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

Had exactly this on Persistence (my narrowboat) last summer. Victron 100/30, 200W of Renogy panels, and a pair of Fogstar 100Ah lithiums that were barely six months old.

The culprit, after an embarrassingly long investigation involving a multimeter and several cups of tea, turned out to be the absorption time being set too short. The MPPT was hitting its voltage target, ticking off the clock, and smugly declaring the job done — despite the batteries being nowhere near actually full.

A few things worth checking:

• Tail current setting — if this is too high, the controller thinks the battery is full before it is. I dropped mine from the default and it made an immediate difference.
• Battery capacity entered in VictronConnect — if it's wrong, the calculated absorption time will be wrong. Simple but easy to overlook.
• Actual battery voltage vs. reported voltage — dodgy or undersized cabling causes voltage drop that makes the controller think it's achieved absorption when it hasn't really.

The fix for me was combining a lower tail current threshold with slightly longer minimum absorption time. Batteries now consistently hit 99-100% SOC by early afternoon on a decent day.

Worth also checking whether your BMS is sending any signals that might be confusing the MPPT if you're running comms between them.

Anyone running the SmartSolar with the VE.Direct to a Cerbo GX — are you seeing the same? Curious whether the networked setups behave differently when DVCC is in the mix.

@ExmoorNomad I had almost identical grief with my garden office setup last spring — Victron 75/15, single 175W panel, Fogstar Drift 100Ah. Turned out my absorption voltage was set a touch low (13.9V instead of the 14.2V Fogstar actually specifies), so the battery was hitting the absorption target almost immediately and the controller assumed it was done.

Worth double-checking your battery profile against Fogstar's own documentation rather than any generic lithium preset in VictronConnect. The presets can be surprisingly conservative.

Also — and this caught me out — if tail current is set too high, the controller interprets normal resting charge acceptance as "battery full." I dropped mine from 2A to 1A and the absorption phase stretched out properly from there.

@ExmoorNomad Oh, the classic "absorption betrayal" — when your MPPT decides the batteries are done and toddles off to float like it's having a little sit-down, whilst your lithiums are still gasping.

Had this exact drama with my emergency backup setup. Turned out my absorption voltage was set about 0.2V too low —

@ExmoorNomad worth being precise here — the MPPT isn't dropping to float early, it's completing absorption correctly based on the tail current setting. If your batteries are transitioning faster than expected, the tail current threshold is almost certainly set too high for your specific Fogstar cells.

In VictronConnect, check your absorption tail current — default is often 2A, which for a 200Ah bank is way too aggressive. Should be closer to 1% of capacity, so 2A is actually borderline acceptable, but if Fogstar specify lower in their docs, defer to that.

Also — absorption voltage matters. What have you actually got it set to? Lithium profiles vary more than people realise between manufacturers.

Had similar confusion setting up my garden office backup system. Sorted it properly once I stopped guessing and actually read the BMS spec sheet.

@GlenDoug raises a fair point about terminology, but the practical outcome is the same — you're not hitting the state of charge you expected. One thing nobody's mentioned yet: tail current threshold. On Victron MPPT units, the absorption phase ends when current drops below a configurable percentage of battery capacity. Default is often set assuming lead-acid, so on lithiums with their characteristically flat voltage curve and low internal resistance, you'll hit that tail current threshold well before the cells are genuinely full.

In VictronConnect, check your tail current setting under the battery parameters. For my shepherd's hut setup with 200Ah of Fogstar Drift cells, dropping it from 4% to 2% made an immediate, measurable difference — verified against a Victron BMV-712 coulomb counter rather than just guessing from panel output. Worth cross-referencing your BMS data too if it's reporting SOC independently.