Solarleistung kommt nicht an

Right, so I've been down this exact rabbit hole with my garden office setup and it's properly maddening — solar showing decent generation on the panels, Victron MPPT sat there looking smug, and absolutely sod-all making it through to the battery.

Turned out my problem was a combination of two things:

1. Dodgy MC4 connectors — crimped slightly off, so resistance was through the roof under load. Looked fine to the eye, measured fine with a basic multimeter, but swap them out and suddenly the MPPT woke up like it'd had three espressos.
2. Wire gauge too thin for the run length — I'd used 4mm² cable on a longer run than I should've, and the voltage drop was eating my lunch before it even reached the controller.

The VictronConnect app is your best mate here — pull up the live MPPT data and watch the panel voltage versus the output voltage. If there's a suspicious gap, something between those two points is robbing you blind.

Anyone else had similar gremlins with off-grid garden buildings? Curious whether Fogstar batteries introduce any quirks when the MPPT is undersupplying — mine seemed to get a bit moody about it.

Also worth checking: is the MPPT actually in bulk/absorb or is it sitting in float prematurely because it's misread the battery state? Seen that catch people out more than once.

What controller and battery combo are you running? Makes a difference to where you start digging.

@BoatPaddy that thread title's in German but the pain is universal 😄

Had this exact thing on my boat last summer — MPPT appeared fine but the battery voltage was barely moving. Turned out to be a corroded negative busbar connection between the MPPT and the battery bank. Looked clean visually but had significant resistance under load.

Worth checking:

• All DC negative connections — not just the obvious ones
• Fuse/breaker between MPPT and battery (some fail partially — pass a trickle but not proper current)
• Whether the MPPT is actually in Bulk or just sitting in Float thinking the batteries are full

Connect a clamp meter directly on the cable from the MPPT output and compare against what VictronConnect is reporting. If they don't match, you've found your culprit layer.

Classic "it's generating but where's it gone" mystery — nine times out of ten it's the MPPT absorption/float voltage set below your battery's actual resting voltage, so the controller thinks it's done and throttles back before it's barely started.

Check your MPPT charge profile in VictronConnect matches your battery chemistry exactly — a Fogstar Drift LiFePO4 wants 14.2V absorption, not the 14.4V AGM default that Victron sometimes ships with.

Also worth checking:

• BMS current limit being hit silently
• High-temp cutback if panels are mounted flat (common on van roofs)
• Cable voltage drop — measure at the MPPT terminals versus the battery terminals under load

Grab the VRM portal history if you've got a GX device; the yield graph will show you exactly where the power vanished.

Check your battery temp sensor isn't lying to you — my Fogstar lithium in the shepherd's hut once had the MPPT throttling itself to near-nothing because it thought the battery was roasting when it was actually 12°C on a drizzly Tuesday in November. 🐑🔋

@BoatPaddy had almost this exact thing with my garden office last spring — MPPT looked happy, batteries barely moving.

Turned out my cable run from the panels was just long enough to introduce meaningful voltage drop. Panels were "generating" fine at source but by the time it hit the MPPT the input voltage had sagged enough that it was barely doing anything useful.

Worth checking:

• Actual voltage at the MPPT terminals vs what the panels should be putting out
• Cable gauge vs your run length (there are decent voltage drop calculators online)
• Any dodgy MC4 connections mid-run — I had one that looked fine but was corroded underneath

@MarineGeoff makes a fair point about absorption settings too but I'd rule out voltage drop first, it's an easy one to miss and properly annoying when you find it.