Can you run an inverter off a boat's starter battery bank without wrecking it?

Stumbled into this forum section after spending the weekend on a friend's narrowboat on the Cam. He's running a fairly basic 12V setup — two 100Ah lead-acid starter batteries (not leisure batteries) and a 300W Victron inverter he's just bolted on. I said that seemed like a bad idea but couldn't fully explain why in the moment.

From what I understand with my static caravan setup, you absolutely need dedicated leisure or LiFePO4 batteries for any sustained inverter load — starter batteries hate deep discharge and the internal resistance is all wrong for it. Is that the same logic on a boat, or is there some marine-specific exception I'm missing?

He's only running a laptop and phone charger off it, so maybe 60–80W sustained. Is that light enough a load that it wouldn't cause real damage over, say, a 3–4 hour evening session? Or is the chemistry just fundamentally wrong regardless of how light the draw is?

Also — if he were to add a proper leisure battery alongside, does the wiring isolation need to be anything special in a marine environment? Wondering if there's a BS standard or similar he'd need to meet if the boat is BSS certified.

@BlownFuse short answer: don't. Starter batteries are designed for a massive burst of current to crank an engine, then get topped up immediately. They absolutely hate deep cycling.

Running an inverter off them will trash them fast — probably within a season if he's doing it regularly.

Seen this exact mistake on a static van I bought secondhand. Previous owner had used a car battery as "backup." Thing was basically dead when I got it.

Your mate needs at least a proper leisure battery setup, ideally AGM or lithium. Fogstar do decent budget lithium if he doesn't want to spend Victron money.

Also worth separating the starter circuit entirely with a battery-to-battery charger so the engine battery stays healthy regardless of what the leisure bank does.

Davo22 | Posts: 847

Worth adding to what @ZFS_OffGrid is saying — even if your mate only uses the inverter occasionally and tries to be careful, the problem is starter batteries genuinely have thinner plates inside. They're optimised for surface area to deliver that crank current, not for repeated deep cycling. You'll kill them surprisingly quickly even with moderate use.

The practical fix for a narrowboat is pretty straightforward though — beg, borrow or buy a couple of second-hand leisure batteries and keep them completely separate from the starter bank. Run the inverter off those exclusively. Victron make decent battery-to-battery chargers that'll top up the leisure bank from the alternator whilst cruising, without dragging the starter battery down.

Canal boats are actually brilliant candidates for a small solar panel or two as well, given you're often moored up in daylight for hours. Might be worth mentioning to him.

Been down this exact road on my own boat. The issue that caught me out — beyond what @ZFS_OffGrid and @Davo22 have already covered — is that starter batteries also tend to sit in the engine bay, and the wiring runs are often undersized for sustained inverter loads. Voltage drop becomes a real problem before you even get to the battery chemistry argument.

If your mate is serious about running an inverter regularly, even a pair of decent leisure batteries would be a massive step up — something like 100Ah AGMs from Fogstar or similar. Doesn't need to be complicated or expensive.

The narrowboat crowd tends to run dedicated domestic banks for exactly this reason. Engine start stays isolated, leisure bank takes the inverter hits. Simple split-charge relay or a basic Victron Orion keeps them topped up from the alternator.

CopperWarden | Posts: 1,243

Good thread this. To add something nobody's touched on yet — starter batteries also tend to have thinner internal plates designed for that brief cranking surge, which means they're physically more prone to warping and shedding plate material when subjected to sustained discharge cycles. That debris accumulates at the bottom of the cells and can eventually cause internal short circuits.

One practical suggestion for your mate: if he's not ready to invest in a proper leisure battery bank just yet, at the very least he should fit a battery-to-battery charger and run the inverter loads off a separate leisure battery that gets topped up from the starter bank while the engine's running. Keeps his cranking ability intact and extends the life of everything considerably. Victron do decent affordable B2B units that'd suit a narrowboat setup nicely. @BlownFuse worth mentioning to him!

FET_King | Posts: 1,204

Good points from @ZFS_OffGrid, @Davo22, and @KevScott — covering the key issues well.

One thing I'd add that often gets overlooked on narrowboats specifically: the Sterling or Victron Battery-to-Battery charger route. If your mate is reluctant to rewire everything, a B2B charger lets him add a proper leisure battery (even just a single 100Ah AGM to start) that charges from the alternator whilst keeping the starter bank completely isolated. The inverter runs exclusively from the leisure side, starter batteries stay healthy, and the engine charging actually works properly rather than the alternator thrashing itself trying to bulk-charge a depleted bank.

It's not the cheapest solution but on a narrowboat it's genuinely the tidiest retrofit option — no major rewiring, no split-charge relay faff, proper charge profiles throughout. Well worth suggesting to him.

DriftWizard | Posts: 387

Right so — and bear with me here — I once tried something spectacularly similar in my van conversion before I knew better. Starter battery, small inverter, "just for a kettle." The battery didn't exactly die immediately, it sort of... declined. Gracefully. Like a Victorian gentleman with consumption.

The thing nobody's mentioned yet: even if your mate's careful with draw limits, starter batteries genuinely hate sitting at partial state of charge. Every hour that inverter's pulling without an alternator running is quietly murdering the plates inside.

The fix is almost embarrassingly simple — a proper leisure battery doesn't cost the earth. Fogstar do decent AGM options that won't have him remortgaging the narrowboat. Even a single 100Ah leisure battery wired separately with a basic battery-to-battery charger off the existing bank would transform the whole situation overnight.