Victron 100/50 only charging 40 watts

Seen a similar head-scratcher with my own SmartSolar 100/30 last summer — worth digging into because there are a few classic culprits here.

If you're getting 21V at the PV terminals but barely any watts going in, the first thing I'd check is whether your panels are wired in series or parallel. Two panels at 21V each wired in series should give you ~42V open circuit — so if you're only seeing 21V at the controller, one panel may effectively be dragging the other down, possibly a shading issue or a duff bypass diode.

Also worth checking: what's your battery voltage doing? If the bank is already sitting at absorption voltage (say 14.4V on a 12V system), the MPPT will naturally throttle back. People often panic thinking the controller is broken when it's actually just doing its job. Pop open VictronConnect and look at the state — is it in Bulk, Absorption, or Float?

The other thing I've seen cause this on UK installs is undersized cabling creating voltage drop between the panels and the controller. Measure VOC at the panels themselves, then again at the controller terminals. Any meaningful difference and you've found your problem.

What battery chemistry are you running? LiFePO4 and lead-acid behave very differently and the charge profile matters a lot here. If someone's set it up on the default AGM settings with lithium cells, you'd see some odd limiting behaviour.

Keen to hear what VictronConnect is actually reporting — screenshot of the live data would tell us a lot. Anyone else had this on a caravan or motorhome setup?

@WheresMeWires classic symptom of the battery already being near full and Victron throttling back — check your absorption voltage in the app, bet it's sitting pretty at 14.4V doing absolutely nothing dramatic.

Also worth checking if your panels are wired in series vs parallel — 21V input on a 100/50 with a 12V battery is a bit sus, you want closer to 30-40V input to give the MPPT something to actually chew on.

My garden office setup had this exact faff until I rewired two panels in series and suddenly the Victron woke up like it'd had three espressos. 🔋☀️

Chuck a screenshot of your VictronConnect history graph in here — the bulk/absorption/float breakdown tells the whole story without any guesswork.

@VoltBarry makes a fair point but if it's early morning and the battery's at 50% that theory goes out the window — more likely your PV cable run is thicker than your ambition but thinner than it should be, causing voltage drop before it even reaches the controller. Also worth checking: are both panels wired in series or parallel? 21V open-circuit from a single 12V nominal panel is normal, but two in parallel gives you the same voltage with double the current — and if your wiring can't handle it, Watts disappear faster than a builder's tea. Pop the Victron Connect app on and check the actual charge state history.

@MarineGeoff had a similar head-scratcher with my cabin setup — turned out my Renogy panel junction box was half-melted and doing a convincing impression of a resistor.

Check your MC4 connectors before going down any rabbit holes; a dodgy crimp will rob you blind and still show "fine" voltage at the terminals because you're measuring with no load.

Victron Connect's history tab will show your peak watts — if that number looks healthy from a previous day, the panels are innocent and you're hunting a connection gremlin.

@SolarNut that melted junction box story rings very true — had almost the exact same thing on my boat last season. Panels were showing reasonable open-circuit voltage but the moment load kicked in, watts collapsed completely.

Worth checking one thing nobody's mentioned yet: partial shading on a single cell. My setup has a Fogstar lithium bank and I noticed my 100/50 behaving oddly one afternoon — traced it back to a bit of bird mess covering maybe 5% of one panel. Because of how series strings work, that tiny shadow was dragging the whole array down disproportionately.

Give each panel a proper visual once-over before pulling connectors apart. Sometimes the answer's literally right there on the glass.