Victron MPPT keeps dropping to float too early — anyone else had this?

Had this exact issue on my motorhome build last summer. Running a Victron SmartSolar 100/30 into a 100Ah Fogstar Drift LiFePO4, and it kept calling float at around 90% SoC according to the BMS app.

Turned out my absorption voltage was set correctly but my absorption time was far too short — I'd left it on the adaptive algorithm and it was deciding bulk was "done" based on previous cycle history that wasn't really representative of my usage pattern.

A few things worth checking:

• Are you using the Battery Preset or a custom profile? Some presets are surprisingly conservative
• What's your tail current set to? If it's too high, the MPPT thinks absorption is complete before the battery is actually full
• Are you on the latest firmware? There was a period where the adaptive absorption algorithm behaved oddly on certain hardware revisions

I ended up switching to a fixed absorption time of around 2 hours and it sorted it immediately. Not ideal for every scenario but it gave me confidence the battery was actually topping off properly.

Also — are you monitoring via VictronConnect or do you have a Cerbo/GX device? The extra telemetry really helps diagnose when exactly the transition is happening and whether it correlates with panel output dropping off (cloud cover can sometimes trigger an early float too).

What battery chemistry are you running and what are your current charge profile settings? Might help narrow it down further.

@WattLiz check your absorption voltage is set correctly for LiFePO4 — Victron's default profile is dead wrong for lithium out of the box, it's basically set up assuming you've got a dusty old AGM from 1987 sitting under your seat 🔋

Mine did exactly this until I set absorption to 14.2V and float to 13.5V in the VictronConnect app, then it stopped throwing a wobbly mid-charge. Also worth checking your absorption time isn't set too short — the tail current setting is your friend here.

@WattLiz this caught me out too on my motorhome setup. The real culprit is often the tail current setting — Victron's default is 1A, which on a 100Ah LiFePO4 is way too low. The BMS is still happily accepting charge but the MPPT sees that low current draw (LiFePO4's flat absorption curve means it genuinely pulls less current near the top) and thinks "job done."

Try bumping tail current to around 4-5A in VictronConnect. That's roughly the 0.05C threshold most LiFePO4 manufacturers actually recommend as a proper charge termination point.

@RetiredNurse49 makes a fair point about absorption voltage too — 14.2V is the sweet spot for most cells including the Fogstar Drift, not the default lithium profile Victron ships with.

Changed both settings on my rig and the early float problem completely vanished.

@LiFePO4Nerd is absolutely on the right track with tail current, but there's another piece of the puzzle nobody's mentioned yet — absorption time.

On a Victron SmartSolar, if you're using the adaptive absorption algorithm, it bases the absorption duration on the previous bulk time. Short bulk = short absorption = premature float. Classic.

For LiFePO4 I'd recommend switching to fixed absorption time in VictronConnect, set to something like 1–2 hours, combined with:

• Absorption voltage: 14.2V
• Float voltage: 13.5V
• Tail current: ~2–4% of capacity (so ~2–4A for your 100Ah Fogstar)

The Fogstar Drift BMS app is notoriously optimistic with SoC readings until the pack has done a few full cycles anyway, so don't chase that number too hard initially.

Once I dialled these in on my cabin setup the difference was night and day.

Really good thread this — had almost the identical headache on my motorhome last year.

One thing nobody's flagged yet: cable resistance can fool the MPPT into thinking absorption is complete. If your wiring between the controller and battery has any meaningful voltage drop, the controller "sees" a higher voltage than the battery actually is, so it thinks the tail current threshold has been met earlier than reality.

Worth doing a proper voltage drop test end-to-end. I found running 6mm² cable and keeping runs short made a noticeable difference on my setup.

Also — are you using the VE.Direct cable to let the SmartSolar talk to a Victron BMV or SmartShunt? Once I linked mine via the VE.Smart Networking, the MPPT used actual battery voltage rather than its own terminal measurement. Night and day difference for accurate charge termination.