Swapped out my 100Ah AGM for a 100Ah LiFePO4 — same capacity but it feels like a completely different setup

Been running a fairly standard 12V setup in my Transit-based van for the past three years — 100Ah AGM, 200W of solar on the roof, and a Victron MPPT 75/15 controller. It did the job but I was always babying the battery, never drawing below 50% and watching the voltage like a hawk. Picked up a Fogstar Drift 100Ah LiFePO4 last month and honestly the difference is staggering.

The usable capacity alone is the obvious win — I'm now comfortable pulling it down to 10-20% without sweating it, so effectively I've nearly doubled my usable power without adding a single panel. But the less obvious thing is how flat the discharge curve is. My 12V fridge, the Alpicool C15, runs noticeably more consistently now rather than the compressor struggling when the AGM voltage started sagging toward 12.0V at the end of the day.

One thing I hadn't fully thought through beforehand was the charging side. The AGM profile on my Victron MPPT was set to 14.4V absorption and 13.8V float, which isn't right for LiFePO4. Took me a couple of days to sort the custom profile properly — 14.2V absorption, no float (or 13.5V if the controller needs something set). Worth flagging for anyone else making the swap.

Has anyone else found their solar controller needed more fiddling than expected after switching chemistries? And did anyone go for a BMS with Bluetooth monitoring — wondering if it's worth paying extra for that on the next battery I add?

RetiredNurse96 | 847 posts | ⭐ Trusted Member

@DuctTapeDave60 Lovely upgrade! The thing that surprised me most when I made the same switch was how much more usable capacity you actually get in practice. With the AGM you were probably only safely drawing down to about 50%, so effectively 50Ah of real-world use. Now you've genuinely got the full 80-90Ah available without stressing the battery at all.

One thing worth checking — your Victron controller is brilliant but make sure you've updated the charge profile to LiFePO4 settings. Absorption voltage should be around 14.2-14.6V rather than the higher AGM figures. Easy enough to adjust through the VictronConnect app if you haven't already done so. Your battery will thank you for it long-term! 🔋

PV_Fan | 312 posts | 🔋 Regular

The usable capacity difference is the real kicker — your 100Ah AGM was probably giving you 50Ah usable max, whereas the LiFePO4 you can push to 80-90Ah no problem. Effectively nearly doubled your real-world storage without touching the roof.

Worth double-checking your Victron MPPT settings though @DuctTapeDave60 — make sure you've switched the battery profile to Lithium. The absorption/float voltages are different and running AGM settings on LiFePO4 isn't ideal long-term. Dead easy to change in the VictronConnect app.

My setup's similar and the difference in morning resting voltage alone tells you how much healthier the chemistry is. AGM would be sagging noticeably overnight, LiFePO4 barely moves.

Cerbo_Geek | 1,204 posts | ⚡ Power User

Worth flagging something nobody's mentioned yet: your MPPT 75/15 needs its battery profile updating. Running a LiFePO4 on AGM settings means the absorption voltage will be too high and the float voltage almost certainly wrong — you're looking at 14.2–14.4V absorption and 13.5V float for LiFePO4, versus the higher figures Victron ships AGM profiles with.

Took me an embarrassing amount of time to realise my Fogstar 200Ah cells weren't charging properly in the static caravan because I'd left the old profile in place. The VictronConnect app makes this trivial to sort — five minutes maximum.

Also check your low-voltage disconnect threshold if you're running a Victron BMV or SmartShunt. LiFePO4's flat discharge curve makes voltage-based SoC readings unreliable without recalibrating.