SmartSolar MPPT 150/70-Tr low PV voltage when connected

Had something weirdly similar with my Victron SmartSolar 100/30 last autumn — brilliant sunny day, panels clearly doing their thing, but the MPPT was reporting embarrassingly low PV voltage through the app. Like, laughably low.

Measured across the disconnected cables at the controller input and got proper voltage. Reconnect them and it just... collapsed. Took me ages to work out what was happening.

Few things worth checking (in rough order of "most likely to least annoying"):

• Wiring gauge — undersized cable will absolutely cause voltage drop under load. What are you running between array and controller?
• MC4 connectors — one of mine had a slightly mis-crimped pin. Looked fine visually, measured fine with no load, fell apart under current draw
• Array wiring — is your 36V array actually wired correctly? Worth double-checking series vs parallel at the junction box end, not just at the controller
• The controller itself — less likely but not impossible

The 150/70-Tr is a chunky bit of kit so I'd be surprised if it's a controller fault, but stranger things have happened.

What's your cable run length? Long runs with marginal cable gauge can behave exactly like this — fine open-circuit, terrible under load.

Also — are you using Fogstar or similar cells, or proper rigid panels? Some of the cheaper flexible panels I've seen have absolutely rotten internal connections that fail the same way.

Anyone else seen voltage collapse like this specifically on the 150/70? Curious whether it's more common than I think.

@EcoFlow_Gal classic symptom of reverse polarity on one string or a dodgy MC4 connection causing partial short — Victron's bulk LED pattern usually gives it away if you know the blink codes.

Also worth checking: if your panels are wired in series and one is heavily shaded (even a bird dropping), the MPPT sees the weakest link and reports the string voltage accordingly — looks daft on a sunny day but that's exactly what's happening.

On my static caravan setup with a Victron SmartSolar 150/35, I spent a miserable afternoon convinced the controller was faulty before realising one MC4 female connector hadn't fully clicked home — voltage drop was staggering.

Run the VictronConnect app and check the PV voltage vs battery voltage ratio in bulk mode; tells you everything.

@Rob1963 has covered the partial short angle well, but worth also checking whether the 150/70-Tr is seeing the panels in full shade briefly at startup — the MPPT algorithm on Victron units can sometimes latch onto a local power point maximum rather than the global one, especially on partially shaded arrays.

Had this exact drama with my garden office setup last winter; turned out one of my Renogy panels had a delaminating bypass diode that was conducting when it shouldn't. PV voltage looked plausible but utterly wrong.

Quick diagnostic: disconnect everything, measure each string's open-circuit voltage with a multimeter in bright conditions before connecting to the controller. Compare against your panel spec's Voc × number of panels. Any string reading significantly low is your culprit. VictronConnect's history tab showing consistently depressed Vmax is the smoking gun here.

@TracyAllen raises a good point about configuration — worth adding that the 150/70-Tr has a 150V open-circuit limit, so if you've strung panels in series and one module has a shading or bypass diode issue, the healthy panels can actually mask the fault voltage-wise whilst dragging down the whole string's output. Classic "looks fine from outside, reads rubbish inside" scenario.

Also check your bulk/absorption setpoints in VictronConnect — I've seen mismatched battery voltage settings cause the MPPT to throttle itself right back, making PV voltage appear artificially low because it's deliberately not pulling power. Ran into exactly this when commissioning my cabin array last spring; had it set to 24V profile on a 12V bank like a complete muppet. Easy fix once you spot it.

Run the history tab in VictronConnect and check your "max PV voltage" figure — that'll tell you immediately whether the panels themselves are healthy.

Great points from @Rob1963 and @DailySolar already. One thing nobody's mentioned yet — have you checked your MC4 connectors for moisture ingress? I had a very similar head-scratcher last spring and it turned out rainwater had worked its way into a connector junction that looked perfectly fine externally. The resistance was enough to drag the reported PV voltage right down under load while the panels themselves were fine.

Worth grabbing a multimeter and measuring voltage directly at the controller's PV terminals versus at the panel end — if there's a meaningful difference, you've got resistance somewhere in the cabling or connectors. Also check your cable run for any pinch points or damage, especially if it passes through any roof penetrations. Victron Connect should show you the PV voltage history too, which might reveal whether it's intermittent or constant.