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

Had exactly this on my narrowboat last summer — Victron SmartSolar 100/30 deciding it was done by 11am like it had somewhere better to be.

Turned out my absorption voltage was set too low for my Fogstar Drift lithium bank, so it was hitting float almost immediately after a shallow discharge. Bumped absorption up to 14.2V and gave it a proper absorption time minimum of 2 hours, problem solved.

Few things worth checking:

• Battery type profile — if you're on a preset rather than user-defined, it might be wildly wrong for your actual cells
• Tail current setting — if this is too high, it'll cut absorption short thinking the battery's full when it isn't
• State of charge at start of day — if your bank barely discharged overnight, a quick absorption is actually correct behaviour, annoying as it feels

The VictronConnect app makes all this dead easy to fiddle with, which is either brilliant or dangerous depending on your Sunday afternoon mood.

What battery chemistry are you running? Makes a big difference to where the blame actually sits — seen people chase MPPT ghosts for weeks when it was a dodgy BMS cutting communication. Anyone else got a favourite tail current percentage for LiFePO4?

@ExTrucker — had almost the exact same drama with my static caravan setup last spring. Victron decided the batteries were "full" before I'd even had my second brew.

Worth checking your battery tail current setting as well — if it's too high, the MPPT thinks absorption is done and boots it into float prematurely. Mine was set at the default 2A, which was way too generous for my Fogstar lithium bank.

Drop it down and let the controller actually work for its living.

Also, if you're running the SmartSolar with a BMV-712 or similar shunt, enable BatteryLife or better yet let the BMV take charge control via DVCC — suddenly the whole system started behaving like a proper adult rather than knocking off at lunchtime.

@ExTrucker @Macca — worth checking your tail current setting too. Mine was defaulting to 4% which on a larger battery bank meant it was triggering float way before the batteries were actually full.

Dropped it to 1-2% on my Fogstar 200Ah LiFePO4 and the issue basically disappeared. The SmartSolar logs in the VictorConnect app are brilliant for diagnosing this — you can see exactly when it's transitioning and whether absorption time looks reasonable.

Does your setup show a very short absorption phase in the history data? That would confirm whether it's a voltage threshold issue like @ExTrucker found, or a tail current issue like mine was.

Great thread this — had a similar head-scratcher with my SmartSolar 75/15 a while back.

One thing nobody's mentioned yet: check your battery temperature compensation if you're running a temp sensor. Mine was pulling the absorption voltage down because the sensor was picking up heat radiating off the engine bay rather than the actual battery temperature. Threw everything out completely.

Also worth having a look at your absorption time settings — if you've got it on adaptive mode, Victron calculates absorption duration based on bulk charge time. Short bulk phase on a partially-topped battery can fool it into thinking the job's done earlier than it is.

@NorfolkExplorer makes a good point on tail current — that's caught plenty of people out.

What battery chemistry are you running @ExTrucker? AGM, lithium or flooded? Makes a big difference to where these thresholds should actually sit.

@OffGridAlan @NorfolkExplorer — both solid points already covered, so just adding one that bit me hard last winter with my garden office setup.

Check your battery temperature compensation if you've got a temperature sensor fitted (or even if you haven't). My Victron was seeing a slightly warm sensor reading and reducing absorption voltage accordingly — so it hit the tail current threshold earlier than expected and bailed to float thinking it was done.

Also worth a look: absorption time if you're running adaptive mode. On overcast days the algorithm can get confused about actual charge state and cut the absorption phase short.

Fogstar Drift LiFePO4 cells here, and once I dialled in the correct charge profile manually rather than trusting the presets, the controller stopped misbehaving entirely. Sometimes the defaults just aren't right for your specific bank.

My Victron on the shepherds hut does this whenever I've got dodgy battery temperature sensor readings — it thinks the pack is warmer than it is and throttles back absorption like a passive-aggressive boiler. Worth checking if yours is actually connected properly or just dangling there looking decorative like mine was for three months. 🙄