MPPT will not turn on

Had a similar head-scratcher with my garden office setup last summer — four panels wired in series, Victron SmartSolar 150/35 sitting there doing absolutely nothing like it was on a tea break.

Turned out my open-circuit voltage was nudging dangerously close to the controller's rated input limit once you factor in a cold morning boost (panels love the cold, inconveniently). Worth remembering that Voc climbs as temperature drops, so if you're measuring 122V on a mild afternoon, you could easily kiss 135-140V on a crisp January morning in the UK — which will happily murder your MPPT if it's only rated to 150V.

A few things worth checking if you're in the same boat:

• Recalculate your Voc at -10°C using the panel's temperature coefficient (it's on the datasheet, usually expressed as %/°C)
• Check your breaker rating — 15A on the PV string side seems low depending on your panel Isc
• Victron's MPPT calculator is genuinely brilliant for sanity-checking this stuff before you fry anything expensive
• Make sure battery voltage is present on the output terminals before the MPPT will initialise — they won't self-start without it

My Fogstar batteries weren't fully charged when I first commissioned mine, and the controller just sat there looking confused until I bumped them up with a bench charger first.

Anyone else had their MPPT play dead and it turned out to be something embarrassingly simple? Feel like half of off-grid troubleshooting is just turning it off and on again but with more multimeter.

@Chopper54 classic symptom — nine times out of ten it's the PV open-circuit voltage creeping over the controller's rated input on a cold clear morning. Four panels in series will do that to you fast. Victron's 150V ceiling sounds generous until you realise manufacturer Voc specs are measured at 25°C; drop to 5°C and you're adding roughly 0.4% per degree depending on your temperature coefficient. Do the maths — that's potentially 8-10V extra across the array. Controller goes into self-protection, looks completely dead.

Check your panel spec sheets, calculate worst-case Voc at minimum expected temperature, and if you're anywhere near that 150V limit you need to reconfigure to parallel or swap for a 250V unit. Victron's MPPT calculator tool handles this properly. What panels are you actually running and what's the rated Voc each?

@FormerMechanic14 makes a solid point, but worth adding — the 150/35 has a 150V absolute maximum input, and four panels in series on a cold morning can easily breach that. PV Voc rises roughly 0.3-0.4% per °C below STC (25°C), so if your panels are rated 38V Voc each, that's 152V nominal — already over-limit before temperature correction.

Use the Victron MPPT calculator or even a spreadsheet with your panel's temperature coefficient (check the datasheet) and your local winter minimum. Here in Devon I've seen panels hit -8°C on clear nights.

Also check: is the battery voltage present at the controller terminals? The SmartSolar won't initialise without a battery connected — it needs that reference voltage to wake up. Caught me out once on my narrowboat install.

@Chopper54 been there with my own Victron setup — one thing neither of the above have mentioned yet: check your battery voltage is actually present at the controller terminals. The SmartSolar won't initialise if it can't sense the battery side first. No battery voltage = no wakeup, full stop.

Worth pulling up VictronConnect if you haven't already — even a dormant controller will usually show something over Bluetooth, and the error codes are genuinely useful rather than vague. Mine once sat inert because a blade fuse in the battery cable had blown silently.

Also double-check your PV polarity before anything else. Sounds obvious but it's caught plenty of people out.

@HazelPaddy that battery voltage point is a good one — I had almost the exact same situation with my van conversion build. The SmartSolar won't initialise if the battery is too flat (below ~10V on a 12V system, from memory). Victron's own docs call it the "start-up voltage threshold."

Also worth checking: is your battery negative actually connected to the MPPT's battery negative, not just the chassis or a bus bar further down the line? I had a dodgy ring terminal that looked fine visually but had enough resistance to fool the controller into thinking nothing was there.

Victron Connect app will often show a fault code even when the unit appears dead — worth plugging in via Bluetooth before assuming it's a hardware failure.

@Chopper54 one thing nobody's touched on yet — the 150/35 won't wake up if your panel open-circuit voltage is too close to battery voltage. The MPPT needs to see a meaningful voltage differential to even bother attempting a charge cycle. Victron's own documentation specifies the panel Voc must exceed battery voltage by roughly 5V minimum, but in practice I've found you want considerably more headroom than that, especially first thing on a cold morning when the battery's already reasonably topped up from the previous day.

On my static caravan setup I had this exact scenario in midsummer — panels facing slightly east, batteries at 13.8V by mid-morning, controller just sat there blinking at me. Rotated the array slightly and sorted it overnight.

Also worth firing up VictronConnect and checking the error log — it'll usually tell you exactly why it's sulking.