Anyone had issues with a Victron SmartSolar 100/30 dropping to float too early on cloudy days?

Just installed a 200W panel (single Risen RSM40-8-200M) on my Transit camper build, paired with a Victron SmartSolar MPPT 100/30 and a 100Ah LifePO4 (Fogstar Drift). Set up the charge profile as per Victron's recommended settings for lithium — absorption at 14.2V, float at 13.5V, absorption time on adaptive.

Problem is, on overcast days the controller seems to rush through absorption and drop into float when the battery is nowhere near full. I'm watching it on the VictronConnect app and it'll sit in absorption for maybe 8–10 minutes, then bail out into float with the battery only showing around 60–65% SOC according to the BMS. Sunny days it behaves fine and gets the battery properly topped up.

I've had a read through the Victron docs and I think it might be related to the tail current setting or the adaptive absorption algorithm getting confused by the low, inconsistent current on grey days. Has anyone run into this with the 100/30 specifically, or with Victron MPPTs in general on lithium? Wondering whether disabling adaptive absorption and setting a fixed absorption time would be the sensible fix, or if I'm barking up the wrong tree entirely.

Reply by CarolWatson:

Hi @ExpertBuild, yes I've seen this on my setup too! The issue is likely your absorption time being set too short for low-current days. On cloudy days the MPPT simply can't push enough current to satisfy the absorption termination criteria, so it bails out early.

In VictronConnect, check your "tail current" setting — for a 100Ah LiFePO4 that's typically around 2-4A. On an overcast day you might only be getting 10-15A from that 200W panel, so the tail current threshold gets triggered prematurely.

Try increasing your minimum absorption time (I use 30 minutes as a floor) or slightly lowering the tail current percentage. Also worth double-checking your battery's actual recommended charge parameters with Fogstar directly — their Drift cells are sometimes a bit fussier than generic LiFePO4 profiles suggest. 🙂

Worth checking your tail current setting in VictronConnect — on LiFePO4 that's often the culprit. If the battery reaches absorption voltage but current drops below the tail current threshold (even briefly due to cloud), the MPPT thinks it's done and skips to float.

I had exactly this on my Fogstar Drift 200Ah. Bumped the tail current down to about 2% of battery capacity and it sorted itself out nicely.

Also double-check you're running the latest firmware — Victron pushed some fixes a while back that improved cloud-cover behaviour on the SmartSolar range.

Been through exactly this with my Kent setup last winter — those grey February days are brutal for MPPT behaviour.

What caught me out was the BMS communication side of things. If your Fogstar Drift has a BMS that's signalling "near full" to the Victron (even via simple voltage sensing), the controller can legitimately decide absorption is done earlier than you'd expect on low-irradiance days.

Worth checking your absorption voltage setting too — LiFePO4 profiles vary wildly between firmware versions on VictronConnect. I had mine set slightly low after a reset wiped my custom values.

Also, @RetiredEngineer86 is on the right track with tail current, but on a single 200W panel in UK winter cloud, you may simply never reach that tail current threshold — so the timer kicks in instead. Check your absorption time limit isn't set too short.

Reply by BitsAndBobs9:

Good shout from @RetiredEngineer86 on the tail current — but also worth checking whether you've got BatteryLife enabled in VictronConnect. On an MPPT connected to a LiFePO4 it can interfere with charge behaviour on low-irradiance days. Should be disabled for lithium generally.

Also, what absorption voltage and time have you set? With a Fogstar Drift, Fogstar actually publish recommended charge parameters on their site — worth cross-referencing those against what you've entered. A slightly low absorption voltage target can cause the charger to think it's done when it really isn't.

@KentVanLifer is right that winter days really expose these edge cases — the panel barely gets going before the MPPT decides it's finished. Sometimes a fixed absorption time (rather than adaptive) behaves more predictably in those conditions.

Something nobody's mentioned yet — check your absorption time setting. If you've got it on a fixed timer rather than adaptive, on a weak day the controller can hit absorption voltage briefly then just... give up and drop to float before the battery's actually full.

Victron's adaptive absorption is worth enabling if you haven't already. It scales the absorption time based on how long bulk took, which makes far more sense on patchy UK days.

My cabin setup did exactly this last November — thought something was broken, turned out fixed absorption was set to 20 mins. Felt a bit daft when I found it tbh. 🙄

Also worth logging a day's data in VictronConnect and actually looking at the voltage curve — tells you a lot more than staring at the LEDs.