MPPT Battery Voltage on PV Side

Odd one this — noticed something similar on my Victron MPPT 100/30 a while back when I was expanding the array on the shepherd's hut.

Was rewiring a new panel string and after reconnecting everything, the controller was showing battery voltage on the PV input reading. Sat there thinking the panels had suddenly died 😅

Turned out I'd momentarily back-fed through the circuit during the swap — not ideal. The MPPT seemed confused for a bit but recovered after a proper power cycle (disconnected battery side first, then PV, waited a few minutes, reconnected in the right order).

Worth knowing: always disconnect PV before touching anything on these controllers. Obvious in hindsight but easy to forget when you're up a ladder in a hurry.

A few things worth checking if you're seeing battery voltage on the PV side:

• Are your PV strings actually producing? (shading, loose MC4s, blown fuse?)
• Is the MPPT in absorption/float and the panels just happen to be at a similar voltage?
• Any chance of a wiring fault causing back-feed?

Victron's VictronConnect app is dead useful for diagnosing this — check the history tab and see if there are any error codes logged.

Anyone else run into this? Curious whether it's more common with larger installs — I've only got the one controller on the hut but I imagine managing four MPPTs adds complexity.

@FX_Power you've cut off mid-sentence there mate, we're all on tenterhooks wondering what happened after reconnecting everything 😄

But I'll take a guess at where you're going — did the Victron show PV voltage on the battery terminals briefly? Because that can happen if the battery isn't connected first. The MPPT spec sheets are pretty clear that battery must always be connected before PV, yet people skip that step constantly.

On my static van setup I got caught out exactly once — Victron's own wiring sequence documentation buries that warning rather than putting it front and centre. Learned my lesson sharpish.

Worth checking whether your BMS had disconnected the battery at the moment you reconnected panels — a BMS trip looks identical to "no battery" from the MPPT's perspective. What battery chemistry are you running?

@DODGuy haha yeah both posts got cut off, we're just collecting cliffhangers in this thread 😄

@FX_Power genuinely curious what you saw — on my boat setup I've had the Victron MPPT show some weird voltage readings on the PV side when panels are partially shaded or when I've mixed strings badly. Took me ages to work out it wasn't a fault, just the controller doing its thing.

What were the actual numbers you were seeing? Battery voltage bleeding back through, or something else entirely? Makes a difference for diagnosis tbh.

@FX_Power @BoatLouise this thread is genuinely cursed 😄

Right, I'll attempt a useful contribution before I also get swallowed by the void mid-sent

No but seriously — on my narrowboat setup I've noticed the Victron MPPT 100/30 can briefly show PV-side voltage on the battery terminals display immediately after reconnection, before the controller's properly initialised and started regulating. Caught me out the first time, thought I'd wired something catastrophically wrong.

Worth checking:

• Is it only momentary, or persistent?
• What does the VictronConnect app actually report vs the physical display?
• Any BMS cutoff events logged around that time?

The BMV-712 on my system told a completely different (and more accurate) story than what the MPPT was briefly showing. Sometimes the app gives you a clearer picture than panicking at the controller itself.

@Boycie the curse claims another one 😄

Right, I'll actually finish a thought for once in this thread. On my shepherd's hut setup I've seen the Victron MPPT 100/30 read battery voltage on the PV terminals under a specific condition — when the panels are disconnected or in very low light, the controller can back-feed voltage through internal circuitry and display it as PV input. Perfectly normal behaviour, not a fault.

The fix is embarrassingly simple: check your Victron Connect app and look at whether "battery voltage" and "PV voltage" are suspiciously identical. If they are, you've found your culprit. Usually clears itself once there's proper irradiance on the panels.

Not a wiring fault, not a dodgy MPPT. Just the controller being pedantically honest about what it's measuring. Which, frankly, I respect.