Converting factory AGM house bank to LiFePO4

Did this swap on my narrowboat last year — went from a tired 200Ah AGM bank to 200Ah LiFePO4 (Fogstar Drift cells, built my own pack). Honestly one of the best things I've done to the boat.

Few things worth flagging if you're planning something similar:

Alternator protection is the biggie. LiFePO4 will hammer an unprotected alternator when it's in bulk — AGMs have enough internal resistance to soften the load, lithium doesn't. I fitted a Victron Battery Protect on the DC side and later added a proper alternator protection relay. Some people go straight to a Wakespeed or Balmar regulator — worth it if you're engine-charging regularly.

Your existing chargers and solar controllers need checking. Anything with a fixed AGM profile will need reprogramming or replacing. My old Sterling charger wasn't lithium-compatible so it got swapped for a Victron IP22.

BMS choice matters more than the cells IMO. I went with a Daly initially, moved to a JK BMS — much better balancing and the Bluetooth monitoring is genuinely useful.

For a twin-engine catamaran setup you've also got the question of whether both alternators charge the same bank or split — worth thinking through your topology before you commit to anything.

Anyone else done this on a liveaboard or cruising setup? Curious whether people are running DC-DC chargers between start and house banks rather than a split charge relay — seems like the cleaner solution with lithium but adds cost.

@NigeCampbell81 great timing on this thread — I've just gone through something very similar getting power sorted for a shepherd's hut build, though my use case threw in an extra wrinkle: I needed to handle occasional EV charging sessions without the battery bank throwing a tantrum.

The thing that really surprised me making the switch was how differently you have to think about state of charge. AGMs you could read voltage and get a rough idea. With LiFePO4 that flat discharge curve means voltage is almost useless mid-range — you're flying blind without a decent shunt and monitor. Fitted a Victron BMV-712 and suddenly the whole picture became clear.

Worth mentioning to anyone doing this: your alternator settings will need revisiting. LiFePO4 will happily gulp current in a way that can cook an older alternator fast.

Really interested to hear how you got on @NigeCampbell81. I made the same switch about 18 months ago on my sailing boat — also Fogstar cells as it happens!

One thing I'd add for anyone considering it: don't underestimate the alternator protection side of things. LiFePO4 will absolutely hammer an unprotected alternator once the BMS cuts out at full charge. I fitted a Wakespeed WS500 regulator and it's been faultless, but I've heard some horror stories from people who skipped that step.

Also worth mentioning — if your boat has a split charge relay setup, that'll need revisiting. LiFePO4 charges at a different profile and the relay logic often doesn't play nicely straight away.

What BMS did you end up going with @NigeCampbell81? I went with a JK and couldn't be happier.

The alternator charging side is where it gets spicy with LiFePO4 — learned that the hard way when my van's alternator nearly cooked itself trying to bulk charge into a pack that just accepts everything until the BMS pulls the rug out 😅 a Victron Orion-Tr Smart DC-DC isolator saved my bacon and I'd not go back.

@BitsAndBobs oh yes, the alternator "surprise" — nothing quite like the smell of expensive regulator letting you know it didn't appreciate a flat LiFePO4 bank demanding everything immediately 🔥

Worth flagging for anyone planning this swap: a proper DC-DC charger between alternator and LiFePO4 bank is non