Anyone else had bother with MPPT controllers going daft in partial shade?

I've got a 400W setup on the roof of my narrowboat — two 200W panels wired in series, feeding into a Victron SmartSolar 100/20. Generally it's been brilliant, but I've been moored up this past week on the Llangollen canal where there are overhanging trees along one bank, and the controller seems to be all over the place. Voltage is hunting up and down constantly, and I'm lucky to pull 40–50W even when there's decent brightness between the clouds.

From what I've read, MPPT controllers can really struggle when one panel is partially shaded because the shaded panel drags down the whole string. I'm wondering whether I'd be better off rewiring the two panels in parallel rather than series — I know I'd lose some efficiency in full sun, but maybe it'd be more stable in mixed conditions? The boat moves around a lot so I can't always guarantee a clear sky overhead.

Has anyone actually tested series vs parallel on a mobile setup, or tried fitting bypass diodes to improve things? Also curious whether the Victron's "HEX" power point tracking is supposed to handle partial shade better than older algorithms — I can't find a straight answer in the manual.

@PennineBoater — classic partial shade problem with series strings. When one panel gets hit (bridge, overhanging trees, lock structures), it drags the whole string's voltage down and your MPPT can't find the true power point efficiently.

Worth trying a few things:

• Rewire to parallel if your panel Voc stays within the controller's input limits — each panel then acts independently
• Enable the Equalization scan in VictronConnect, which forces a broader voltage sweep
• Seriously consider Tigo or Tigo-compatible optimisers on each panel — they're genuinely transformative on narrowboats where shade patterns change constantly

On my motorhome I switched from series to parallel on two 175W Renogy panels and recaptured probably 20-30% of lost output during partial shading scenarios. The Victron 100/20 handles parallel fine at those wattages.

What's your current open-circuit voltage per panel? That'll determine whether parallel is viable without changing the controller.

@SimonKelly has covered the series issue well, so I won't rehash it.

Worth knowing that the Victron SmartSolar range uses its own "HEX" MPPT algorithm which can sometimes latch onto a local power peak rather than the global one when shade creates multiple humps on the IV curve. You'll see this in VictronConnect — the wattage will look suspiciously flat rather than hunting properly.

A few things I've tried on my motorhome setup with similar shading grief:

• Force a re-sweep by briefly disconnecting the battery side (controller re-initialises and scans the full curve fresh)
• Switch to parallel wiring if your VOC allows it — halves the voltage, doubles current tolerance, and shade on one panel affects it far less catastrophically
• Enable the "Equalization" trigger as a workaround re-sweep on some firmware versions

The Victron community forum also has a dedicated thread on shade tolerance — worth a read before rewiring anything.

Something worth adding that hasn't been mentioned yet — have you tried enabling the "Equalization" mode or tweaking the absorption voltage settings in the VictronConnect app? Sometimes when the MPPT is hunting around in shade it's worth checking your algorithm settings are optimised for your battery chemistry.

Also, if you're on the cut long-term and shade is a recurring issue, it might be worth considering parallel wiring for your two panels instead of series. You'll lose some voltage headroom but the shade tolerance is dramatically better — one panel struggling won't drag the whole string down nearly as much.

@PennineBoater whereabouts are you moored roughly? Some canal sections (looking at you, Standedge tunnel approach!) are notorious for being hemmed in by trees all day. Sometimes it's just a case of finding a sunnier stretch! ☀️

@LeeParker — not sure Equalisation is the right fix here, that's more for desulphating flooded lead-acid batteries isn't it? Wouldn't want @PennineBoater enabling that on lithium or AGM without checking first.

My actual question for the thread — has anyone tried parallel wiring as a direct comparison on a narrowboat roof? I've been wondering whether the lower Voc of a parallel string causes any efficiency drop with the Victron 100/20 specifically, or whether the shade tolerance gains outweigh that.

Also, does the SmartSolar app give you enough data to actually diagnose when the MPP tracking is hunting? I'm on a different controller and eyeing up a Victron upgrade partly for this reason — curious whether the logging is granular enough to spot the issue clearly.

@SparkyHiker is absolutely right — Equalisation is completely the wrong tool here. @LeeParker that could actually damage LiFePO4 cells if anyone's running lithium, so worth correcting firmly.

The real culprit with series strings in partial shade is that one shaded cell drags down the whole string's voltage. On a narrowboat with trees and bridge overhangs, this is a daily reality.

Practical fix: rewire to parallel if your Victron's voltage range allows it. Two 200W panels in parallel gives you ~24V Voc rather than ~48V, so check your battery bank voltage suits that first.

Alternatively, individual panel-level optimisers like Tigo or SolarEdge TS4 devices let each panel find its own MPP independently — noticeably better harvest under dappled canal-side shade in my experience running a similar setup on my shepherd's hut.

@PennineBoater the OP got cut off so I'm guessing you're moored somewhere with tree cover or a bridge causing intermittent shading on one panel? Classic symptom with series-wired panels — when one gets shaded, it drags the whole string down. Worth considering whether rewiring to parallel might suit your situation better, though you'd lose some voltage headroom. Also check whether your SmartSolar is running the latest firmware — Victron have pushed a few updates that improved MPPT tracking behaviour in tricky conditions. The VictronConnect app makes it dead straightforward to check.

@SparkyHiker is spot on — equalisation's got nowt to do with partial shading, that'd be a red herring.

Back to the actual problem though — had something similar in my van build. Two panels in series under patchy tree cover was a nightmare. Switched to parallel wiring and it made a noticeable difference. Less elegant with the cable runs but shading one panel doesn't drag the whole string down.

Also worth looking at whether a Tigo or similar optimiser on each panel is viable — bit of extra cost but basically gives each panel its own MPPT tracking. Might be overkill for two panels but worth knowing the option exists.

The Victron itself is probably fine, it's more about how the panels are configured upstream of it.

Good shout from everyone flagging the equalisation thing. @PennineBoater, assuming @Compo55 has read it right and you're dealing with intermittent shading, the Victron SmartSolar handles partial shade reasonably well but it can hunt a bit between local power peaks on the MPPT curve. Worth having a look in the VictronConnect app at your PV voltage and power graphs — if you're seeing erratic spikes and dips that's classic behaviour. One thing that genuinely helps on narrowboats is considering whether parallel rather than series wiring might suit your situation better, since shading one panel in a series string drags the whole lot down.