherwise you're just asking for fried BMS circuits and terminal corrosion.

That said, for a narrowboat you'd be better off with a dedicated leisure battery charger wired to your engine. Much cleaner, voltage-regulated, and you can walk away without babysitting it. Cost isn't much more than a quality DC-DC converter when you factor in the labour.

The alternator sits in a drawer at my place now. Not worth the grief for what amounts to £100-150 of proper kit.

Mate, I tried this on my narrowboat and ended up with a £400 Victron alternator controller just to stop it frying my LiFePO₄ — might've been cheaper to just buy a proper marine alternator in the first place.

The lads above nailed the voltage and current issues, but what killed it for me was the inefficiency. You're spinning that engine hard to charge properly, and the fuel cost versus what a modest 400W solar panel generates... let's just say the maths doesn't love you.

If you're genuinely stuck without solar/shore power though, pair it with a decent MPPT-style regulator (Victron or Fogstar make them). Just don't expect miracles and definitely don't leave it unattended — I've seen one go full thermal on someone's leisure batteries.

What's your actual charging situation on the boat? Might be worth exploring alternatives before committing to the alternator faff.

Mate, the alternator controller is where your money actually goes, not the alternator itself. You'll be looking at Victron or Redarc kit to manage the charge curve properly, at which point you might as well have bought a proper marine alternator from the start.

That said, if you've already got the alternator and a decent leisure battery bank (not LiFePO like @EssexNomad found out the hard way), it's a decent emergency backup. I've got one on my caravan setup purely for when the solar's having a laugh in winter — but it's wired through a Victron controller because I'm not entirely mad.

The real question is: how often will you actually be running the engine? On a narrowboat especially, if you're only moving it monthly, you're better off investing in a decent panel array or a small wind turbine. Cheaper long-term and your neighbours won't hate you for the noise.

Yeah, the alternator's cheap but the controller's where it gets pricey. You basically need something like a Victron or Redarc to manage the charge curve properly — otherwise you're hammering your leisure batteries or watching the alternator cook itself.

Done this on my caravan setup. The old car alternator works, but only if your engine's running consistently. Soon as you switch off, you lose the charge input entirely. With solar I get trickle charging all day even when stationary.

If you're doing a narrowboat though, might be worth it just for engine-running periods (pumping, cruising). Just budget for the controller — that's 70% of your actual cost. Cheap alternator + dodgy DIY = dead battery pack faster than you'd think.

What's your main power draw like on the boat? That'd help work out if it's even worth the faff.

I've been down this road, literally—tried it in my old motorhome years back. The alternator itself is free, but here's the rub: a standard car alternator isn't designed for the charging profile leisure batteries need. It'll dump current whenever the engine's running, which absolutely hammers lithium cells and even ages lead-acid faster than it should.

The real problem comes when you're parked up. Your engine's off, alternator's useless. So you're looking at fitting a split-charge relay or (better yet) an intelligent regulator. That's where the money vanishes.

What actually worked for me was fitting a proper leisure alternator with an integrated regulator—cost me about £300 fitted to the motorhome. Not glamorous, but it actually understands what a leisure battery needs. If you're committed to the free alternator route, budget for a decent controller anyway. You'll spend it either way.

Honestly, if it's a static caravan or cabin, just run solar panels and a small inverter. Less hassle, no engine hours to count. That's what I use now and I haven't looked back.

Spot on what @DaleSpirit's touched on. I've been there with my motorhome—the real issue isn't the alternator, it's managing the charge properly so you're not cooking your leisure batteries or draining your engine battery flat.

The alternator will happily dump whatever current it fancies into your setup, which is great for starting an engine but murder on leisure batteries. You need a proper split-charge relay or MPPT controller between the two. That's where your actual cost sits, and honestly, once you factor that in, you might be better served by solar paired with a decent battery management system.

What I found worked best was keeping the alternator charging as a backup only—fitted a battery isolator relay to my van's leisure circuit. Charges whilst driving, keeps them isolated when parked. Costs about £30-60 for a decent relay versus £300+ for a smart controller.

What's your engine? Some older setups the alternator output is too modest anyway to make it worthwhile. If you're running a narrowboat, you've likely got engine time available—worth doing it properly rather than bodging it.