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

Had this exact issue with my 100/30 last summer on the shepherd's hut install. Drove me absolutely mental for weeks.

Turned out to be a combination of things in my case:

• Absorption time set too short — the "adaptive" algorithm can be a bit aggressive if your battery isn't pulling much current
• Tail current threshold — if you're using Victron's default settings with Fogstar Drift lithiums (as I was), the tail current was triggering float far too early because the BMS was already doing some of its own management

Worth connecting up VictronConnect and having a proper look at your charge history graphs. The app shows you exactly when it transitions and you can see if absorption is even lasting more than 20-30 minutes.

What battery chemistry are you running? Makes a big difference here. With my LiFePO4 setup I ended up:

1. Disabling the adaptive absorption entirely
2. Setting a fixed absorption time manually
3. Dropping the tail current percentage right down

Also check whether you've got any partial shading hitting panels mid-morning — I had a situation where a shadow would cause a brief dip that the controller was interpreting as the battery being nearly full. Took me ages to spot that one.

Which MPPT model are you on and what's your panel wattage vs battery capacity? That ratio matters quite a bit for how the algorithm behaves.

Plenty of people on here have cracked this before so hopefully someone with a different setup can chime in too — curious whether it's more common with certain configurations.

StormyNomad | 847 posts | ⚡ Off-Grid Enthusiast

@ValleyTony classic Victron rabbit hole that one! I'd add something nobody mentions — check your battery temperature sensor if you're using one. Mine was positioned too close to a heat source in the battery compartment and was feeding false high temps to the MPPT. Controller thought the batteries were warmer than they were, applied temperature compensation, and kept cutting absorption short. Took me ages to figure out because everything looked fine in VictronConnect. Repositioned the sensor away from the inverter and suddenly my absorption times made sense again. Worth checking the BMS comms cable too if you've got lithium — corrupted data packets can confuse the charge state calculation something rotten.

Good shout from both of you. One thing neither has mentioned yet — check your battery temperature compensation settings if you're running a BMV or SmartShunt alongside the MPPT via VE.Smart networking.

I had my 100/20 on the narrowboat dropping to float embarrassingly early last winter, and the culprit was the temperature sensor skewing the voltage thresholds downward in cold weather, making the controller think absorption was complete when it absolutely wasn't.

Pull up VictronConnect and look at the absorption voltage actually being applied versus what you've set — if those numbers don't match, temperature compensation is almost certainly your villain.

Also worth checking: tail current settings. If you've got it configured for a higher tail current than your battery chemistry warrants, it'll exit absorption prematurely. Fogstar Drift cells in particular seem fussy about this compared to their non-LiFePO4 equivalents.

Worth adding — double-check your BMS communication if you're running lithium. Had this exact headache with my Fogstar Drift cells last year. The MPPT was reading a full battery signal from the BMS via the VE.Direct link and dropping to float prematurely, even though the cells weren't actually topped off.

Also, on the absorption time front (I know @ValleyTony mentioned it) — if you're using the adaptive absorption algorithm, Victron calculates the next day's absorption time based on the previous cycle. If something interrupts a charge cycle, it can skew the adaptive calculation for days afterwards.

Connect it up to VictronConnect and pull the history graphs — the pattern in the data usually tells you immediately whether it's a voltage spike triggering early termination or something deeper in the settings.

Good points from everyone above. One thing I'd add from my garden office setup — check your absorption voltage is actually set correctly for your battery chemistry. Victron's defaults aren't always right out of the box, especially if you've got Fogstar or similar cells.

Also worth looking at the "tail current" setting in VictronConnect. If it's set too high, the MPPT thinks the battery is full when it's not — drops to float prematurely. I had mine at 4% which was way too aggressive for my 200Ah bank. Dropping it to 1-2% sorted it completely.

You can monitor it properly with a SmartShunt if you haven't already — seeing the actual current tail off in real time makes diagnosing this much easier.