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

Just picked up a 200W panel (a Renogy mono, 37.8Voc, 8.03Isc) paired with a Victron SmartSolar MPPT 100/30 to charge a 100Ah lithium (a BMS-protected LiFePO4 from Fogstar). I've set absorption to 14.2V and float to 13.5V as recommended, absorption time set to 2 hours. On sunny days it's cracking — hits absorption, holds it, job done.

The problem is on overcast days. We've had a run of grey weather here in the south-west and I keep noticing the controller dropping into float after maybe 20–30 minutes in absorption, even though the battery is clearly nowhere near full (the Victron BMV-712 is still showing 60–70% SoC). My gut says the panel just isn't pushing enough current to satisfy the absorption exit condition, so it's timing out early, but I'm not entirely sure that's how the 100/30 logic actually works.

Has anyone else seen this with Victron kit on LiFePO4 in low-light conditions? I've had a poke around in VictronConnect but I'm not sure whether tweaking the tail current setting is the right move, or whether I'm missing something obvious. Any pointers much appreciated.

SomersetOffGrid | ⚡ Solar Tinkerer | Posts: 847

@DanPhillips this is almost certainly your tail current setting catching you out. On cloudy days the charge current is already low, so the SmartSolar hits the tail current threshold thinking the battery's full when it isn't.

In VictronConnect, go into the battery settings and check your "tail current" value — it defaults to 1A on some firmware versions which is far too aggressive for a 100Ah LiFePO4 in low-light conditions. Try bumping your absorption time up to a fixed 2 hours rather than adaptive, or drop the tail current threshold to around 0.5A.

Also worth double-checking your absorption voltage hasn't crept down — Fogstar recommend 14.2V which you've got, but make sure temperature compensation is disabled as that can reduce it unexpectedly on cold days.

Hey @DanPhillips, worth checking your absorption time settings in VictronConnect — if you've got "adaptive absorption" enabled, the controller calculates absorption duration based on the previous night's discharge. On a cloudy day where the panel's struggling to push decent current, the SmartSolar can interpret the low tail current as the battery being full and bail out to float prematurely. Try setting a fixed absorption time of around 2-3 hours rather than relying on adaptive mode. Also double-check your tail current threshold — it defaults to 2A on some firmware versions, which on a 100Ah LiFePO4 could trigger float way too early. With Fogstar cells specifically I'd set it closer to 1A or even lower. The Bluetooth logging in VictronConnect is brilliant for spotting exactly when it's transitioning — definitely worth reviewing a day's worth of data.

ExTrucker32 | 🔋 Off-Grid Enthusiast | Posts: 1,203

@DanPhillips had the exact same head-scratch moment with my setup last year. On overcast days your panel simply isn't pushing enough current to satisfy the absorption cycle properly, so the controller can interpret the low current as the battery being full and kicks into float early.

Worth having a look at your "minimum absorption time" setting in VictronConnect — I'd bump it up to around 1 hour as a floor so it doesn't bail out too soon on dull days. Also double-check your battery capacity is set correctly in the app, as Victron uses that figure to calculate certain thresholds.

The Fogstar cells are decent quality so I doubt it's a BMS comms issue, more likely just the controller being a bit eager on a grey UK afternoon! 😄

BaySoul | 🚐 Motorhome Wanderer | Posts: 312

My Fogstar/Victron combo did the exact same thing until I realised my absorption voltage was set correctly but the time was so short it was basically waving at bulk and sprinting past it — check your minimum absorption time isn't still on the default AGM-friendly setting, because LiFePO4 wants longer up top before it earns its float.