Possible Panel Failure - MPPT Effect on Panel Output

So I've been down a bit of a rabbit hole this week and wondered if anyone else has had a similar head-scratcher with their setup...

My van's roof-mounted 200W array (two 100W panels wired in series) has started behaving oddly. The Victron SmartSolar is reporting perfectly reasonable numbers on paper, but my 200Ah Fogstar lithium is barely gaining charge on days that should be decent — even accounting for the typical British "sunshine" 😅

Here's what's confusing me: does a partially failing panel actually drag down the whole MPPT's performance, or does the controller compensate reasonably well? I know with series wiring, shading on one panel kills the whole string, but I'm wondering if a degrading panel behaves similarly — essentially bottlenecking the output even when there's no obvious physical damage.

I've checked connections, cleaned the panels, and the Victron app shows the MPPT hunting around the operating point more than usual. That feels suspicious to me.

A few questions for the more experienced folks here:

• Is there a reliable way to test individual panel output without specialist kit?
• Would switching to parallel wiring expose the duff panel more clearly?
• At what point do you just bite the bullet and replace rather than diagnose forever?

I'm half convinced one panel is on its way out, but equally it could be something boring like a dodgy MC4 connector doing its best impression of a resistor.

Anyone had a panel quietly degrade rather than fail outright? What were the signs, and how did you eventually pin it down?

Hey @PeakCamper, interesting one! Just to flag something worth checking before you go too far down the fault-finding rabbit hole — when your MPPT controller is actively working, you should see the panel voltage pulled down from its open-circuit Voc to somewhere around the maximum power point voltage (Vmp). So if your two 100W panels in series have a combined Voc of say 44V, you might only measure 36-38V at the controller input under load. That's completely normal behaviour, not a fault!

What readings are you actually seeing, and at what point in the circuit are you measuring? Would help narrow things down considerably. Also worth noting what the solar irradiance was like at the time — a cloudy day can make perfectly healthy panels look suspiciously underperforming. 😄

Had this exact issue with my shepherds hut setup last summer. One panel in a series string dragging the whole array down is a classic symptom.

Worth checking whether your Victron (or whatever MPPT you're running) is logging anything useful — the VictronConnect history graphs can show you if the array voltage is dropping unexpectedly at certain times.

Also, grab a multimeter and test each panel individually in decent sun. Voc and Isc readings will tell you pretty quickly if one's underperforming. A healthy 100W panel should give you ~22V open circuit and ~5-6A short circuit roughly speaking.

Shading is an obvious culprit too — even a tiny shadow on a series string hammers output badly.

Hey @PeakCamper, welcome to the rabbit hole! 😄 Worth grabbing a multimeter and testing each panel individually under load — disconnect from the MPPT and measure open-circuit voltage (Voc) and short-circuit current (Isc) on each panel separately on a clear day. If one panel's Isc is significantly lower than its rated spec, that's your culprit. Could be cell degradation, delamination, or even micro-cracks from road vibration — vans are particularly prone to that compared to static installs. Also check your bypass diodes if you suspect shading is triggering them unexpectedly. @TracyKnight is spot on about series strings — one weak panel really does drag everything down disproportionately. What readings are you actually seeing from your MPPT display? Knowing your actual vs expected wattage would help us narrow it down further.

@PeakCamper one thing nobody's mentioned yet — shade or soiling on a single cell in series will current-limit the entire string, so before you start swapping hardware, get up there with a cloth and check for bird deposits, because I once lost 40% output on my garden office array to one pigeon's breakfast.

Also worth pulling the Victron MPPT history logs if you've got a SmartSolar — the yield graph will show you exactly when the drop started, which narrows down whether it's a sudden failure (cell crack, diode gone) versus a gradual degradation issue.

Bypass diodes failing in a Renogy panel is not unheard of either — they'll let current flow one way fine but kill your Voc asymmetrically, so measure each panel's open-circuit voltage individually in full sun before condemning the controller.