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

Had this exact faff with my 100/30 last summer — turns out my Fogstar batteries were lying about their state of charge like a teenager claiming they've done their homework.

Worth checking a few things before you go down the rabbit hole of blaming the Victron:

• Battery voltage sense — is your MPPT actually measuring at the battery terminals or relying on cable voltage drop to gaslight itself?
• Absorption time settings — the adaptive algorithm can get a bit too clever and cut short if it thinks the battery's full based on tail current
• BMS communication — if you're running lithium with a BMS that's chatting to the Victron via VE.Direct or DVCC, the BMS might be the one calling time early

Mine fixed itself once I ran proper remote voltage sensing cables directly to the battery posts — went from dropping to float at 70% to actually doing a proper full charge. Game changer, embarrassingly simple solution.

What battery chemistry are you running? Lithium and lead-acid behave completely differently here and the fix is usually different. Also worth chucking the VictronConnect app on and watching the live graph during a charge cycle — you'll see exactly where it bottles it.

If you're on a narrowboat or static (like my setup) and the batteries are far from the controller, voltage drop through the cables is almost certainly the culprit. Longer the run, worse the problem.

Anyone else found the Renogy units handle this better or worse? Genuinely curious whether it's a Victron-specific quirk.

@MoorRuss the "lying battery" problem is a real one — worth clarifying why it happens though, because it's usually not the Fogstar cells themselves but rather the BMS cutting communication or reporting nominal voltage rather than true SoC.

A few things I'd check before blaming either component:

• Absorption voltage threshold — if it's set too low in VictronConnect, the MPPT will declare "done" prematurely
• Tail current setting — mine was defaulted to 4A on a 200Ah bank, which is far too aggressive
• Battery temperature — cold Fenland mornings meant my voltage readings were consistently optimistic

The Victron/Fogstar combination can work brilliantly but only if you're running the Fogstar with proper lithium profile settings rather than the generic preset. What absorption voltage are you currently running?

@FenlandSolar go on then, enlighten us — meanwhile my Victron's been dropping to float faster than my motivation to read a 47-page BMV manual on a Sunday morning.

@FenlandSolar @FET_Fan while you two sort that out — can someone explain what voltage the 100/30 actually uses to decide absorption is done? Is it purely time-based after hitting the absorption voltage threshold, or is it watching tail current too?

I've got a similar setup on my static caravan, Fogstar 100Ah LiFePO4, and my MPPT drops to float within about 20 minutes of hitting 14.2V. Feels wrong. Battery's nowhere near full judging by what I can pull from it afterwards.

Is this a case of tweaking the absorption time manually in VictronConnect, or is there a proper way to set tail current cut-off? I can't find a clear answer in the manual — it reads like it was translated by someone who'd rather be somewhere else.

@BayTim the 100/30 uses absorption voltage to decide when to shift — by default it's looking for the current to tail off to around 2% of battery capacity. Problem is, if your voltage sensing is at the controller rather than the battery terminals, you've got cable voltage drop fooling it into thinking the battery is fuller than it is.

Fix: run a proper voltage sense wire directly to the battery, or at minimum check your cable sizing isn't causing a significant drop under load.

@FenlandSolar is right that it's usually a sensing issue rather than the battery "lying" — though with LiFePO4 the flat discharge curve does make SoC estimation genuinely tricky without proper battery comms (Bluetooth/VE.Direct to a BMS that knows what it's doing).

Had this exact issue on my garden office setup until I sorted the sense wiring.

@CE_Builds is on the right track, but worth adding — the 100/30 has a default absorption time that's time-based unless you're using the adaptive algorithm, which estimates based on the previous day's bulk duration. If your Fogstars had a shallow cycle the day before, it'll assume they're already nearly full and cut absorption embarrassingly short.

In VictronConnect, check Charger → Expert mode → look at your maximum absorption time setting. I had mine on 1hr default for my garden office setup, which was nowhere near enough after a cloudy week.

Also cross-reference the controller's reported state of charge against actual resting voltage — let the bank sit disconnected for a couple of hours and measure at the terminals. If it's reading 95% but resting voltage tells a different story, your Fogstars may have a calibration drift worth resetting.