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

Had exactly this with my 100/20 on the shepherd's hut last summer. Turned out the default absorption time was set far too short relative to my battery bank size — I was running 200Ah of Fogstar Drift LiFePO4 and the controller was dropping to float after maybe 45 minutes of absorption. Nowhere near enough.

Worth checking a few things systematically:

• Battery capacity vs absorption duration — Victron's adaptive algorithm bases absorption time on the bulk phase duration. If your panels are oversized relative to the bank, bulk finishes fast and absorption gets cut short accordingly.
• BMS communication — if you've got a smart BMS talking to the MPPT via VE.Direct or VE.Bus, check whether it's prematurely signalling the charge complete flag. I had this issue when my JK BMS firmware was outdated.
• Temperature compensation — a cold battery will hit voltage thresholds earlier than you'd expect, potentially triggering float prematurely.

In VictronConnect, go into the charger settings and try switching from adaptive to fixed absorption time temporarily — just to eliminate the algorithm as the culprit. Also cross-reference your actual state of charge in the app versus when float kicks in. If it's dropping to float at 85-90% SoC, something's definitely misconfigured.

What battery chemistry are you running and what are your current absorption voltage/time settings? That'd help narrow it down considerably. Others on here have had similar gremlins — particularly with certain LiFePO4 profiles that ship with overly conservative defaults.

@SomersetVanLifer classic Victron — it's basically telling your batteries "right, you've had enough, off to bed" while they're still absolutely gagging for more charge 😄

Worth checking your absorption voltage too, not just the time. I had mine set to the default 14.4V on my Fogstar Drift cells and

@SomersetVanLifer had the exact same drama with my motorhome setup — check your absorption voltage too, because if it's hitting the target voltage prematurely (dodgy sensor, high ambient temp fooling the controller), it'll barge straight into float like an impatient landlord.

Worth diving into VictronConnect and enabling adaptive absorption if you haven't — it calculates absorption time based on the bulk phase duration, so a lazy solar morning won't shortchange your Fogstar cells.

Also double-check your battery temperature compensation settings; a warm battery reads artificially high voltage, convincing the MPPT it's done when it absolutely isn't.

Great thread, this. @SomersetVanLifer I'd also look at your tail current setting in VictronConnect if you haven't already — it's the one that often gets overlooked. Essentially it tells the MPPT to end absorption once charge current drops below a certain percentage of your battery capacity, regardless of time. On lithium like the Fogstar Drift, getting that dialled in properly can make a massive difference. Default settings are really geared toward flooded lead-acid, so they don't translate brilliantly to LiFePO4.

Also worth double-checking your battery capacity is entered correctly in the app — I've seen folk enter 100Ah when they're running 200Ah, which throws the whole algorithm off from the start. Small thing but easy to miss when you're rushing to get set up! 🙂

@HighlandDweller tail current sorted my garden office setup overnight — but also worth checking your battery temperature if you've got a sensor fitted, because my Victron was seeing a warm battery and deciding it was fuller than it actually was, basically marking its own homework.

@SomersetVanLifer had this exact faff on my narrowboat — turns out my adaptive absorption was doing its own optimistic maths based on a suspiciously sunny previous day, so worth disabling that and setting a fixed absorption time manually via VictronConnect until you've got a proper baseline.

Rule of thumb I use: roughly 1 hour per 100Ah at your bank size, so you're looking at 2 hours minimum for 200Ah Fogstar Drift — though I'd creep it up until your cells stop sulking.

Also double-check you've got the correct battery preset selected; "lithium" vs "user defined" catches people out more than it should, and Victron's default lithium profile is quite conservative out of the box.