Mppt100/30 troubleshooting

Been scratching my head with my MPPT 100/30 for the past couple of days and wondering if anyone's had similar.

I've got a pair of 200W panels wired in series giving me around 72V open circuit, feeding into the controller. Before making the final PV connection I always check voltage at the terminals first — good habit I picked up from this forum actually.

Here's where it gets odd. With my multimeter negative probe on the Victron's PV negative terminal and positive probe on the incoming PV positive cable (not yet connected), I'm reading 36-37V rather than the ~72V I'd expect. Panels are definitely getting decent light, and I've confirmed the voltage reads correctly when measured directly at the panel junction box.

My setup:

• 2x 200W mono panels, series wired
• Victron MPPT 100/30
• 2x 100Ah Fogstar Drift LiFePO4 in parallel
• Battery voltage sitting at around 26.4V

A few things I'm wondering:

1. Could there be some kind of back-voltage from the battery side influencing the reading at the PV terminals before connection?
2. Is this a grounding issue with how I'm taking the measurement?
3. Or is the controller itself doing something funny?

The system does seem to be charging once everything's connected — VictronConnect shows it working — but I'd rather understand what's happening before assuming it's fine. Feels like the kind of thing that could indicate a wiring gremlin lurking somewhere.

Anyone else noticed unusual pre-connection voltage readings on their Victron controllers? Is this actually normal behaviour I've just not come across before?

@LazyBodger that 72V OC is absolutely fine for the 100/30 (rated to 100V input), so that's not your issue.

Classic gotcha with series strings into that controller though — what's your actual battery voltage and what absorption/float targets have you set in VictronConnect? If you've accidentally left it on the default lithium profile when you're running AGM (or vice versa), it'll behave really oddly.

Also worth checking:

• Wire gauge between controller and battery — undersized cable causes voltage drop that confuses the MPPT algorithm
• Temperature compensation settings if you've got the temp sensor fitted
• Whether you're seeing the issue only on partial shading days (series strings hate uneven shading)

What's the actual symptom you're seeing? Bulk not completing? Weird absorption times? Controller showing one thing, battery showing another? The more detail the better — this rabbit hole goes deep. 🐇

Went through almost exactly this with my shepherd's hut build last autumn. After ruling out all the obvious stuff, mine turned out to be a dodgy MC4 connector — looked perfectly fine on the outside but had a slightly oxidised pin inside causing intermittent resistance. Swapped it out and suddenly my absorption voltages were behaving properly again.

Worth checking your battery temperature compensation settings too if you're running lead-acid. The 100/30 can act strangely if it thinks the battery is warmer than it actually is and starts pulling absorption voltage down prematurely.

Grab VictronConnect and watch a full charge cycle in real time — the history tab will show you exactly where it's dropping off. That live data view saved me hours of guesswork on the hut.

Worth checking your battery temperature sensor if you've got one wired in — mine was giving daft readings and the MPPT was pulling back absorption voltage thinking the battery was roasting when it wasn't. Pulled the sensor off, let it default to 25°C compensation, problem sorted.

Also, what's your cable run from panels to controller? Long thin cables will drop your actual input voltage more than you'd expect, especially on cloudy days when the panels are already struggling. I had 4mm² over about 6 metres in my shepherd's hut and was losing more than I'd calculated.

What does the Victron Connect app actually show? Specifically interested in whether it's going into absorption at all, or just sitting in bulk indefinitely. That narrows it down considerably.

My Fogstar 12V lithium laughed at me for three weeks before I realised the BMS was quietly disconnecting under charge and the MPPT was just sat there having an existential crisis — worth ruling that out before you go mental checking cables.

Good shout from @BrookLover there — BMS disconnects are a sneaky one that catches loads of people out.

One thing nobody's mentioned yet: check your actual cable losses between the MPPT and battery. If your cabling's undersized or you've got a dodgy connection, the controller can think it's hitting absorption voltage at the terminals whilst the battery's actually seeing significantly less. The MPPT then backs off too early and your battery never properly charges.

Grab a multimeter and measure voltage simultaneously at the controller output terminals and at the battery terminals under charge. Any more than about 0.3V difference and you've found your culprit. Also worth giving every connection a proper wiggle and checking for any green corrosion on crimps — especially if any of it's been exposed to moisture.

What gauge cable are you running between the controller and battery, and what's the run length?