How do I replace LA battery with LFP in 12v system?

Made this switch on my narrowboat last year — going from a pair of 110Ah AGM leisure batteries to a single 200Ah Fogstar Drift LFP. Honestly one of the better decisions I've made.

Few things worth flagging for your setup:

Charging sources need checking first

Most older split-charge relays and basic solar controllers will technically charge LFP but not properly. You really want a charger with a dedicated LFP profile — flat absorption, no float or a very low one (~13.5V). Victron's SmartSolar range handles this well. If you've got a B2B charger off your engine alternator, check it's LFP-compatible before wiring anything up.

BMS is non-negotiable

If you're buying a self-contained LFP battery (Fogstar, Enjoybot, etc.) the BMS is built in, so less to worry about. Going with raw cells is a different conversation entirely.

State of charge monitoring

LA batteries give you voltage as a rough SOC guide. LFP has a very flat discharge curve — voltage tells you almost nothing until the battery is nearly empty. A proper shunt-based monitor (Victron BMV-712 or similar) makes a real difference here.

Weight and space

LFP is significantly lighter for equivalent capacity. My Fogstar freed up a surprising amount of space in the battery box.

One thing I'd ask — what's your current charging setup? Alternator only, solar, shoreline? That'll determine how much work the transition actually involves. Happy to go into more detail on any of the above if useful.

Did the same swap on my motorhome and the Fogstar Drift basically paid for itself in smugness alone — though do check your alternator isn't going to cook itself trying to charge a hungry LFP, a DC-DC charger (Victron Orion is the go-to) between the vehicle alternator and the leisure bank is worth every penny.

Great thread — done almost exactly this swap on my narrowboat too, so happy to add a few bits.

One thing nobody's mentioned yet: your charging sources need to be LFP-compatible. My old Victron MPPT was fine after a profile tweak, but my engine alternator was a different story — older "dumb" alternators can cook themselves trying to charge LFP because the battery accepts current so readily without the voltage rise they expect. Worth fitting a Battery-to-Battery charger (B2B) between alternator and LFP bank if you're cruising a lot.

Also watch your low-voltage cutoff settings. LFP sits at a flatter voltage curve than AGM, so any 12v appliances calibrated to AGM behaviour might cut out prematurely — or worse, never warn you before the BMS pulls the plug hard.

Great timing on this thread — just finished a very similar swap on my static setup last month.

One thing I'd flag that hasn't come up yet: check your alternator charging situation carefully if you're running an engine. LFP batteries accept charge so quickly that older alternators can overheat trying to keep up — they're essentially working flat out the whole time rather than tapering off as a lead-acid would. A battery-to-battery charger (B2B) between your alternator and the LFP bank is often the sensible solution, and it'll also mean your alternator isn't seeing the full load directly.

@AshChild and @BurnSophie — did either of you address this on your narrowboats? Curious whether the Fogstar Drift's internal BMS handles it gracefully or whether you took additional precautions. Mine's purely shore power and solar so I sidestepped the issue entirely.

Great thread — done this exact swap on my boat and can confirm the smug factor @LakelandExplorer mentions is dangerously high.

One thing worth flagging that I don't think's been covered: your alternator. LFP will happily hoover up charge at whatever your alternator can throw at it, unlike the AGMs that e