ALERT - Alarm on Temperature, MPPT stopped charging with heavy battery loads

Had something similar crop up on my van build last summer and it's been bugging me ever since.

Running a Victron SmartSolar 100/30 into a 100Ah Fogstar Drift LiFePO4, and during a particularly warm spell parked up in direct sun, the MPPT threw a high temperature alarm and stopped charging. Fair enough in principle — nobody wants thermal runaway. But here's the thing: the battery was actively discharging at the time running the compressor fridge, so the cells were actually pulling away from any dangerous voltage ceiling, not approaching it.

Felt like the protection logic was a bit blunt. Temperature alone triggered the cutoff, but surely discharge under load is a fundamentally different risk scenario compared to charging into an already-warm battery at high SOC?

A few questions for those who've dug into this:

• Is there a way in VictronConnect to set separate temperature thresholds for charge cutoff vs. just triggering an alarm?
• Does the BMS in the Fogstar Drift communicate temp data back to the MPPT via VE.Direct or is the MPPT using its own internal sensor?
• Would adding a proper battery temperature sensor (rather than relying on ambient) actually help here, or make it more aggressive?

I've got a garden office build in the pipeline with a larger bank and more consistent loads, so want to get my head around this before I scale up. Suspecting the default settings are quite conservative and there's room to tune them without voiding any warranties or doing anything daft.

Anyone dealt with this properly rather than just ignoring the alarm?

@StuKnight classic thermal derating behaviour — the MPPT is protecting itself, not the battery.

The 100/30 has an internal temperature sensor and will throttle output (sometimes to zero) when its own internals get cooked. In a van in summer, especially if the unit's tucked away with poor airflow, that controller can easily hit 40–50°C internally.

Few things worth checking:

• Mounting orientation — vertical with fins running vertically is critical for convection
• Ambient vs. case temp — grab a cheap IR thermometer and shoot the heatsink mid-charge
• VictronConnect history — look at the "Charge cycle" tab for throttling events

Mine did identical in my tiny house build until I added a 40mm 12V fan on a thermal switch behind the unit. Virtually eliminated the issue for about £3 in parts from Amazon.

The Fogstar itself is almost certainly fine — LiFePO4 laughs at UK summer temperatures.

Really interesting thread this — I had something weirdly similar on my narrowboat last summer with a Victron SmartSolar 75/15 (smaller setup I know, but still).

Mine was cutting out when the engine was running and the solar was pulling hard simultaneously. Took me ages to figure out it wasn't the MPPT temperature at all — it was actually the BMS on my Fogstar tripping because the combined charge current was pushing past its limits momentarily.

Worth checking: are you monitoring actual battery temperature or just the MPPT internal temp? The Victron app shows both if you've got a Smart battery sense dongle fitted.

Also — whereabouts is your MPPT physically mounted? Mine was tucked in a poorly ventilated cupboard which definitely didn't help matters. Moving it somewhere with a bit more airflow made a noticeable difference straight away.

@LindaClark90 curious what narrowboat setup you're running — I've got a similar situation on mine. The thing people often overlook is where the MPPT is physically mounted. Mine was in the engine bay initially and the ambient temp was pushing the unit into derating territory even before solar load kicked in.

Moved it to a ventilated cabinet under the gunwale, massive difference. The Victron Connect app will show you the internal temperature history if you dig into the trends — worth checking whether the alarm is correlating with peak afternoon heat rather than actual charging current.

@StuKnight also worth double-checking your battery temperature sensor is actually connected properly. Had a loose JST connector on mine that was feeding the BMS duff readings and causing phantom protection trips.

@StuKnight had almost identical grief with my cabin setup last July. What nobody's mentioned yet — check where your MPPT is physically mounted. Mine was bolted to an interior wall with basically zero airflow behind it, and the internal temp sensor was reading ambient as 45°C+ before it even started working hard.

Moved it to a shaded external enclosure with a bit of ventilation and the thermal throttling practically vanished overnight.

The 100/30 manual does mention a derating curve starting around 40°C internal temp — it'll quietly wind back the charge current without throwing any obvious errors, which makes diagnosing it a right faff.

Worth grabbing the VictronConnect app and watching the internal temperature log during a charge cycle. If it's climbing fast early on, it's an airflow problem, not a battery or BMS issue.