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

Had exactly this with my 100/30 last summer when I was doing a long stint parked up in the Lakes. Battery would be sitting at maybe 70% and the Victron app would cheerfully announce it had hit float. Drove me absolutely mental.

Turned out my absorption voltage was set slightly too low for my Fogstar 100Ah lithium — the MPPT was seeing the voltage creep up under light load and deciding "job done." Classic false positive.

Few things worth checking:

• Battery voltage sense — if you're relying on the controller terminals rather than a proper voltage sense wire run directly to the battery, you'll get voltage drop skewing the readings
• Absorption time settings — the adaptive absorption on Victron kit can be really aggressive if you haven't tailored it to your actual battery chemistry
• BMS communication — if you've got a smart BMS, is it actually talking to the MPPT via the VE.Direct port? Mine wasn't, and the MPPT was essentially guessing

Once I sorted the voltage sense wiring and dialled in the absorption settings manually through VictronConnect, it stopped doing it almost immediately. Took me an embarrassingly long time to find though — I'd been chasing ghosts thinking it was a panel issue.

What battery are you running? That'd help narrow it down. Also, are you on the latest firmware? There was a period where certain versions had quirks with lithium profiles that got patched out.

Anyone else found the adaptive absorption to be more trouble than it's worth on smaller systems? I've half a mind to just fix the absorption time manually and be done with it.

@Gazza classic Victron behaviour — the controller looks at your battery and thinks "close enough, that'll do" like a teenager tidying their room by shoving everything under the bed 😄

Seriously though, worth double-checking your absorption voltage setting. If it's set a touch low, the MPPT hits that target voltage way too quick under light load and

@Gazza this is almost certainly a voltage spike fooling the controller into thinking absorption is complete. The 100/30 is particularly sensitive to this if your wiring has any resistance — even a decent cable run can create a voltage drop that reads backwards under light load, making the battery appear fully charged at the controller terminals.

What absorption voltage have you got set? For most lithium setups I'd want to see genuine time-in-absorption before float kicks in. Check your Min Absorption Time in VictronConnect — factory default is often embarrassingly short.

Also worth enabling the Tail Current setting if you're on a supported firmware. That forces the controller to only drop to float once charge current falls below a percentage of capacity rather than purely on voltage. Made a massive difference on my shepherd's hut setup with Fogstar Drift cells.

@LesWood78 makes a good point about voltage spikes — worth checking your cable run length and gauge too, as voltage drop can work both ways and cause odd readings at the controller end.

Practical question: have you actually measured the battery voltage directly at the terminals with a decent multimeter when this happens? I ask because I went through something similar with my van build and the Victron app was reading noticeably different to what my battery was actually sitting at. Turned out to be a connection issue rather than a controller setting problem.

Also, what battery type are you running and have you set a custom absorption voltage in VictronConnect, or left it on the default profile? The defaults can be quite conservative depending on your chemistry.

@Gazza this bit catches people out — the default absorption end condition on Victron MPPTs is tail current, not time. Head into VictronConnect and check your "tail current" setting under the battery parameters. If it's set too high (say 4-5% of battery capacity), the controller will exit absorption the moment charge current drops below that threshold, even with a half-full bank.

On my shepherd's hut setup with a 200Ah Fogstar lithium, I had to drop the tail current right down to around 1-2% before absorption completed properly. Also worth checking your absorption voltage is correctly matched to your battery chemistry — Victron's factory defaults are somewhat conservative for LiFePO4.

What battery type and capacity are you running? That changes the diagnosis considerably.