Victron MPPT keeps dropping out in the afternoon — shading issue or something else?

Had the exact same drama on my narrowboat last summer — turned out my panels were getting a sneaky shadow from a bridge I'd moored under for three days without realising, felt like a proper idiot 🙄

A few things worth checking though:

• Temperature derating — afternoon sun bakes the controller itself, and if your Victron MPPT isn't got decent airflow it'll throttle back to protect itself. Mine was mounted inside a poorly ventilated box and was basically having a little lie-down every afternoon at 2pm.
• Battery voltage ceiling — if your bank is nearly full by afternoon (lucky you), the MPPT will naturally back off absorption/float. Looks like a dropout but it's just doing its job.
• PV voltage spikes — on a hot day, weirdly, VOC can shift enough to cause fits if you're right on the edge of your controller's input range.

What does the VictronConnect history actually show? The daily graphs are genuinely useful here — if you see a clean cliff-edge drop at the same time each day, shading is almost certainly your culprit. If it's erratic, I'd lean toward thermals or a dodgy MC4 connection.

Also worth checking whether anyone else on here with a shepherds hut setup has seen this — fixed roof installs seem to cop afternoon shading from tree lines more than you'd think.

What's your panel orientation and what MPPT model are you running?

Reply by LutonLife:

@AmyChapman89 Ha, don't feel bad about that — shading gremlins are notoriously sneaky! Worth mentioning for anyone troubleshooting this: even partial shading on just one cell can drag down the whole string dramatically, especially if you haven't got bypass diodes doing their job properly.

Before assuming it's shading though, I'd check whether the dropout correlates with panel temperature — afternoon sun means panels are running hot, and if they're poorly ventilated (lying flat on a roof with no airflow underneath) you'll lose a surprising chunk of output to thermal derating. Victron's VictronConnect app will show you the actual figures if you dig into the history graphs. What's the MPPT model, OP? Some of the older units had firmware quirks that caused similar behaviour.

@AmyChapman89 That bridge story is painfully relatable — I spent nearly a week chasing what I thought was a dodgy Victron SmartSolar before realising my rooftop panel array was getting clipped every afternoon by my own roof vent housing. Classic case of RTFD (Read The Flipping Data).

One thing worth checking that nobody's mentioned — pull up the Victron Connect app and look at your historical yield graphs. If it's dropping out at roughly the same time each afternoon, that's almost certainly a consistent shadow source rather than a controller fault. Consistent pattern = external obstruction. Random dropouts = more likely a wiring or temperature issue. The MPPT itself is rarely the culprit in my experience.

@AmyChapman89 had something similar with my shepherd's hut — afternoon dropouts that drove me mental for about a fortnight. Ended up being a nearby tree I'd completely overlooked because it only cast a shadow after about 2pm when the sun shifted west.

Worth checking your VOC at the controller during the dropout — if it's spiking oddly rather than just dropping, could be a partial shade issue triggering the MPPT to hunt. My Victron SmartSolar logs are brilliant for this, you can pull the history and see exactly what it was doing hour by hour.

Also double-check your MC4 connections if you haven't already — had a dodgy crimp cause similar symptoms once.

@AmyChapman89 afternoon dropouts specifically are almost always partial shading — the fact it's time-specific is the giveaway. I had this with my home setup where a neighbour's new fence post was catching the bottom edge of one panel from about 2pm onwards. Took me ages to spot because it was only a sliver of shadow.

Worth checking your VRM portal if you've got it connected — the yield graphs will show you exactly when it's dipping and by how much. If it's a consistent cliff-edge drop at the same time each day, that's shade rather than a controller fault.

Also, are your panels wired in series or parallel? A single shaded cell in a series string will hammer the whole array output far more than people expect.

@Mark1978 is right but also check your temperature — my static caravan panels on a south-facing roof hit 65°C+ on a hot afternoon and the Victron throttles back like it's having a nap, took me ages to figure out it wasn't shading at all but just the controller doing exactly what it's supposed to do, which made me feel like an idiot for swearing at it for three weeks straight.