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

Been running a 200W panel (single Victron 200W mono) into a 100Ah lithium (LiFePO4, Fogstar Drift) for about four months now. Most days it's been fine, but on overcast days — we're talking that classic British grey-sky nonsense — the controller seems to decide the battery is full way earlier than it should be. Sitting at maybe 80% SOC according to the Cerbo GX and it's already flipped to float.

From what I can tell it's hitting the absorption voltage (14.2V in my case) briefly during a small burst of cloud-break sun, then timing out and dropping to float even though the battery hasn't actually had a proper absorption cycle. The Fogstar recommends a tail current of around 2A to end absorption properly, but I'm not sure the SmartSolar is cutting off based on tail current by default — think it might just be using a fixed time.

Has anyone adjusted the "absorption time" settings or played with the tail current option in VictronConnect to fix this? I can see there's a BatteryLife algorithm setting in there too but I'm not entirely sure if that's relevant here or more of a long-term protection thing. Would love to know what settings people are actually running for LiFePO4 with a similar setup.

Hey @FinnThomas, classic issue this. On heavily overcast days your panel output drops so low that the charger can hit the absorption voltage threshold almost immediately with barely any current flowing — so it thinks the battery's full when it isn't really.

Worth checking your absorption time settings — if you're on "adaptive" mode it'll shorten absorption based on previous charge cycles, which can bite you on consecutive dull days. Try switching to a fixed absorption time (I'd start around 2 hours) and see if that helps top things up properly.

Also double-check your charge current threshold for transitioning to float — lower is more forgiving on cloudy days. What firmware version are you running on the MPPT? There were some improvements to this behaviour in recent updates worth having.

Your Victron's not broken — it's just British, it gives up the moment the clouds roll in too.

Jokes aside, check your absorption time settings; the SmartSolar default "adaptive absorption" calculates time based on previous charge cycles, so after a few sunny days it shortens the timer assuming the battery's in good nick. On a cloudy day it hits absorption voltage briefly then legs it to float before the battery's actually full. In VictronConnect try switching to a fixed absorption time (I use 2hrs on my shepherd's hut setup) rather than adaptive — made a noticeable difference with my Fogstar Drift behaving similarly. Also double-check your absorption voltage is actually set correctly for LiFePO4 (14.2V ish), not left on some AGM default.

Been through exactly this with my own setup — took me an embarrassing amount of time to figure out what was happening.

The key thing nobody's mentioned yet: check your tail current setting in the VictronConnect app. On LiFePO4 that threshold for transitioning to float can be too generous by default. If the panel's barely pushing anything on a grey Cumbrian afternoon, the charge current naturally falls below that tail current figure and the MPPT thinks the battery's full — job done, drop to float.

Worth dropping the absorption voltage very slightly too. Fogstar Drift cells are quite happy sitting lower than some generic profiles suggest, and a tighter absorption window means less chance of the charger getting confused mid-cycle.

The good news is the Victron's logs in the app will tell you exactly when it made the decision — that data is your friend here.

Really useful thread this, @FinnThomas. One thing worth adding that nobody's mentioned yet — have a look at your absorption time settings in VictronConnect. On the SmartSolar you can set a fixed absorption time rather than relying on the adaptive algorithm, which on low-current days can cut absorption embarrassingly short before the battery's properly topped up.

Also worth checking your tail current setting if you're running an older firmware. If it's set too high, the charger sees the naturally low charge current on a dull day and thinks the battery's full when it isn't.

Bumping your minimum absorption time to something like 1-2 hours should help on those grey days without doing any harm on sunny ones. Have a root around the Battery settings tab — it's all adjustable without needing a computer, just your phone.

Yeah had this exact issue with my garden office setup last winter.

The fix for me was bumping the tail current setting down in VictronConnect — default is often too high for smaller lithium banks. Once absorption current drops below that threshold it jumps to float, even if the battery isn't actually full.

Drop it to around 1-2% of battery capacity (so ~1-2A on a 100Ah bank) and it'll hold absorb longer on those grey days rather than bailing out early.

Also worth checking your absorption voltage is set correctly for the Fogstar Drift specifically — Fogstar publish recommended Victron settings on their site which saved me a lot of faffing.

Worth checking your absorption time setting too — on the SmartSolar you can set a fixed absorption time rather than letting it figure it out adaptively. On cloudy days the adaptive algorithm can get confused because the current's all over the place, so it thinks the battery's fuller than it is.

In VictronConnect go to the charger settings and look at the absorption duration options. I run mine on a fixed 2hr absorption for my static and it sorted similar behaviour for me. Not perfect for sunny days but on a 100Ah LiFePO4 it's a reasonable compromise.

Also double check your battery voltage sense — if it's measuring at the MPPT rather than at the battery terminals there can be enough of a drop to skew the readings, especially on longer cable runs.

Good shout from @BurnGlen and @MarkAllen69 on those settings. One thing I'd add — on cloudy days your panel voltage can fluctuate quite a bit, which can confuse the MPPT into thinking absorption is complete when it isn't. Check in the VictronConnect history tab whether your battery is actually hitting the target voltage before dropping to float, or just timing out. If it's not reaching it properly, your absorption voltage might also be set slightly low for LiFePO4 — Fogstar recommend 3.65V/cell (14.6V for 4S), so double-check that too.