Anyone else had their MPPT controller randomly drop to float way too early on cloudy days?

Been scratching my head over this one for a couple of weeks now. Running a Victron SmartSolar 100/30 with two 200W panels wired in series (so roughly 44V Voc) feeding a 200Ah lithium (LiFePO4) battery from Fogstar. On a decent sunny day it charges absolutely fine, hits absorption at the right voltage and holds it properly before dropping to float. No complaints there.

The issue is on overcast days — typical British summer, basically. The controller seems to jump into float sometimes when the battery is only at about 60-70% SoC according to the Victron app. I've got the absorption voltage set to 14.2V and float at 13.5V, with a 2-hour absorption time. My hunch is that on low-irradiance days the panel voltage is drooping enough that the controller thinks it's done something it hasn't, but I'm not entirely sure that's how the logic works.

Has anyone else seen this with Victron kit specifically, or is it more of a general MPPT thing? I've been poking around in VictronConnect but there are so many settings I'm not sure what I should actually be tweaking. Wondered if the "tail current" setting might be relevant here, or whether I need to look at the absorption time settings differently for lithium.

NorfolkWanderer | 847 posts | ⚡ Solar Enthusiast

@DuctTapeDave62 Classic Victron behaviour this, and it caught me out too when I first switched to LiFePO4. Check your absorption voltage setting - if it's set too close to your float voltage, the controller thinks it's "done" the moment current tapers off even slightly on a dull day.

With lithium you want a much tighter absorption window than lead-acid. I'd also double-check your tail current setting in VictronConnect - that's the current percentage at which it decides absorption is complete and drops to float. Mine was defaulting to something daft like 4% which on a cloudy day with reduced panel output it was hitting almost immediately.

Worth connecting via Bluetooth and watching the live graphs during a grey afternoon - you'll likely see exactly where it's making the wrong decision.

Your absorption timer is almost certainly cutting short because on a cloudy day the charge current never hits the threshold Victron uses to start the absorption clock properly — check your tail current setting in VictronConnect, it's probably set too high relative to your actual panel output on a grey Norfolk afternoon.

Also worth enabling BatteryLife or just manually bumping the absorption time — lithium profiles on Victron kit are notoriously conservative out of the box.

My Fogstar 200Ah does exactly this when cloud cover drops input below ~5A; float kicks in embarrassingly early and the battery sits at 80% looking smug.

Dizzy83 | 312 posts | ☀️ Off-Grid Convert

@DuctTapeDave62 Worth checking your tail current setting in the VictronConnect app - this is what I'd look at first before anything else. If it's set too high (say 4% or above), the controller sees the reduced cloudy-day current and thinks the battery is already nearly full, so it bails out of absorption early and drops to float. For a 200Ah LiFePO4 I'd set tail current around 2A absolute maximum, possibly lower. Also double-check your absorption voltage is actually correct for your specific battery - LiFePO4 values vary between manufacturers and an incorrect setting can cause all sorts of odd behaviour. What battery are you running exactly? Some of the cheaper cells have slightly different recommended charge parameters that catch people out.