Swapped out the old alternator on my narrowboat – worth going bigger?

Not on a narrowboat myself — static caravan setup — but I went through a similar decision process when I upgraded my charging sources last year, so hopefully some of this is useful.

The short answer is almost always yes, go bigger, but with caveats.

A larger alternator is only worth it if the rest of your system can handle the input. If you're running a decent bank of lithium (Fogstar Drift cells are popular right now), a bigger alternator makes a real difference because lithium will happily gulp down whatever you throw at it during the bulk phase. Lead-acid is a different story — too aggressive and you're cooking the batteries.

The other thing people miss is protection. A bigger alternator pushes more heat and current, so if you haven't got a proper DC-DC charger like a Victron Orion-Tr Smart sitting between the alternator and your house bank, you're potentially hammering both the alternator and the batteries. Narrowboats especially seem to run everything through a basic split charge relay and then wonder why things fail.

What did you go from and to, size-wise? And what's your battery bank — lithium or lead? That changes the advice quite a bit.

Also curious whether you're seeing much difference in charge times at cruising RPM vs. idle. That's usually where the real-world gains show up.

@CharlieStevens85 — static caravan here too, so similar thought process on charging sources, though obviously the alternator side doesn't apply to me directly.

One thing worth flagging that often gets missed in these discussions: charging efficiency drops off sharply once your bank is above ~80% SOC, regardless of alternator size. A bigger alternator won't necessarily mean more usable charge if your BMS or battery chemistry is already pushing into absorption phase.

What battery chemistry are you running? LiFePO4 will accept charge much more aggressively than AGM, which changes the calculus entirely on whether upsizing is actually worth it versus, say, improving your solar input or DC-DC charging setup.

@CharlieStevens85 mate, a static caravan weighing in on alternator upgrades is a bit like me giving advice on curtain fabric — technically adjacent, but not quite the same sport 😄

That said, I'll give you the actual narrowboat answer since we're on a Marine & Boat forum and all.

Went from a

Great thread – @CharlieStevens85 makes some solid points even if the context is a bit different. From what I've seen discussed here and elsewhere, going bigger on the alternator is usually worthwhile if your regulator can handle it properly. A smart external regulator like a Balmar or Wakespeed paired with a larger alternator makes a real difference, especially if you're running lithium. The key thing people overlook is heat management – a bigger alternator pushing hard in an engine bay with poor airflow will throttle back anyway, so you can end up no better off than before. Worth checking your belt setup too; a single V-belt often can't transfer the torque a larger alternator wants to put out. What alternator did you end up fitting, and are you running lead-acid or lithium? That changes the advice quite a bit.

Ha, @PanelSteve's curtain fabric comment made me snort my tea! Fair point though.

I'll add something nobody's mentioned yet — whatever size alternator you go for, make sure your wiring is actually rated to handle the increased output. It sounds obvious but I've seen folk on here fit a beefy 150A alternator and leave the original undersized cables in place, which rather defeats the purpose and creates a genuine fire risk.

Also worth checking whether your battery bank can actually accept charge at the rate the new alternator can deliver — particularly relevant if you're running AGMs or lithium. A big alternator pushing into a full or undersized bank just means the regulator's doing overtime.

A smart external regulator (Balmar etc.) is worth every penny if you're going significantly bigger. Pays for itself in battery longevity alone.