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

Had this exact issue with my 100/20 last summer running the garden office setup. Drove me absolutely mental for weeks.

Turned out my battery voltage sense was coming from the controller terminals rather than the battery itself — so with any cable resistance, it was over-reading voltage and triggering float prematurely. Classic gotcha.

Few things worth checking:

• Temperature compensation — if you've got a BMV or Smart Shunt in the network, make sure the MPPT is pulling temp data from that rather than its own internal sensor. The internal one is notoriously inaccurate if the controller's in a warm enclosure.
• Absorption time settings — are you using fixed or adaptive? Adaptive can cut short if the algorithm decides the battery is "full enough," which on a partially sulphated battery can be misleading.
• Tail current — check if you've set this. Even a slightly high tail current threshold will boot you into float early.

What batteries are you running? Fogstar Drift cells behave quite differently to a decent AGM in terms of how they accept charge near the top. If you're on lithium with a BMS, sometimes the BMS itself is throttling charge current and the MPPT interprets the resulting voltage spike as full.

VictronConnect logs are your friend here — pull the last 30 days and look for a pattern in when it drops to float. Morning drops are usually temperature-related, afternoon drops are more likely a settings issue.

What firmware version are you on? There were some absorption/float transition quirks in older builds that Victron quietly addressed.

@JackeryNerd classic Victron trap — the controller thinks the battery's full because it's reading voltage at its own terminals rather than the battery, so cable voltage drop is basically lying to its face 🔌

Enable remote voltage sensing via the VE.Direct or wire the dedicated sense terminals straight to the battery posts — sorted mine out in the garden office overnight, went from a suspiciously early float to a proper bulk/absorb cycle like nature intended.

@JackeryNerd @RockyMender both right about the sense wiring, but there's another culprit nobody's mentioned yet — absorption time configuration.

By default on a lot of Victron MPPTs the adaptive absorption algorithm calculates absorption duration based on the previous day's discharge depth. If your battery wasn't heavily discharged overnight, it'll blitz through absorption in 20 minutes flat and punt to float.

On a narrowboat or van setup this catches people constantly — shore power topped you up overnight, next morning the solar thinks the job's already done.

Worth checking in VictronConnect under Battery → Expert Mode:

• Set a fixed absorption time (I use 2hrs minimum on my 206Ah Fogstar Drift LiFePO4)
• Confirm your tail current threshold isn't set too aggressively low

The adaptive algorithm is clever in theory, brutal in practice when you've got mixed charging sources.

@ExSquaddie49 go on then, don't leave us hanging — absolute what? Absorption time settings?

I'll guess anyway: if your absorption time is set too short (or you're using adaptive absorption and the battery was already at a decent state of charge when the sun came up), the MPPT will blitz through absorption and dump it into float well before the battery's actually full.

Had this exact thing on my shepherd's hut setup. Victron defaults can be wildly optimistic depending on your battery chemistry. Running Fogstar Drift LiFePO4 here and had to manually tweak the absorption duration — the adaptive algorithm was essentially useless until I gave it a proper charge history to learn from.

Worth connecting VictronConnect and actually watching a full charge cycle before assuming it's a wiring issue.

@ExSquaddie49 is clearly the guy at the pub who says "I know exactly what's wrong with your car" and then walks off to order another pint 🍺

But yeah @ExPostie your absorption time hunch sounds right — if you've got the adaptive algorithm enabled on a Victron and your panels are undersized for the bank, it'll clock off early every single time because it never saw enough tail current to satisfy itself.

Happened on my setup before I realised my Fogstar cells needed the absorption threshold dialling right back in VictronConnect anyway.

@SamFrost73 haha that's exactly it — bloke's been haunting this thread like a ghost who forgot what he came to say.

Had this same drama on my boat last year. Spent a whole weekend convinced my Fogstar cells were dud, nearly ripped the whole installation out. Turned out to be the absorption timer — Victron defaults are quite conservative and if your bank isn't pulling decent current at the absorption voltage threshold, it just... gives up and goes to float early like a lazy apprentice.

Check your absorption time setting in VictronConnect. There's also a tail current setting which tells the controller "job's done" if current drops below X amps — that one catches people out constantly.

@ExSquaddie49 is this where you were heading? Because if not, genuinely curious what you were about to say 😄