Victron MPPT keeps dropping to float mid-afternoon — panels are clean and it's sunny

Had exactly this on my narrowboat last summer and it drove me mad for weeks. Turned out my battery SOC was genuinely hitting 100% by early afternoon — the absorption stage was completing faster than I expected because I'd recently added two extra Fogstar 100Ah lithiums and the bank was just... full.

Worth checking a few things:

• What's your battery bank size vs panel output? If your array is oversized relative to the bank, float by 2pm isn't necessarily wrong
• Are you running any loads during the day? On the boat I had the fridge and inverter pulling enough to mask this, but on a still day at the marina it was minimal draw
• Check your absorption voltage and time settings in VictronConnect — if absorption completes early because voltage holds steady, it'll drop to float correctly

The Victron MPPT is pretty clever about this. It's not always a fault — sometimes it's just doing its job properly and we're second-guessing it.

That said, if your batteries definitely aren't full (check your BMV or SmartShunt if you have one), then it could be a wiring issue causing a false voltage reading at the controller. Loose connections or undersized cable between the MPPT and battery terminals can cause voltage spikes that trick it into thinking absorption is done.

What size is your panel array and battery bank? And are you on lead-acid or lithium? Makes a big difference to how the charge profile behaves.

@RustyNomad that's a good point about SOC, but worth ruling out a few other causes first:

• What are your absorption voltage and time settings? If absorption time is set too short, the controller bails out early even if the battery isn't truly full
• Are you running Victron's adaptive absorption? It scales duration based on the previous night's discharge, so a shallow discharge = short absorption = early float
• What battery chemistry? LiFePO4 genuinely does hit full quickly on a sunny day — that's actually normal behaviour

On my motorhome with a Fogstar 200Ah LiFePO4, I see float by midday regularly in summer. Initially thought it was a fault, but the BMS data confirmed it was legitimately full.

Could you share your absorption voltage target and what batteries you're running? That'd help narrow it down considerably.

@RustyNomad @WattLiz had almost the exact same thing with my shepherd's hut setup — genuinely thought something was broken for ages.

One thing worth checking that nobody mentions: tail current. Victron SmartSolar will exit absorption and drop to float when the charge current falls below a set percentage of the battery capacity, even if the timer hasn't expired. If your bank is smaller or the panels are slightly oversized for it, absorption can complete very quickly on a bright afternoon.

Jump into VictronConnect and look at your absorption end trigger settings. Also check your battery capacity setting is actually correct — I'd fat-fingered mine and had it set way too low, which made the tail current threshold trigger far too early.

Fogstar batteries have specific charge profiles too if you're running lithium, worth cross-referencing their recommended settings.

@WattLiz makes a fair point about ruling things out systematically, but everyone's dancing around the elephant in the room: temperature compensation.

If your battery bank is in a well-insulated cabin or engine bay, it'll warm up considerably by early afternoon. Victron's default temp coefficient will lower the absorption voltage target as temp rises — meaning absorption completes "early" not because the batteries are genuinely full, but because the voltage threshold has dropped to meet ambient conditions.

Check your MPPT history in VictronConnect. If absorption duration is consistently short (under 2 hours on a decent bank), that's your tell.

I've got a SmartShunt on my garden office setup precisely because the MPPT history alone doesn't tell the whole story — cross-referencing SOC against the charge stage timing is where you actually find the culprit rather than just poking at settings randomly.

@SomersetNomad go on then, what's the elephant — my Fogstar batteries doing it in my static and I've been blaming the Victron like it's the neighbour's dog.