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

Went through this exact decision last year on my 57ft trad. Ran a standard 70A for years and it was perpetually losing the battle against a 400Ah LiFePO4 bank — engine would need to run for hours to make any meaningful dent.

Swapped to a 120A unit with an external Victron Alternator Regulator and the difference is significant. Charge acceptance is properly managed now rather than just hammering voltage at the bank and hoping for the best. The VAR lets you set a current limit so you're not cooking the alternator during sustained cruising either, which with LiFePO4's flat absorption curve is genuinely important — these batteries will take whatever you throw at them until they're nearly full.

A few things worth considering before just going bigger:

• Belt and pulley sizing — a 120A+ alternator under load puts serious strain on a single belt. Worth checking pulley ratios and upgrading to a twin-belt setup if your engine allows it
• Cable runs — many older narrowboats have undersized wiring from the alternator. I ran new 35mm² cable direct to the battery bus
• Heat — engine bays on boats run warm already. Alternators don't like sustained high-current output in confined spaces

I've also seen people fit 150A+ units and regret it because their engine simply doesn't have the torque to drive them efficiently at tick-over speeds common on canal running.

What engine are you working with and what's your bank capacity? That'll determine whether 120A is sufficient or whether you need to look at DC-DC chargers from a secondary alternator as a cleaner solution.

@RiverRunner the hours-of-idling battle is so familiar — had exactly the same frustration in my static before I sorted the charging properly.

On the motorhome I eventually fitted a Victron Orion-Tr Smart DC-DC charger alongside a beefier alternator, and the difference was night and day. The Orion handles the LiFePO4 charge profile properly rather than just throwing amps at it randomly.

Worth considering though — oversizing the alternator without protecting it from a hungry LiFePO4 bank is asking for a cooked unit. Those batteries will just drain whatever you throw at them until the alternator gives up.

A B2B charger limiting charge current combined with a sensibly-sized alternator (100-120A) tends to outlast the "just fit a massive alternator" approach considerably. The alternator runs cooler, lives longer, and your engine isn't labouring unnecessarily at tickover either.

Really useful thread this. One thing worth adding — if you do go bigger, make sure your alternator-to-battery connection cabling is rated for the extra current. Surprising how many people fit a 150A alternator and leave the original undersized wiring in place, which can get warm in a hurry.

Also worth considering a decent battery-to-battery charger (B2B) rather than connecting your lithiums directly to the alternator. Lithiums will happily pull everything an alternator can give right up to absorption, which can cook the alternator over a long cruise. A B2B limits that draw and protects the alternator nicely. Victron and Sterling both do solid units. Bit of extra cost upfront but far cheaper than a replacement alternator mid-canal somewhere near Middlewich! 😄

Really interesting thread, cheers @RiverRunner. One thing I'd add from my own experience on a 60ft widebeam — don't overlook the regulator situation when you upsize. I jumped from a 70A to a 115A and initially kept the standard internal regulator, which meant the alternator was still essentially charging at the same lazy rate as before. Fitting an external smart regulator (I went with a Wakespeed) made an enormous difference — it actually pushed the alternator properly and I went from needing 3+ hours of engine time down to around 90 minutes for a meaningful top-up on my 300Ah LiFePO4 bank. The cost of the regulator wasn't trivial but honestly it transformed the whole setup. Worth factoring into your budget from the start rather than retrofitting later like I did!

Good timing on this thread — I've been wrestling with something similar, though on land rather than water.

Running a 120A Victron Alternator Controller on my tiny house setup and the difference it made to charge curve behaviour was the real revelation, not just the raw amperage. The alternator thinks it's charging lead-acid unless you tell it otherwise — LiFePO4 will just gulp down whatever it's offered with no back-pressure, and that's where alternators cook themselves.

@HeatherMandy's point about not oversizing blindly is spot on. I'd add — look at your engine's rated alternator bracket load before anything else. Jumped too big and you're stressing belts and bearings every time you spin up.

On a narrowboat I'd probably settle somewhere around 100-120A with a proper external regulator rather than chasing maximum numbers.

Interesting crossover between land and water setups — the principles are pretty much identical once you strip it back.

My own journey with this started in the garden office context rather than a boat, but I eventually ended up consulting a mate who lives aboard a 62ft cruiser stern. He'd upgraded to a 120A Victron alternator with an external regulator and said the difference was night and day — particularly with LiFePO4, where you need something smart enough to handle the flat absorption curve rather than just hammering away like it's flooded lead-acid.

The external regulator is the bit people overlook. A bigger alternator with a dumb internal regulator is arguably worse for your cells long-term than a smaller smart one.

@Brummie — worth considering even on your land setup if you're running alternator charging as a backup source.