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

Had this exact issue last summer with my 100/30 on the static. Drove me absolutely mad for a few weeks before I tracked it down.

Turned out my battery voltage sense was off — the MPPT was reading a slightly higher voltage at the controller terminals than what was actually at the battery. Small resistance in the wiring was enough to fool it into thinking the batteries were fuller than they were. Ended up running a dedicated voltage sense wire direct to the battery terminals and it sorted it almost immediately.

Worth checking a few things:

• Battery type settings — make sure absorption voltage is actually appropriate for your chemistry. Fogstar Drift LiFePO4 cells need very different settings to AGM
• Tail current setting — if this is set too high it'll drop to float early thinking the battery is full
• Cable resistance — even a few milliohms can skew the readings

Also had a mate with a 75/15 where the BMS was causing brief current spikes that the MPPT was misinterpreting as the absorption phase completing. Took ages to figure that one out.

What battery type are you running? And have you got VictronConnect logging enabled? The detailed graphs are genuinely useful for spotting where in the charge cycle it's bailing out early — you can usually see exactly what triggered the transition to float.

Might also be worth checking if there's a firmware update pending. Victron have tweaked the charge algorithm a few times over the past couple of years.

@Max1970 classic one that. Worth also checking your absorption time settings in VictronConnect — if it's set too short it'll bail out early regardless of actual charge state.

Also had this on my setup with a duff temp sensor giving false high readings. MPPT thinks battery's warm, cuts absorption short to protect it.

Few things to check:

• Tail current setting — if set too high it triggers float prematurely
• Battery voltage calibration (sounds like you've sorted this)
• Wiring resistance between controller and battery — voltage drop fools it

Mine's a 150/35 on a Fogstar 200Ah LiFePO4 and I had to dial the tail current right down before it behaved properly. VictronConnect logs are your friend here — pull the history and you'll see exactly where it's dropping off.

Had this on my boat last season — different cause though. Turned out one of my battery terminals had a very slight amount of corrosion building up underneath the lug. Looked fine visually but was causing enough resistance to skew the voltage readings. MPPT thought it was fully charged when it clearly wasn't.

Worth doing a proper load test if you've already ruled out @Max1970's voltage sense issue and @Wez1961's absorption time. Pull some decent amps and watch what the terminal voltage actually does in VictronConnect's graph view.

Also — what battery chemistry are you running? If someone's previously had the unit on AGM and you've switched to lithium without updating the charge profile, it'll drop to float embarrassingly early. Seen that catch people out more than once.

@BayTim that's a good shout on the terminals — resistance creep is sneaky.

Worth mentioning another culprit nobody's said yet: tail current setting. If it's set too high the controller thinks the battery's full when it isn't. In VictronConnect check Expert Mode → tail current should typically be around 1-2% of battery capacity. Had mine set wrong on my Fogstar lithium setup and it was dropping to float with 20% still to go.

Also if you're on LiFePO4, double-check your battery preset matches the actual BMS charge parameters — Victron's default lithium profile isn't always right out the box.