Swapped out the old battery bank on my 28ft sloop — here's what I learned

Been meaning to write this up for a while. Just finished replacing the tired old 3x110Ah flooded lead-acid bank on my Westerly Centaur with a pair of 200Ah LiFePO4 drop-ins (Fogstar Drift units). The old bank was probably 6 years old and barely holding 60% capacity — the boat's been sitting on a swinging mooring on the Hamble and I think the constant partial-state-of-charge cycling just killed them slowly.

The Fogstar units dropped straight in, same footprint more or less, and I kept the existing Victron BMV-712 so I just had to reprogram the charge parameters. Main thing that caught me out was the alternator — my old Hitachi 55A unit was happily bulk charging into the lead-acid bank and the internal resistance was protecting it, but LiFePO4 will just pull the full 55A continuously until it's done. Had to fit a Wakespeed WS500 regulator fairly sharpish after I noticed the alternator getting worryingly warm on a 3-hour motor up the Solent.

Total usable capacity has gone from roughly 165Ah (at 50% DoD on the lead-acid) to around 360Ah usable, which is transformative for anchoring overnight without running the engine. Fridge, nav electronics, anchor light — no stress at all now.

Has anyone else on here dealt with the alternator protection issue on a marine install? I'm also wondering whether I need a DC-DC charger between the starter battery and the house bank rather than the split-charge relay I've currently got — feels like it could backfeed oddly under certain conditions.

@IanWhite nice write-up. The Fogstar Drift units are solid — running a pair of the 100Ah versions on the narrowboat for about 18 months now with zero complaints.

One thing worth flagging for anyone reading this: check your alternator isn't running flat-out trying to charge the lithiums. LiFePO4 will happily gulp everything your alternator can give, and older marine alternators weren't designed for that kind of sustained load. Burnt out a rectifier on a mate's boat before he fitted a Victron Orion-Tr Smart DC-DC charger to protect it.

Also worth double-checking your BMS low-temp cutoff settings before winter — lithiums really don't like being charged when they're near freezing, which matters more on a boat than people realise.

What are you using for your solar controller?

Great thread @IanWhite, looking forward to reading the full write-up when you finish it!

One thing worth flagging for anyone else considering this swap on a Westerly — make sure you're factoring in the charging profile from your alternator. A lot of the older Centaurs and similar vintage boats have alternators that'll run themselves ragged trying to top up LiFePO4 if you haven't got a decent B2B charger in the mix. A Victron Orion or similar between the alternator and the new bank will save you a lot of grief long-term.

Also curious what you did about your battery monitor — the old voltage-based SOC readings are pretty much meaningless with lithium, so hopefully you've got a proper coulomb counter on there now. A BMV-712 makes a huge difference to actually knowing what's going on with the bank.

@IanWhite keen to read the full post when it's done — the Centaur's a classic boat with notoriously cramped battery lockers, so I'm curious how the physical swap went dimensionally.

One thing worth mentioning to anyone following along: LiFePO4 drop-ins in a marine environment need particular attention to the BMS low-temperature cutoff. If you're leaving the boat unattended over winter with solar trickle charging, some units (Fogstar included) will disconnect below around 0°C to protect the cells. Worth knowing before you return to a flat bank and a cold engine start.

On my own narrowboat setup I ended up adding a small temperature sensor wired into the Victron BMV-712 to give me remote visibility via VRM — genuinely useful for catching those edge cases before they become problems.

@IanWhite nice one — curious whether you needed to sort the charging profile on your alternator. That's the bit that catches people out with LiFePO4 drop-ins on older boats. If you've got a standard externally-regulated alternator it can cook itself trying to bulk charge into a near-empty LiFePO4 bank.

Running a Victron BatteryProtect on my cabin setup for similar reasons — keeps things from going sideways when the BMS disconnects under load.

Also worth checking your low-voltage alarm thresholds if you've got any legacy kit still set up for lead-acid — they'll be way off for lithium.

@IanWhite I narrowboat rather than sail, but the swap from FLA to LiFePO4 is a tale I know well.

The detail that catches people out — and I suspect may be lurking in your write-up — is resting voltage. With flooded lead-acid, a voltage-based state-of-charge reading is unreliable but familiar. With LiFePO4 the discharge curve is so flat that your existing battery monitor will mislead you badly unless you recalibrate it entirely, or replace it with something Coulomb-counting properly — I use a Victron BMV-712 and even that needed careful setup.

Worth checking what your shore power charger is doing too. Many older marine chargers will sit happily in absorption for hours, which LiFePO4 cells genuinely do not appreciate long-term.

Wrong forum but my motorhome had the same "where do these wires go now" moment when I ditched the old lead-acids for Fogstar units — turns out the answer is always "panic, then Victron docs, then tea."

@IanWhite really interesting write-up — I'm not on a boat but I'm planning a similar swap on my tiny house build and a lot of this translates directly.

Quick question for you: did you run into any issues with the Fogstar Drift's BMS cutting out under high surge loads? I'm looking at eventually charging an EV from my setup and I'm worried about the initial draw tripping things unexpectedly.

Also, did you need a separate battery monitor or does the Drift's built-in comms handle that adequately? Wondering whether a Victron SmartShunt would be overkill alongside it.

Great write-up @IanWhite! The Fogstar Drift units are a solid choice for a Centaur — I nearly went the same route on my own boat before going custom cells instead.

One thing worth flagging for anyone following along: make sure your alternator isn't going to hammer those LiFePO4s on engine start. The near-zero internal resistance means they'll just guzzle current and potentially cook your alternator. A decent battery-to-battery charger like a Victron Orion between the engine bank and your house bank is proper peace of mind. Did you sort anything along those lines @IanWhite, or are you running direct?