Chute tension panneau

Noticed something on my motorhome setup recently that had me scratching my head for a good while, so thought I'd throw it out here.

I've got a Victron SmartSolar MPPT 100/20 paired with a 100W panel (Voc around 22V) and a Fogstar 12V 100Ah LiFePO₄ battery. Everything looked right on paper, but the moment I connected the panel, the voltage was collapsing — dropping right down to sit almost at battery voltage rather than holding anywhere near what I'd expect from open circuit.

The weird bit? As soon as I plugged in a second small load or another panel in parallel, it all settled down and behaved perfectly normally. Classic case of the MPPT not quite "waking up" properly on its own, I reckon — something about the impedance not being quite right for the controller to latch onto.

I've read bits and pieces suggesting this can happen when the panel wattage is quite low relative to the controller's minimum operating threshold, or when the LiFePO₄ is sitting at a high state of charge and presenting too little voltage differential to kick things off.

Has anyone else on here run into this with smaller panel setups on a motorhome or campervan? Particularly curious whether it's a wiring length issue causing voltage drop before the controller even gets a chance to do its thing, or whether it's genuinely a quirk of how the Victron firmware handles cold starts with lithium.

Would love to know if there's a setting in VictronConnect I've missed, or whether a dummy load on startup is the accepted workaround. Seems daft if so!

Reply by Scouse16:

@MarineVicky Looks like your post got cut off there - "100W pa..." - panel? Presumably you were about to describe the actual symptoms you're seeing?

That said, "chute tension panneau" is French for panel voltage drop, so I'm guessing you might be seeing your panel voltage sagging under load? Really common on motorhome installs, especially if the cable run between panel and controller is longer than it looks once you account for routing around the roof edges.

First thing I'd check is your cable gauge and the actual measured length of the run. With a 100W panel you want to keep voltage drop under 3% ideally. What diameter cable are you using and roughly how far is it from panel to MPPT?

Also worth checking your MC4 connectors for any corrosion or a loose crimp - they're notorious for causing intermittent voltage drops. 👍

Yeah @MarineVicky your post got truncated so we're all guessing here, but assuming it's a standard 100W panel (Renogy or similar), voltage drop on the cable run is the first thing I'd check before anything else.

What's the distance between your panel and the MPPT? Even a few metres of undersized cable can cause a noticeable drop in Voc readings versus what's printed on the panel spec sheet. The SmartSolar app will show you your actual open-circuit figures if you disconnect the battery side temporarily.

Also worth checking — is this a reading discrepancy you're seeing in the VictronConnect app, or are you actually measuring it with a meter at the panel terminals? Makes a big difference to how you'd diagnose it.

While we wait for @MarineVicky to finish that sentence, "chute tension panneau" is French for panel voltage drop — classic symptom of undersized cable runs on a motorhome roof where you've got a decent length between panel and MPPT.

On my shepherd's hut I learned the hard way: even a couple of metres of 2.5mm² cable causes enough voltage drop under load to confuse the SmartSolar's tracking algorithm. Victron's own cable sizing tool is worth five minutes of your time.

Quick checklist until @MarineVicky clarifies:

• Check cable cross-section — 4mm² minimum for most 100W setups
• Inspect MC4 connectors for corrosion
• VRM or Bluetooth app will show actual panel voltage vs expected Voc

Bet it's the cable. It's always the cable.

@MarineGeoff cheers for the translation — my French extends to "croissant" and mild panic at French toll booths, so that's genuinely useful 🥐

@MarineGeoff useful to know the translation cheers — seen this crop up on French forums when I was researching my shepherd's hut build.

Common culprits for voltage drop on a 100/20 setup:

• Undersized cable run — even a metre or two matters more than people realise
• Dodgy MC4 connectors — had a nightmare with some no-brand ones before I switched to proper Victron-spec
• Panel junction box connection going bad

The SmartSolar app should show you input vs output figures pretty clearly. Worth checking your Voc at the panel terminals directly vs what the controller's actually seeing. Big gap there tells you it's a wiring/connection issue rather than the panel itself.

Waiting for @MarineVicky to repost the full message but betting it's cable gauge given it's a motorhome install where runs can get awkward.