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

Had this exact issue on my van build last summer. Victron 100/30 was hitting float after barely 2 hours on a decent sunny day — batteries nowhere near full.

Turned out my absorption voltage was set slightly too high for my Fogstar lithium cells, so the controller was "seeing" the battery as full earlier than it actually was. Dropped absorption by 0.2V and it sorted itself out almost immediately.

Few things worth checking:

• Battery voltage calibration — is the MPPT actually reading the same voltage as your battery monitor? Even 0.1V difference can cause early float
• Tail current setting — if this is too high it'll exit absorption early. Worth lowering it in VictronConnect
• Temperature compensation — if you've got a temp sensor fitted and it's reading wrong, it'll skew everything
• Absorption time (fixed vs adaptive) — adaptive mode can cut short on partially discharged banks

What battery chemistry are you running? Lithium and AGM behave very differently here. My emergency backup setup at home runs AGM and I had to go fully fixed absorption time because the adaptive algorithm was all over the place with a mixed age bank.

Also worth checking if you've got VE.Smart networking — if your BMV or SmartShunt is talking to the MPPT it can override the charge decisions, which is usually great but can cause weird behaviour if the shunt calibration is off.

What does your absorption voltage sit at and what's the battery spec say it should be? Might help narrow it down.

@PanelNige — welcome to the forum, glad you found us! Always good to have someone who's already diagnosed their own issue joining in, that kind of hands-on experience is exactly what this community needs.

One thing worth checking if others hit this problem: the battery temperature sensor. Without one fitted, the Victron assumes a fixed temperature and can compress the absorption phase quite aggressively depending on your settings.

Also worth looking at tail current in VictronConnect — if it's set too high relative to your battery capacity, the controller thinks the bank is full long before it actually is. I had something similar on my narrowboat with a 200Ah Fogstar lithium setup until I dialled that in properly.

What battery chemistry are you running? Makes a big difference to where those thresholds should sit.

@PanelNige mine did exactly the same thing on my static — turned out the absorption time was set to "adaptive" and it was using the previous cycle's dodgy data to calculate it, basically gaslighting itself into thinking the job was done.

Force it to fixed absorption time (I do 2.5 hrs minimum) and set your tail current properly — Victron default of 2A is laughable on anything over 100Ah.

Also worth checking your battery temp sensor if you've got one; a warm battery reads artificially high voltage and the MPPT thinks it's fuller than it is.

VRM portal will show you exactly where it's cutting out if you've got the dongle — genuinely useful once you stop rage-staring at the app wondering why your Fogstar cells are at 60%.

@PanelNige classic Victron gotcha — the tail current setting is the sneaky one nobody reads about, if your battery's even slightly warmer than expected it'll bail out of absorption early thinking the job's done 🌞

Check your tail current % in VictronConnect and bump it down — mine was set to 4% on Fogstar lithiums and dropping to float embarrassingly fast until I sorted it.

@SolarNut had the same tail current ambush on my Fogstar-fed setup in the van — also worth checking your battery temperature if you've got a sensor fitted, because a warm battery looks "fuller" to the MPPT and it'll bail into float sharpish thinking it's done a grand job when it absolutely hasn't.