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

Had this exact issue on my narrowboat last summer with a 100/30 and a 100Ah Fogstar lithium. Battery sitting at around 80% but the controller would declare it "full" and drop to float by early afternoon.

Turned out my absorption voltage was set correctly but the tail current was the culprit — mine was configured far too high (something like 4% default), so the MPPT was interpreting the naturally low charge acceptance of lithium as "battery full" and bailing out early.

Dropping the tail current threshold right down (I went to around 1%) made an enormous difference. The controller now actually works through a proper absorption phase before floating.

A few things worth checking if you're in the same boat (pun intended):

• Tail current setting in VictronConnect — is it appropriate for your battery chemistry?
• Absorption time — are you using adaptive or fixed?
• Battery temperature — cold lithium charges differently and some BMSs will throttle acceptance, which can fool the MPPT

Also worth asking: are you using a Victron BMV or SmartShunt? If so, have you enabled DVPE so the battery monitor is actually talking to the MPPT and giving it a more accurate state of charge picture? That changed things considerably for me.

Curious whether others have seen this more on lithium setups specifically, or whether it happens just as often with AGM? My previous boat had lead-acid and I don't recall it being nearly as fussy.

@TrevorHughes yes! Had almost identical on my narrowboat last season. Turned out my absorption voltage was set slightly too low — the Fogstar was hitting that threshold before it was genuinely full.

Worth checking a few things:

• What's your absorption voltage set to? Fogstar recommend 14.2V for their lithiums IIRC
• Have you got the tail current configured in VictronConnect? That's the one that actually tells the MPPT "job done" based on amps rather than just voltage
• Is your battery temp compensation throwing things off?

The tail current setting made a massive difference for me — once I set it correctly the controller stopped declaring victory too early. What firmware version are you running on the MPPT? There were some absorption-related quirks in older versions that got patched.

@TrevorHughes mine did exactly this in the shepherd's hut until I realised the tail current setting was the culprit — Victron defaults are written for lead-acid dinosaurs, not lithium, so drop that tail current threshold way down and suddenly your MPPT stops knocking off early like it's clocked out for lunch 🔋

@BatteryPaula nailed the tail current thing, but worth also checking your absorption time isn't set to a fixed value — if it's on fixed rather than adaptive, the Victron just gives up after that timer regardless of actual SoC, which on a smallish 100Ah Fogstar will bite you every cloudy morning followed by a brief sunny spell. Flip it to adaptive and let the algorithm earn its keep. Also double-check your battery voltage sense — if it's reading from the controller terminals rather than the battery posts, voltage drop on dodgy cable connections will fool it into thinking you're fuller than you are. Done this in my van conversion more times than I care to admit.

@BatteryPaula and @Rob1963 have covered tail current and absorption time well, so I'll add something nobody's mentioned yet: temperature compensation.

If your Fogstar cell BMS is reporting a slightly elevated temperature (common in narrowboat battery compartments), the MPPT will lower its target voltage accordingly — sometimes enough to trigger an early float transition before the cells are genuinely full.

Check your VictronConnect app → MPPT settings → Battery temperature sensor. If you're running the Smart Battery Sense dongle or have the VE.Smart network active, make sure the compensation coefficient matches Fogstar's spec (typically 0mV/°C for LiFePO4 — disable compensation entirely for lithium).

Had exactly this on my own setup before I caught it. Disabled temp compensation, sorted immediately.

Worth logging a few charge cycles in VictronConnect and comparing the absorption termination voltage against your actual BMS state of charge readout.

@RetiredSquaddie oh brilliant, the suspense of that trailing sentence is genuinely keeping me up at night — meanwhile @TrevorHughes, have you checked your battery temperature sensor is actually connected properly, because my Victron on the van once decided a 12°C Welsh morning meant the battery was "full" and knocked off early like it had somewhere better to be.