Adding a LifePo4 battery

Been going back and forth on this for a while now — got a 200Ah AGM bank on the narrowboat at the moment and wondering whether it's finally time to make the jump to LiFePO4.

The main thing holding me back is the upfront cost, but I keep reminding myself that cycling down to 20% on the AGMs is basically destroying them slowly, whereas with lithium you can use the full capacity properly. It's almost false economy keeping the lead acid stuff going at this point?

Currently running a Victron SmartSolar MPPT which I think is lithium-compatible — just needs a profile change, right? Has anyone had any issues with that side of things?

Been looking at Fogstar Drift for the cells — the 12V 100Ah seems decent value and their UK support reputation seems solid. Tempted to do two in parallel for a 200Ah bank, which should genuinely feel like more usable capacity than my current 200Ah AGMs by a massive margin.

Few questions rattling around in my head:

• BMS integration — do most people here just rely on the built-in BMS on something like the Fogstar, or are folk adding external ones too?
• Heating — on a narrowboat it can get properly cold over winter. Do I need a heated battery enclosure or will a decent install location handle it?
• Charging from the alternator — got a 70A alternator on the engine, do I definitely need a DC-DC charger in between to protect it?

Anyone who's done a similar swap on a boat (or even a static, the principles are similar I'd imagine) — would love to hear how you got on and what you'd do differently.

@ThingamyBob I made exactly this jump last year on my narrowboat — went from 200Ah AGM to a pair of Fogstar Drift 100Ah 12V cells wired in parallel and honestly it's transformed things.

The bit nobody really talks about beforehand though — your alternator setup needs serious thought. Running LiFePO4 off a standard alternator without a DC-DC charger in between is asking for trouble. What engine have you got? I fitted a Victron Orion-Tr Smart and it's been brilliant.

Also curious — what's your main use case? Are you continuous cruising or mostly on moorings? That changes the equation quite a bit for whether the upfront cost actually makes sense for you.

What's your current inverter/charger situation? That might need updating too depending on what you go with.

@ThingamyBob One thing nobody's mentioned yet — check your alternator situation before you commit. On a narrowboat your engine is probably doing a fair bit of charging, and LiFePO4 will absolutely hammer an unprotected alternator. The battery accepts charge so fast it can cook older alternators that were perfectly happy with AGMs.

Worth looking at a DC-DC charger (Victron Orion-Tr Smart is the go-to) between your alternator and the lithium bank. Adds cost but saves your alternator.

Also — what's your inverter/charger situation? Some older units aren't properly lithium-compatible even if they claim to be. Had this headache myself on my static caravan setup before I got the right profile dialled in on my Victron Multiplus.

What alternator are you running?

@LisaStewart71 raises an absolutely critical point — worth expanding on. Most narrowboat alternators are 70–120A units running straight to the battery bank, which is fine for AGMs but can destroy a LiFePO4 if the BMS suddenly disconnects under full charge current. The alternator sees an open circuit and the voltage spike can fry the rectifier diodes instantly.

You have two sensible options:

1. Battery-to-battery charger (Sterling, Victron Orion) — limits charge current and isolates the alternator properly
2. A smart alternator regulator like the Balmar or Sterling AtoB

I've got a Sterling B2B on my boat feeding a Fogstar Drift 200Ah and it's been rock solid for two seasons. Don't skip this step thinking the BMS alone will protect things — by the time the BMS acts, the damage is already done to your alternator.

@DevonDweller @LisaStewart71 good point on the alternator — but what's the actual fix here? Are people just fitting a DC-DC charger (Victron Orion seems to be the go-to) between the alternator and the LiFePO4 bank, or is there a better approach?

I've got a similar dilemma with my van conversion — been putting off the AGM-to-LiFePO4 switch partly because I don't want to fry anything. The Orion solution sounds clean but adds cost and complexity.

@ThingamyBob also worth asking — what's your inverter situation? Some older inverter-chargers have charge profiles that don't play nicely with LiFePO4 without a firmware update or full replacement. Found that out the hard way looking at setups for my garden office backup system.