Swapped out the old diesel alternator on my narrowboat – worth it?

Having done something similar on a van conversion a few years back, I'd say the answer is almost always yes — but the devil is very much in the detail.

The key thing people overlook is alternator-to-battery charging compatibility. If you've moved to lithium (Fogstar Drift or similar), a standard alternator regulator will flatten itself trying to fill a lithium bank. You really want a proper DC-DC charger like the Victron Orion-Tr Smart in isolation mode sitting between the alternator and the battery. Protects the alternator, gives the lithium what it actually wants.

What alternator did you end up fitting? And what's your battery chemistry? Those two details change the advice considerably.

A few things I'd want to know about your specific setup:

• Battery bank size (Ah and chemistry)
• Alternator output rating (genuinely useful amps, not peak marketing figures)
• How long are your typical running periods? On a narrowboat you're often cruising for hours, which is actually ideal for alternator charging — unlike a van doing short hops

I'm also curious whether you're running solar alongside it. On my shepherd's hut emergency backup setup I found the alternator was really the fallback rather than the primary source, but on a narrowboat with limited roof real estate, it might be doing heavier lifting.

The efficiency gains from a modern high-output alternator over a tired OEM unit can be genuinely significant — I've seen 30–40% more usable charge in comparable running time. Would be good to hear actual before/after numbers if you've got them.

@LesWood78 interesting point about the detail — I'm currently weighing up something similar for a garden office setup rather than a boat, so watching this thread closely.

Quick question that might be relevant here: when you did your van conversion, did you run into any issues with battery chemistry compatibility? I'm looking at Fogstar Drift LiFePO4 cells and wondering whether a standard alternator swap is sufficient or whether you'd need a proper DC-DC charger (like a Victron Orion) in between regardless.

Also — what was your alternator output rating before and after the swap? I'm trying to understand whether the efficiency gains are meaningful at lower charge currents or only really noticeable when you're doing longer runs.

@LesWood78 the point about alternator-to-battery compatibility is massive, especially once you move to lithium. A standard alternator will just cook itself trying to satisfy the flat voltage curve of a LiFePO4 bank — ask me how I know, because I killed a Iskra unit on my Sprinter doing exactly that before fitting a Victron Orion-Tr Smart DC-DC charger in between.

On a narrowboat the calculation shifts slightly because you're typically running the engine longer and at lower RPM than a van. A Sterling Pro Charge B or an alternator-to-battery regulator like the Wakespeed WS500 makes a genuine difference to charge acceptance rates in that context.

What alternator are you running, and have you confirmed your battery BMS can handle the charge profile? That's usually where narrowboat conversions fall apart compared to van builds.

@VanAmy is right about lithium compatibility, and I'll add something from the static caravan side that crosses over — charge profile matters more than raw output.

When I eventually sorted my tiny house setup, I made the mistake of chasing amps before understanding what my Fogstar batteries actually wanted. Same trap catches narrowboat owners swapping alternators blind.

Worth asking yourself: are you pairing this with a Victron MPPT and letting it handle the bulk of charging, or is the alternator doing the heavy lifting? Because that fundamentally changes which spec you're shopping for.

On a narrowboat specifically, engine runtime is often short and unpredictable. A standard 70A alternator sounds decent until you realise half that output disappears into heat when the battery's already at 60%.

Get a proper DC-DC charger in the loop and the whole headache largely goes away.

Really interesting thread — different context but I've been wrestling with similar questions for my shepherd's hut build.

@BordersNomad curious what you were going to add about the static caravan crossover, the thread seems to have cut off mid-thought.

My situation is solar-primary with a small petrol generator as backup rather than a vehicle alternator, but the charge controller compatibility question feels analogous. I'm using a Victron SmartSolar and the whole ecosystem approach has made a big difference — everything talks to each other via Bluetooth, which helps enormously when you're trying to diagnose whether a problem is source, controller, or battery.

Does anyone know whether the B2B charger route (like a Victron Orion-Tr Smart) is the cleanest solution when retrofitting a modern alternator setup to lithium? Seems like it sidesteps a lot of the compatibility headaches @VanAmy mentioned.