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

Had exactly this with my 100/30 last summer. Battery would barely be at 80% and it'd swan off into float like it had done a full day's work. Drove me absolutely mad.

Turned out my absorption time was set too short. If you're using the fixed time mode rather than adaptive, Victron will just run absorption for however long you've told it — doesn't matter if the battery's actually full or not. Check what yours is set to in VictronConnect.

Also worth asking: what batteries are you running? I've got Fogstar Drift LiFePO4 and the settings I pulled off a random forum (probably this one, to be fair) were slightly off for my specific cells. Ended up going through Fogstar's recommended charge profile and dialling it in manually.

A few things to check:

• Absorption voltage — is it actually high enough for your battery chemistry?
• Tail current setting — if this is too high, it'll exit absorption prematurely
• Battery temperature — cold batteries will behave oddly and some controllers compensate aggressively

The tail current one catches people out constantly. Victron's default can be quite conservative, so if your bank is on the larger side it'll hit that current threshold before the battery's properly topped up.

What size is your array vs battery bank? If you're seriously undersized on panels, you might genuinely be hitting the absorption time limit before the bank is full — in which case the controller is doing its job, just not the job you want.

Post your settings and I'll have a proper look. Someone on here will know what's wrong.

TerryLewis | 847 posts | Shropshire

@GoldenSocket — absorption time is almost certainly your culprit, yes. Worth also checking whether you've got the battery capacity set correctly in the VictronConnect app. If the Victron thinks it's charging a 100Ah bank but you've actually got 200Ah, it'll calculate the tail current threshold against the wrong figure and cut to float far too early.

Also worth checking your wiring for any resistance — a dodgy connection can fool the controller into thinking voltage has risen sufficiently when it actually hasn't at the battery terminals themselves. Sense wires directly on the battery posts sort that nicely if you haven't already done it.

What battery chemistry are you running? AGM and lithium behave quite differently here and the default profiles aren't always ideal straight out of the box.

Seen this exact thing with my setup. Worth checking your tail current setting in VictronConnect — if it's set too high, the MPPT thinks the battery's full before it actually is. Mine was defaulting to something daft like 4A on a 100Ah Fogstar lithium, which is way off.

Also double-check your battery capacity is entered correctly in the charger settings. The absorption termination logic is partly calculated off that figure, so if it's wrong you'll get early float every time.

@GoldenSocket what battery chemistry are you running? LiFePO4 and AGM behave quite differently here, and the default profiles in VictronConnect aren't always suitable out of the box — especially for the cheaper Chinese cells that don't quite match the spec sheet voltage curves.

Also check your battery temperature — if you've got no temp sensor fitted, the Victron just assumes a balmy day and cuts absorption short accordingly. Slapped a Smart Battery Sense on mine for the narrowboat bank and it transformed the whole charging profile. Fogstar cells in particular seem to need the full absorption cycle in cooler weather or they just never quite get there.

@ExFarmer raises a good point on the temp sensor — I'd also add that if you're running adaptive absorption and your previous charge cycles were short, Victron essentially "learns" a shorter absorption time and you end up in a perpetual cycle of undercharging.

Fixed mine on the shepherd's hut system by switching to fixed absorption time and setting it manually to match my Fogstar battery spec sheet rather than trusting the algorithm to figure it out.

Sometimes the smart features are a bit too clever for their own good.