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

Had this exact issue last summer with my 100/20 running the garden office setup. Battery would barely be at 80% and the MPPT would decide it was done and drop to float. Drove me absolutely mad for weeks.

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

1. Absorption time set too short — I'd left it on the default "adaptive" mode which was clearly making some optimistic assumptions about my Fogstar Drift LiFePO4s
2. Tail current threshold too high — the controller was seeing the current drop and thinking job done

Fixed it by switching to fixed absorption time (2 hours for my bank) and dropping the tail current setting down to around 1-2% of battery capacity. Night and day difference.

Worth checking in VictronConnect what your absorption voltage is actually hitting too — mine was also set slightly low, so the controller was reaching the target voltage quickly on a partially charged pack and calling it a win.

What battery chemistry are you running? LiFePO4 and AGM behave quite differently here. If you're on a managed lithium with a BMS talking to the MPPT via VE.Direct, that changes things completely — the BMS can effectively tell the MPPT to back off early if it's not configured right.

What firmware version are you on? There were some oddities in older builds that affected this behaviour.

ExTrucker63 | Posts: 847 | Location: North Yorkshire

@FormerMariner24 mate I've been down this rabbit hole! Check your battery voltage absorption setpoint first — if it's set even slightly low the controller thinks it's finished when it absolutely hasn't.

Also worth looking at your tail current setting. Victron defaults are sometimes too generous for smaller battery banks and it'll drop to float the moment current tapers off, regardless of actual state of charge.

What battery chemistry are you running? If it's AGM and you've accidentally got it configured for a sealed lead acid profile that can cause exactly this. Connect it up to VictronConnect and have a proper look at the charge algorithm settings — took me about ten minutes to sort mine once I actually sat down with the app properly.

My Fogstar Drift LiFePO4 cells were doing the same thing until I realised my absorption voltage was set 0.2V too low — Victron just thought the job was done when it absolutely wasn't.

Robbo | Posts: 312 | Location: Array

Had this exact headache with my cabin setup last year. Worth checking whether your battery's BMS is actually communicating properly with the MPPT — or not communicating at all. Mine was running in standalone mode essentially guessing the battery state based on voltage alone, which is about as accurate as reading tea leaves.

What battery are you actually running? If it's a cheaper LiFePO4 with no VE.Direct or Smart networking capability, the MPPT just sees a very flat voltage curve and panics into float early.

Also — have you checked the tail current setting? Default is something daft like 2% for a lot of profiles. Bump it down and you might find it stays in absorption properly.

Victron's own battery profiles are weirdly conservative out of the box too. What firmware version are you on?

GoldenSocket | Posts: 1,203 | Location: Array

One thing nobody's mentioned yet — check your absorption time setting. If you're on a fixed absorption timer rather than adaptive, and it's set too short, the MPPT will bail to float regardless of actual state of charge.

Victron's adaptive mode is far more reliable for this. Go into VictronConnect, set absorption to adaptive, and let it calculate based on bulk duration.

Also worth checking: are you running temperature compensation? A warm battery reads artificially high voltage, so the controller thinks it's done when it isn't. Slap a BMV-712 or SmartShunt on if you haven't already — the difference in accuracy is night and day.

@SilverHiker's point about absorption voltage is valid too, but I'd sort the timer issue first. It's the more common culprit in my experience with my garden office setup.