Victron MPPT cutting out mid-afternoon on hot days — temperature compensation issue?

Running a 400W panel array into a Victron SmartSolar 100/30 on my static caravan, paired with a 200Ah Fogstar Drift LiFePO4. All worked brilliantly through winter and spring, but since we've had proper warm weather the controller keeps dropping into what looks like float early — usually around 1–2pm when the panels are cooking. Battery's only sitting at maybe 70–75% SOC at that point, nowhere near full.

I've been digging through the VictronConnect app and I'm wondering if my absorption voltage is set slightly too conservative (currently 14.2V) combined with the MPPT pulling back because the panel temps are spiking. The controller itself is mounted in a south-facing cupboard which was a daft decision in hindsight — I've measured 48°C in there on a warm afternoon with a cheap IR thermometer.

Has anyone dealt with this specifically? I'm torn between relocating the MPPT to a shadier spot versus tweaking the charge profile. The Fogstar documentation says absorption at 14.6V is fine, so I might bump that too. Curious whether the built-in temperature compensation on the SmartSolar is actually doing more harm than good here if there's no battery temperature sensor fitted — I've read it defaults to assuming 25°C ambient, which clearly isn't accurate in my setup.

@SaltySocket worth checking whether it's actually cutting out or just hitting absorption/float earlier than expected on hot days.

A few things to look at:

• Panel Voc at temperature — panels lose voltage as they heat up, but your 100/30 should handle that fine
• Throttling due to high controller temp — if the MPPT itself is in direct sun or a poorly ventilated space, it'll derate to protect itself
• Battery temp sensor — if you're using the built-in Bluetooth temp sensing rather than a dedicated sensor cable, it's reading controller temp, not battery. Could be skewing compensation badly

Check the VictronConnect history tab — it'll show exactly what triggered the state change. The MPPT logs are genuinely useful here rather than guessing.

Where's your controller mounted physically? That's usually the culprit in my experience.

@SaltySocket The clue's right there in your post title — you've correctly diagnosed it yourself!

A few things to verify systematically:

1. Check your battery temp sensor — if you're running the Victron Smart Battery Sense (or the built-in BMS temp reporting via VE.Smart networking), confirm it's actually paired correctly in

@SaltySocket one thing nobody's mentioned yet — check your cable runs. On a hot day the voltage drop across warm cables increases, which can confuse the MPPT's calculations and trigger odd behaviour.

Also worth checking: is the SmartSolar mounted somewhere with decent airflow? Mine throttles noticeably when cable-tied to a sun-baked bulkhead on the boat. Moved it inside to a ventilated locker and the issue disappeared.

In the Victron Connect app, pull up the history graphs and look for any correlation between the cutouts and panel voltage spikes — occasionally it's actually an overvoltage condition that only manifests when panels are cool and producing peak Voc.

My Fogstar in the shepherd's hut does exactly this in summer — the MPPT reads the battery as "hotter than it is" if your temp sensor is poorly placed, so it backs off charging thinking it's protecting cells that are actually perfectly happy.

Check where your temperature sensor is physically sitting — if it's near the inverter or any heat source rather than actually on the battery, you're basically lying to your Victron and it's doing the sensible-but-wrong thing.

Had this exact issue with my motorhome setup last summer. Worth checking your absorption voltage setting in the VictronConnect app — if it's set for flooded lead acid defaults it'll be too high for the Fogstar Drift anyway, and the BMS will disconnect to protect itself.

For LiFePO4 you want absorption around 14.2V, float around 13.5V. Temperature compensation should ideally be disabled entirely for lithium — lead acid needs it, lithium doesn't, and it can actually cause problems like you're describing.

Quick fix: in VictronConnect go to Charger > Expert mode and set temp compensation to 0mV/°C.

@OffGridHamish makes a fair point about sensor placement too — remote temp sensor directly on the battery is worth doing if you haven't already.