Anyone else had their Victron MPPT drop to near-zero amps on a clear day? Think I've found the culprit

I've been scratching my head for a couple of weeks now. My 100/30 SmartSolar has been throwing out maybe 1–2A on what should be brilliant harvesting days — panels are 2x 175W Renogy monos wired in series giving me around 44Voc, and the battery bank is a 200Ah 12V LiFePO4 sitting at roughly 13.4V mid-afternoon. By rights I should be seeing 20A+ easily.

Turns out the culprit seems to be a dodgy MC4 connector on the positive panel lead. Crimped it myself about 18 months ago and the pin had worked itself slightly loose inside the housing — enough to create resistance but not enough to break the circuit completely. Swapped it out for a proper Stäubli connector yesterday and I was immediately back up to 22–24A in similar conditions. Gutted I didn't check it sooner.

My question is — has anyone got a reliable way of diagnosing high-resistance connections without a proper clamp meter or thermal camera? I used the VictronConnect app to spot the anomaly in the charge data, but that only told me something was wrong, not where. I did eventually use a basic multimeter across each connector pair but the resistance difference was tiny and I nearly missed it. Wondering if there's a smarter workflow for tracking these things down before they quietly rob you of harvest for weeks.

My Victron did exactly this and the culprit was a single corroded MC4 connector acting like a 300W resistor — swapped it out and suddenly I was a solar genius again.

Really interesting thread — does the MPPT show anything unusual in the VictronConnect history graphs when this happens? I'm thinking specifically whether the Vpv reading is dropping or staying roughly where you'd expect it.

On my narrowboat setup I had something similar and it turned out to be a dodgy inline fuse holder between the panels and the controller — the spring contacts had oxidised and were creating enough resistance to absolutely throttle the current. Worth checking those too if you have any inline fuses in the string.

Also — is there any partial shading hitting even a corner of one panel? With series wiring the whole string suffers badly. Would the 100/30 be happier with those panels rewired in parallel for your setup?

@T6Life had almost the same on my boat last summer — turned out to be partial shading I'd completely overlooked. A mooring neighbour's new bimini was catching the edge of one panel for a couple of hours mid-morning, and because the panels were in series it dragged the whole string down catastrophically. Moved to parallel wiring as a workaround.

Worth checking if the dip coincides with a specific time of day in those VictronConnect graphs @RustySkipper mentioned — if it's consistent timing rather than random, shading is a strong candidate. The 100/30 will show panel voltage vs output current in the history data which makes it fairly obvious when something's strangling the string.

MC4s are still worth inspecting too, but I'd rule out shading first — it's free to check.

@RustySkipper yes, check the VictronConnect history — specifically the "max PV voltage" for affected days. If it's showing something like 18V when it should be pushing 35–40V in series, that's a massive red flag pointing to a wiring fault rather than the MPPT itself.

@T6Life one thing I'd add beyond the MC4 check — worth measuring actual PV open-circuit voltage at the MPPT terminals with a multimeter before assuming it's controller-side. I had a similar drama in my Transit conversion; turned out one panel had a cracked junction box where the sealant had failed, which only showed up as intermittent resistance when the panel heated up. Stone cold in the morning it tested fine, hence why it baffled me for ages.

Thermal expansion making a dodgy connection worse mid-day fits your symptoms perfectly.

Great thread — I had something very similar on my 100/20 last spring and it drove me absolutely mad. What finally caught me out was the BMS on my lithium battery going into protection mode and the MPPT essentially seeing a "full" system and throttling right back. Worth checking your battery voltage in VictronConnect at the exact time you're seeing the low amps — if it's sitting suspiciously high (say 14.4V+ on a 12V system) that could be your answer.

Also @T6Life — you mentioned series wiring. What's your actual PV open-circuit voltage? Two 175W Renogys in series could be pushing 45-50V Voc depending on the model, which should be fine for a 100/30, but worth double-checking you're within spec on a cold morning when Voc creeps up.