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

Had exactly this on my narrowboat last summer. The Victron SmartSolar 100/30 kept pushing into float around 1pm on decent days, with the batteries nowhere near properly full. Drove me mad for about a fortnight before I tracked it down.

Turned out my absorption voltage was set correctly (14.4V for my Fogstar lithium drop-ins) but the absorption time was set far too short — I'd accidentally left it on the default "adaptive" setting which was clearly miscalculating based on some early-morning cloud cover and deciding the batteries were more charged than they actually were.

A few things worth checking in VictronConnect:

• Absorption duration — adaptive vs fixed, and what your fixed time is set to
• Tail current setting — if this is too high, the controller decides absorption is "done" and drops to float prematurely
• Battery capacity entered — if it's undersized, the algorithm gets confused early

On my static caravan setup I actually switched to fixed absorption time (around 2 hours) and the problem completely vanished. Less elegant than adaptive, but far more reliable for my use case.

One other thing — check whether your battery temperature sensor is reading correctly. A false high temperature reading will cause the controller to back off charging voltage, which can trigger an early float transition.

Worth posting your VictronConnect screenshots if you can — the history graph often tells the whole story immediately. Has anyone else found the adaptive algorithm behaving oddly specifically on partly-cloudy days? I wonder if it's a known quirk or just something in my setup.

@RetiredNurse had this exact issue on my cabin setup — turned out my absorption voltage was set too low. Worth checking your battery profile in the VictronConnect app. If you're on lithium (Fogstar Drift cells in my case) the defaults can be way off.

Also, what's your absorption time set to? There's a "tail current" setting that tells the MPPT to exit absorption early if charge current drops below a threshold — could be kicking in prematurely if the value is too aggressive.

One other thing — are your battery voltage sense leads connected properly? If the MPPT is only reading panel-side voltage it can think batteries are fuller than they are.

@RetiredNurse classic one — also check your absorption time in the settings, because Victron's adaptive algorithm will cut it short if it thinks the battery's full based on tail current, and if your tail current threshold is set too high it'll bail out early like a teenager at a family dinner 🙄

On my shepherd's hut setup I had to drop the tail current from the default down to around 1–2% of battery capacity (C) before it started doing proper full absorptions — made a massive difference to my Fogstar cells staying genuinely topped up.

Also worth checking: are you on the BatteryLife algorithm? That deliberately limits charge on smaller banks. Disable it if you're not on a critical load scenario and watch it behave properly.

Something else worth looking into @RetiredNurse — check whether your battery temperature sensor is reading correctly, or if you're even running one. If the Victron thinks the batteries are warmer than they actually are, it'll shorten the absorption phase considerably. On a narrowboat the sensor placement matters a lot; tucked too close to the engine bay and you'll get skewed readings all day.

Also worth connecting via the VictorConnect app and checking your charge history — there's a handy graph showing each phase duration. If absorption is consistently cutting short, that'll confirm it's not just a one-off. Once you've ruled out what @AlexJones and @BoatPaddy have mentioned, the temperature compensation settings are usually the next culprit in my experience.

Welcome to the forum @RetiredNurse — good first post, straight to the point.

Others have covered the obvious culprits, so I'll add one more: check your tail current setting. Victron uses this to decide when absorption is "done" — it's the current threshold below which the MPPT decides batteries are full and drops to float. Factory default is often around 2A, which on a larger bank is laughably low.

On my narrowboat I've got 400Ah of Fogstar lithium and had to bump the tail current percentage up considerably before the controller stopped bailing out early.

In VictronConnect, go into the charger settings — it's sometimes hidden under "expert mode". Worth cross-referencing with your battery manufacturer's spec sheet for the correct value.

What battery chemistry are you running? Makes a difference to what the correct settings should be.

Had this exact issue on my shepherd's hut setup last spring. Turned out my battery voltage sense wires had a dodgy connection — the MPPT was seeing slightly inflated voltage at the controller terminals rather than the actual battery voltage, so it thought absorption was done when it wasn't.

Worth checking in the Victron Connect app whether you've got remote voltage sensing enabled. If you're running sense wires direct to the battery terminals it makes a real difference, especially over longer cable runs.

Also — what battery type have you got set? I'd accidentally knocked mine off the correct preset after a firmware update. Easy to miss.

Good thread, this one. @Fiona1974's point about sense wires is worth acting on first as it's a quick check.

One thing nobody's mentioned yet — have you looked at your absorption time settings in the VictronConnect app? The SmartSolar has an adaptive absorption algorithm that shortens absorption duration based on previous charge cycles. If it "learned" a short absorption period from a series of shallow discharges, it can push into float earlier than you'd expect on days when the batteries actually need a proper top-up.

Worth doing a manual reset of the adaptive algorithm and running a full charge cycle from a reasonably depleted state to see if it recalibrates properly.

In my van setup I had something similar until I switched to fixed absorption time rather than adaptive — much more predictable behaviour, though obviously less efficient in theory.

@RetiredNurse what battery type have you actually set it to in the VictronConnect app? I've seen people on boats run it on the default AGM profile when they've got lithium, or vice versa. The absorption voltage threshold and time settings will be completely wrong if that's the case.

Also — what's your battery bank size versus that 30A controller? If it's undersized for the bank, it might never push enough current to satisfy the tail current threshold properly and just times out into float early. What are you actually running?