Charging 24volt Lifpo battery with mppt

Got me thinking about my own boat setup — my Victron SmartSolar does this lovely gradual voltage ramp during absorption and I've always just trusted it to do its thing, but now I'm curious whether anyone's actually tuned the charge profile step increments on theirs or if we're all just leaving it on the default LiFePO4 preset and hoping for the best? 🤔

My Fogstar 24v cells seem perfectly happy but I do sometimes wonder if a more aggressive ramp would get me to full SOC faster on short sunny days — we get precious few of those up here.

Specifically interested in whether the charge current limit on the MPPT is the real lever here rather than the voltage stepping — my understanding is the MPPT naturally regulates voltage rise based on available PV watts anyway, so trying to manually replicate a slow ramp from an AC charger might be solving the wrong problem entirely?

Anyone running a custom absorption voltage curve on their Victron, or are you lot all on VictronConnect just clicking "lithium preset" and cracking on?

@VickyWard the absorption ramp is worth understanding properly rather than just trusting blindly.

On my system I've got the SmartSolar logging to VRM and what you'll actually see is that absorption isn't just a fixed voltage hold — the controller is watching current draw and tapering based on what the cells will accept. With LiFePO4 specifically, that acceptance current drops off quite sharply compared to lead-acid, so absorption phases can be deceptively short.

Key thing to check: your absorption voltage setting. For a 24V LiFePO4 bank you want around 28.4V (3.55V/cell × 8), not the 28.8V that Victron sometimes defaults to for lead-acid equivalents. Running it too high repeatedly stresses the top of the charge curve even if your BMS doesn't immediately complain.

What capacity is the bank and do you have a Cerbo or just the SmartSolar in isolation?

@QJ_Builds fully agree on understanding it rather than just hoping for the best.

On my narrowboat I spent a whole winter season just watching what the Victron SmartSolar was actually doing via the app — absorption voltage, tail current, the lot. Genuinely eye-opening compared to just glancing at a percentage readout.

The bit that caught me out initially was confusing absorption duration with absorption completion. The MPPT doesn't just time out — on a properly configured system it's watching that tail current drop to know when the cells are actually satisfied.

With LiFePO4 specifically, I've got mine set so absorption is fairly short and tight. The chemistry doesn't want to sit there being topped up like old lead-acid does. Worth checking your BMS logs alongside the MPPT data if you can — the two together tell a much clearer story.

@VickyWard one thing people miss with LiFePO4 and absorption — it's not like lead acid where you need a long absorption phase. Most decent BMS units will just hit the voltage ceiling and taper current naturally. Your Victron will handle it fine but honestly you can often shorten absorption time considerably without any ill effect on cell longevity.

On my shepherd's hut setup I dropped absorption from the default 2 hours down to 30 minutes with Fogstar cells and haven't had a problem in two years. The BMS is doing the real protection work anyway.

Worth logging a few charge cycles in VictronConnect and actually looking at the current tail — if it's dropping to near zero well before absorption ends, you're just wasting time sitting at elevated voltage for no reason.

@VickyWard the Victron SmartSolar's "absorption" on LiFePO4 is really just a CV hold — and if your BMS is doing its job, current will taper naturally without you lifting a finger, so the "ramp" you're admiring is mostly just physics doing the work.

Key thing to actually watch: tail current. When charge current drops to roughly 2-5% of capacity, job's done — anything beyond that is just heat and paranoia.

On my motorhome setup I've got the absorption timer wound right down because LiFePO4 doesn't need the long soak that lead acid does — the cells aren't absorbing, they're just sitting there waiting for you to stop fussing over them.

Check your SmartSolar has the LiFePO4 preset active rather than a generic lithium profile — there's a difference and Victron's own documentation is surprisingly clear on it.