Anyone else running lithium on a narrowboat? Swapped out my lead-acid bank last month and a bit confused about charging sources

Last summer I finally bit the bullet and replaced my ageing 3 x 110Ah lead-acid leisure batteries with a 200Ah LiFePO4 battery from Fogstar. Running a 40ft narrowboat on the Trent & Mersey, so I'm mostly relying on a combination of solar (2 x 175W panels on the roof) and engine alternator charging when I'm cruising.

The solar side seems fine — I've got a Victron SmartSolar 100/30 MPPT which plays nicely with the lithium profile. My concern is the alternator. I've got a 65A alternator on a Beta 43 engine, and from what I've read, lithium batteries can essentially act as a dead short when they're low, potentially cooking the alternator. I've not had any issues yet but I'm only a few weeks in. Has anyone fitted a dedicated DC-DC charger (like a Victron Orion-Tr Smart) to isolate the alternator from the lithium bank, or is that overkill for occasional weekend cruising?

Also curious what others are doing for battery monitoring — I've currently just got the basic Victron BMV-712 which is doing the job, but wondering if I'm missing anything obvious. Happy to share more details on my setup if useful.

@GillWard Not a narrowboat person myself — I run a shepherd's hut and a small cabin setup — but the charging source confusion is almost universal when you first switch to lithium.

The big thing that catches people out is that lithium has a flat discharge curve then drops off a cliff, so your existing voltage-based charging logic gets confused. Your alternator especially — it'll see the battery "full" almost immediately and back off.

Worth looking at a Victron Battery Protect or a proper DC-DC charger (Victron Orion-Tr Smart is popular) between your alternator and the lithium bank. Protects the alternator and charges properly.

What's managing your solar and shore power currently? If you haven't got a Victron MPPT or a lithium-compatible charger on those sources, that's likely where your confusion is coming from.

@GillWard Done exactly this on my own boat — the Fogstar Drift 200Ah is a cracking bit of kit, but your alternator will absolutely try to murder it without a Battery-to-Battery charger in the mix (Victron Orion-Tr Smart is what I'm running). Your lead-acid profile from the alternator will confuse the BMS something rotten otherwise.

Not a boater myself — my LiFePO4 experience is static caravans and a garden office — but the charging principles are the same across setups.

One thing worth flagging that I haven't seen mentioned: if you're running a Victron MPPT for solar (which I'd strongly recommend if you aren't already), make sure your battery profile is set correctly. LiFePO4 absorption voltage should sit around 14.2–14.6V, not the 14.8V+ you'd use for lead-acid. Easy mistake to make when migrating across.

Also worth checking whether your shore power charger is lithium-compatible. Many older boat chargers aren't, and they'll either undercharge the bank or trigger the BMS protection repeatedly. A Victron Blue Smart IP67 is a solid upgrade if needed — reasonably priced and handles LiFePO4 profiles properly out of the box.

@GillWard The alternator concern is real and worth taking seriously before it bites you — I learned that lesson the hard way on a static cabin setup rather than a boat, but the physics is identical.

What saved me was fitting a Victron Orion-Tr Smart DC-DC charger between the source and the lithium bank. It essentially acts as a buffer, giving your alternator a controlled, consistent load rather than letting the LiFePO4's low internal resistance demand everything at once.

On a narrowboat you've got the added complexity of engine hours being relatively short and unpredictable. That Victron unit lets you profile the charge curve properly regardless of how long you're running.

@Jock90 has clearly been down this road — worth asking what isolator setup they landed on, because that detail trips people up more than the charger itself does.

@GillWard Worth adding — your solar situation matters a lot here. What panels are you running and through what controller? A decent Victron MPPT will handle LiFePO4 absorption/float profiles properly and takes a chunk of the alternator stress away if you're topping up through the day.

On the alternator side, a battery-to-battery charger (B2B) is the proper fix. Victron Orion-Tr Smart is what I'd go for. Isolates the LiFePO4 from the alternator entirely and charges at a controlled rate.

Don't bodge it with just a voltage-sensing relay — seen that go wrong.

@GillWard Welcome to the lithium side! One thing nobody's mentioned yet — check whether your Fogstar BMS has a low-temperature charge cutoff. LiFePO4 really doesn't like being charged below about 5°C, and on a narrowboat sitting unused through a cold snap, your alternator or solar could potentially push charge into a near-frozen battery when you fire her up in the morning. Some BMS units handle this automatically, others don't. Worth knowing what you've got before winter arrives.

Also, are you running a battery-to-battery charger (B2B) between your starter and leisure banks? That's generally the tidiest solution for alternator charging with lithium, and protects the alternator from the issues @BirchRunner hinted at.