Anyone else noticed their MPPT readings go completely haywire in partial shade?

Running a 400W array (2x200W panels in series) on a Victron SmartSolar 100/30 into a 100Ah lithium, and I've been scratching my head at some odd behaviour lately. When the oak tree at the back of my plot starts throwing shade across the bottom corner of one panel around 2pm, the whole string just falls off a cliff — talking drops from 18A down to 3-4A even though the other panel is still in full sun. Reckon I'm losing maybe 40-50% of my afternoon generation because of it.

I've been reading a bit about the bypass diodes inside the panels doing their thing, and how series strings handle shade so badly compared to parallel. Tried switching to parallel wiring last weekend to test it — same Voc but doubled the current capacity obviously — and honestly the shading performance was noticeably better, though I've now got to keep an eye on the wire gauge handling the extra current on the run back to the controller.

Has anyone gone down the optimiser route for a small setup like this? I've seen the SolarEdge P300 and Tigo TS4 crop up in conversation but I'm not sure they're worth the cost and complexity for a single van/cabin setup. Or is the real answer just to trim that tree?

Hey @24V_Geek, classic partial shade nightmare with series strings! The problem is that one shaded cell drags down the whole series string disproportionately - your 400W array can suddenly behave like a 60W one.

Worth considering whether you could rewire to parallel instead of series if your panel Voc allows it within the 100/30's input limits. Parallel configurations handle partial shade much more gracefully since shading one panel doesn't throttle the other.

Also, have you enabled the "Panel voltage" logging in the VictronConnect app? Watching those voltage graphs during shading events is genuinely eye-opening and helps confirm what's actually happening.

Longer term, some people swear by adding bypass diodes if the panels haven't got adequate ones built in, though on modern panels that's usually not the issue. What orientation are your panels facing?

Great topic @24V_Geek! To add to what @MarineSam was getting at - this is exactly why bypass diodes exist in panels, but they only help so much with series strings.

Have you considered rewiring to parallel instead? With 2x200W panels you'd halve your Voc but the shaded panel won't drag the healthy one down nearly as badly. You'd need to check your battery voltage still falls within the MPPT's input range though.

The other option worth looking at is whether Victron's "Equalize" or shade-optimisation settings in VictronConnect are configured sensibly - sometimes tweaking the absorption voltage and scan intervals helps the algorithm find the true MPP faster when shade conditions are shifting.

Long term, panel-level optimisers like SolarEdge or even cheap eBay DC optimisers can make a surprising difference if the tree's not going anywhere! 🌳

Hey @24V_Geek, just to add something the others haven't touched on yet - have you tried logging the data through the VictronConnect app and comparing MPPT efficiency across different times of day? The SmartSolar keeps decent historical records and you might spot a pattern tied specifically to when that oak casts its shadow.

The other thing worth considering is whether switching those two panels to parallel rather than series might help your situation. You'd drop your Voc but the shade impact becomes far less dramatic since one struggling panel won't pull the other down. At 24V nominal you should still be fine going into a 100Ah lithium with that controller.

What's your battery voltage - 12V or 24V? That'd help figure out whether parallel wiring is actually viable for you. 🙂

@24V_Geek I ran into exactly this on my boat last summer — partial shading from the mast was killing my output in a similar way. Switched from series to parallel on my two panels and the improvement was night and day. Yes, you need heavier cable to handle the lower voltage/higher current, but partial shade tolerance is dramatically better.

Also worth knowing: Victron's MPPT algorithm does struggle a bit with multi-peak power curves that shade creates in series strings. Some people swear by the Renogy Rover for shaded installs purely because its scanning algorithm is slightly more aggressive, though I'd be reluctant to ditch a SmartSolar personally.

If rewiring isn't feasible, @JakeDavies is right about the logging — check your VRM portal history for the voltage dips and you'll see exactly which periods the shading hits hardest.

Good shout from @JakeDavies on the VictronConnect logging — worth doing.

One thing nobody's mentioned yet: panel orientation within your series string matters a lot with partial shade. If the shaded panel is your "upstream" one, it can drag down the entire string disproportionately compared to if it's downstream. Not always predictable.

Also worth checking your VOC in shade conditions via the MPPT history. If it's dropping below the controller's minimum operating voltage, the 100/30 may be briefly disconnecting and reconnecting, which shows as erratic readings rather than smooth reduced output.

I had something similar on my shepherd's hut setup — turned out splitting into parallel rather than series (with appropriate wire sizing) made a significant difference once I had regular partial shading. Loses some efficiency in ideal conditions but far more predictable behaviour when things get messy.

Slightly different angle here — I've got a static caravan setup with panels that get partial shade from a neighbour's tree in the afternoon, and I ended up splitting my array into two separate strings, each on its own MPPT. Night and day difference.

I know that's not always practical depending on your controller, but it completely eliminated the "one shaded cell drags everything down" problem.

Also worth asking: are your bypass diodes actually functioning? Had a duff diode on one of my panels that made partial shade far worse than it should've been. @CrispyTrekker's point on orientation might be linked to this too — if certain cells are consistently hitting shade at the same angle, a failed diode would absolutely cause erratic MPPT hunting.

Has anyone tested their diodes directly? Curious whether it's a common issue or just my bad luck with budget panels.