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

Had this exact issue on the narrowboat last summer. Victron 100/30 kept floating by midday even though the batteries weren't anywhere near full.

Turned out my battery capacity setting was way off — I'd never properly configured it after swapping to a Fogstar 200Ah lithium. The MPPT was using the old absorption voltage threshold and bailing out early because it thought the job was done.

Few things worth checking:

• Absorption voltage — is it set correctly for your battery chemistry?
• Tail current setting — if it's too high, float kicks in prematurely
• Battery temperature — Victron adjusts thresholds with a temp sensor, and if it's reading wrong you'll get odd behaviour
• State of charge history — connect via VictronConnect and look at the trends, it'll show you exactly what's happening

Also worth checking if you've got the adaptive absorption feature enabled. It can shorten absorption time based on previous cycles, which sounds clever but confused the life out of my setup initially.

What controller and battery combo are you running? Makes a big difference to what the fix might be. Lithium vs AGM behaves completely differently in terms of how the MPPT decides when to transition.

Anyone else on here running Fogstar lithiums with Victron kit? Curious whether others had to fiddle with settings for a while before things settled down properly.

@HelenThompson this is a really common one and worth digging into properly.

The capacity setting alone won't cause early float — the MPPT doesn't actually track state-of-charge directly. What it does use is the absorption voltage threshold combined with tail current. If your tail current is set too high (or left at default), the controller interprets the reduced afternoon current as "battery full" and drops to float prematurely.

Check your absorption tail current in VictronConnect — on a 100/30 the default is often 1A, which is far too aggressive for anything over 100Ah. On my narrowboat setup with 400Ah of Fogstar Drift LiFePO4, I run it at 4A.

Also verify your battery voltage sense — if it's reading from the controller terminals rather than the battery, cable voltage drop will cause it to think absorption is complete earlier than it actually is.

@DefenderAdventure finish your thought mate, got cut off there 😄

@HelenThompson worth checking your absorption voltage and time settings too. My Fogstar Drift cells needed me to drop absorption voltage slightly compared to Victron's defaults — was finishing absorption way too quick because the BMS was already happy and the voltage was climbing fast with low internal resistance.

Also — tail current. If you're using a AGM/GEL profile on lithium that setting can be way off and it'll kick to float prematurely.

What battery chemistry are you running?

@HelenThompson had similar on my static. Turned out the absorption time was set too short — Victron defaults can be a bit optimistic depending on which battery type you've selected.

Worth checking in VictronConnect whether you're on a preset or custom profile. If you're running lithium (Fogstar or similar) someone may have selected the AGM preset which behaves completely differently.

Also — what's your actual resting voltage when it drops to float? If it's still sitting at 12.6-12.7 that's suspicious. Could be a dodgy voltage sense reading rather than a capacity issue at all.

@HelenThompson another thing worth checking is your tail current setting in VictronConnect. If it's set too high, the MPPT thinks the battery is full because charge current has dropped to that threshold, even when it hasn't actually completed absorption properly.

On my setup I had it at the default 4A which was far too aggressive for my battery bank size — dropped it to around 1-2% of battery capacity and it made a massive difference. Particularly noticeable on overcast days when the panel output is naturally lower and fluctuating. The controller was basically giving up prematurely every time a cloud passed over! Worth having a look at that alongside what the others have mentioned. 😊