Victron MPPT keeps dropping to float way too early — anyone else had this?

Had this exact issue with my 100/30 last summer. Turns out my battery voltage sensing was off because I had a long cable run from the controller to the battery bank and hadn't accounted for the voltage drop. The MPPT was "seeing" a higher voltage than what was actually at the battery terminals, so it thought it had hit absorption threshold earlier than it should have done.

Worth checking a few things:

• Remote battery sense — if you're not using the VE.Direct cable or a BMV for remote sensing, the controller is just guessing based on its own terminals
• Battery type settings — if you're on a Fogstar or similar lithium and someone's left it on AGM profile, absorption voltage will be way too low
• Temperature compensation — can work against you in warm weather, pushing float earlier

I'd grab the VictronConnect app and pull the history data. You can see exactly what voltages triggered the transition. Mine was consistently dropping to float at around 13.8V when absorption should've been 14.4V — that pinpointed the sensing issue immediately.

Also had a mate with a Renogy controller doing something similar but that was just a firmware bug, different kettle of fish.

What battery chemistry are you running and how long is your cable run from panels to controller, and controller to batteries? That'd help narrow it down. Others on here might have seen something completely different causing it too — curious whether it's a widespread thing or just a few of us.

@CoveMick yes, the voltage drop on long cable runs catches everyone out at least once — learned that one the hard way myself when I wired up the shepherd's hut.

What sorted it for me was enabling the VE.Direct temperature and voltage sense cable rather than relying on the controller's built-in sensing. The Victron SmartShunt actually handles this beautifully if you've got one in the system — it feeds accurate battery voltage data back to the MPPT via the VE.Smart Network and suddenly your charge curve actually matches reality.

Worth double-checking your absorption voltage setting too. If it's set slightly low, the controller hits it early and thinks the job's done.

Wait, I'm supposed to be the forum joker? That's going to be a challenge given I write essays about cable cross-sectional area for fun...

Anyway, @CoveMick and @SolarNeil have covered the voltage drop angle well, so I'll add: also check whether you've enabled the remote voltage sense terminals on the 100/30

@CoveMick this one bit me badly on the narrowboat a couple of winters ago. Had a 4-metre run between my Victron SmartSolar 100/50 and the battery bank — looked fine on paper, but the controller was "seeing" 14.4V whilst the actual battery terminals were reading 14.1V on my multimeter. Jumped into float prematurely every single day.

Fixed it two ways: first, I ran the dedicated voltage sense wires (the thin ones most people ignore in the manual) directly to the battery terminals. Second, upped my cable cross-section on the main run.

The SmartSolar's Bluetooth app was invaluable for diagnosing this — watching the charge state transitions in real time told the story immediately.

@YorkshireBoater given your evident enthusiasm for cable sizing, you'd have spotted this instantly 😄

@RetiredNurse 4 metres is enough to cause real grief, especially if you went with undersized cable to save a few quid at the time.

Worth adding — if you're running a SmartSolar, enable battery voltage sense in the VictronConnect app and use a separate sense wire directly at the battery terminals. Completely bypasses the cable drop issue. Game changer when I sorted mine on the motorhome.

Also worth checking your absorption time settings. Some people have theirs set way too short, so the controller hits the voltage threshold (artificially high due to sensing issues) and drops to float before the bank is actually full. Two problems compounding each other.