Victron MPPT keeps dropping to float mid-afternoon — anyone else had this?

Had exactly this on my narrowboat last summer — drove me mad for weeks before I tracked it down.

The culprit in my case was the absorption time being set too short combined with the battery voltage spiking briefly due to a light load period. The Victron saw the voltage hit the float threshold, assumed the batteries were full, and dropped straight down. Perfectly logical from the controller's perspective, completely wrong in practice.

A few things worth checking:

• Absorption time — if you're using fixed rather than adaptive, try bumping it up and see if the behaviour changes
• Battery temperature — on cooler afternoons my Fogstar lithium cells report lower temps which shifts the charge curve slightly
• Tail current setting — this is the big one for lithium. If your tail current threshold is set too high, it'll terminate absorption prematurely
• VE.Direct / Bluetooth logs — pull the full charge cycle data from VictronConnect and look at the actual voltage and current curves rather than guessing

What battery chemistry are you running? The answer changes significantly between lead-acid and lithium. On my setup (200Ah Fogstar Drift LiFePO4 with a Victron SmartSolar 100/30) I had to dial the tail current down to around 2% of capacity before absorption would hold properly through the afternoon peak.

Also worth checking whether you have any other devices communicating over the VE.Smart network — occasionally a temperature sensor or SmartShunt sending odd data can influence the charge profile unexpectedly.

What does your VictronConnect history tab actually show for the transition point? That'll narrow it down considerably.

@MeadowDweller had this exact issue on my shepherd's hut setup last year. Worth checking your tail current setting in VictronConnect — if it's set too high, the MPPT thinks absorption is "done" and drops to float prematurely even with partially charged cells.

Also check whether you've got adaptive absorption enabled. On smaller Fogstar lithium banks it can behave oddly if the BMS is occasionally nudging the voltage, confusing the controller into thinking the battery's fuller than it is.

Mine turned out to be a dodgy voltage sense reading — temperature compensation was pulling the target voltage down on warm afternoons right when solar was peaking. Disabled temp comp temporarily and the problem vanished immediately.

What battery chemistry are you running? Makes a big difference to how aggressive you can set the absorption parameters.

Great thread — worth adding that temperature can play a sneaky role here too. If you've got the Victron temperature sensor fitted, double-check it's actually making good contact and reading accurately. I had mine reporting ambient as though the batteries were already warm, which caused the controller to back off absorption voltage earlier than it should have done.

Also worth having a look in VictronConnect at your history data — specifically the absorption time logged for recent days. If it's consistently short, that'll confirm the controller thinks the batteries are satisfied when they're not. Could point to a dodgy cell or a connection with higher resistance than expected causing a false voltage reading at the controller terminals.

@OldSparky18 's point about tail current is spot on too — often overlooked on the initial setup.

@MeadowDweller had this on my narrowboat too — turned out my battery voltage sense wires had more resistance than my will to live, so the MPPT thought it was fuller than it was and bailed early. Chuck a proper Victron Smart Battery Sense on it and let the Bluetooth do the heavy lifting instead of trusting cable runs through a damp bilge.

@MeadowDweller worth checking whether you've got adaptive absorption enabled — if the previous day was overcast, Victron calculates a shorter absorption time the next day, so a run of dull weather can leave your batteries perpetually undercharged and dropping to float embarrassingly early.

Fixed mine by switching to fixed absorption time temporarily while I diagnosed things. Also double-check your absorption voltage threshold is actually appropriate for your battery chemistry — I had mine set for AGM when I'd switched to Fogstar lithium and hadn't updated the profile. Complete schoolboy error but it'll cause exactly the symptoms you're describing.

VictronConnect history graphs are brilliant for this — look at the voltage curve in the hours before it drops to float and you'll see pretty quickly whether absorption is actually completing properly.

@Nessa73 raises a good point on adaptive absorption — that caught me out badly on my narrowboat the first winter I had the system running.

What nobody's mentioned yet: check your tail current setting in VictronConnect. If it's set too high relative to your battery bank capacity, the controller decides absorption is "done" prematurely and drops to float before the batteries are actually full.

My setup runs a 200Ah lithium bank through a Victron SmartSolar 100/30. I had the tail current at the default 2A, which was far too aggressive for that bank size. Bumping it to match roughly 1% of battery capacity made an immediate difference — mid-afternoon float drops disappeared entirely.

Worth logging a few days through the VRM portal before changing anything, so you've got a proper baseline to compare against.