Anyone else finding their Victron MPPT wildly over-optimistic with absorption time this time of year?

Running a 200Ah Fogstar Drift LiFePO4 in the van, paired with a Victron SmartSolar 100/20. Set absorption at 14.2V with a 1-hour timer, but the controller seems to want to sit in absorption for ages even when the battery's clearly near full — SOC showing 97%+ on the Cerbo but the MPPT just refuses to tail off quickly.

Wondering if it's a seasonal thing — less consistent irradiance through autumn means the voltage is bouncing around more, so the controller resets its absorption clock repeatedly? Had no issues over summer but it's been a nightmare since October.

Has anyone dialled in a shorter fixed absorption time for LiFePO4 specifically, or switched to using the tail current setting to exit absorption? Not sure if tail current is even reliable enough to trust when cloud cover is causing the input to fluctuate constantly.

ExFirefighter59 | Posts: 847

@MuddyRanger Worth checking whether you've got the BMS tail current feature enabled in VictronConnect - if the MPPT can't "see" the actual charge current dropping off, it'll just keep burning through that absorption timer regardless.

Also this time of year with weak sun, your panels might barely be pushing enough current to ever properly trigger an end-of-absorption condition, so it keeps resetting or extending the timer.

I had exactly this with my 100Ah Fogstar over winter - dropped my absorption voltage slightly to 14.0V and cut the timer to 30 minutes. Honestly for LiFePO4 chemistry you don't need long absorption anyway, it's not like AGM. The Drift's BMS will handle any top balancing itself.

What's your panel wattage going into that 100/20?

KN_Builds | Posts: 312

@MuddyRanger The issue is likely your absorption timer - fixed time absorption on LiFePO4 is a bit of a blunt instrument honestly. With Victron gear and a Fogstar Drift, I'd strongly recommend switching to the tail current termination method instead. Set your tail current to around 2-4% of battery capacity (so roughly 4-8A for your 200Ah bank) and let the controller end absorption when charge current naturally drops to that level rather than relying on a timer.

Worth connecting via VictronConnect and having a proper look at your charge history graphs too - you can see exactly what the battery's been doing. This time of year with lower solar input the controller can behave oddly with fixed timers because it's trying to compensate for slower charging. The tail current method is far more appropriate for LiFePO4 chemistry anyway.

Stu1991 | Posts: 156

My Fogstar on the narrowboat laughs at my Victron's optimism every November — switch to "end amps" termination instead of a fixed timer and let the BMS tail current actually do the maths, otherwise you're just sitting in absorption like a lost tourist waiting for a bus that's already full.

FionaShaw | Posts: 203

@MuddyRanger Something worth considering that nobody's mentioned yet — at this time of year your panels are hitting much lower irradiance levels, so you're potentially starting absorption at a lower state of charge than you realise. The Victron sees 14.2V but the battery hasn't actually had the amps pushed through it to back that voltage up properly.

Have you got the VictronConnect app logging your charge current during absorption? If you're only seeing 2-3A going in when absorption kicks off, that tells you the bulk stage did most of the heavy lifting earlier. Worth looking at your tail current relative to your battery capacity — for a 200Ah pack you'd typically want something around 2-4A as your end-amps threshold rather than relying purely on the timer.

The winter sun just doesn't give the controller enough to work with sometimes!

JimButler | Posts: 847

@MuddyRanger Worth checking your tail current setting if you haven't already. On LiFePO4 you want absorption to end when charge current drops to around 2-5% of your battery capacity — so roughly 4-10A for a 200Ah pack. That's far more reliable than a fixed timer, especially in winter when your panels are delivering inconsistent current throughout the day anyway.

In the Victron app, look under "Expert mode" — you can set a tail current threshold that'll cut absorption short once the battery's actually satisfied rather than just waiting out the clock. Makes a noticeable difference this time of year when you're getting patchy sunshine and the controller can get a bit confused about where it actually is in the charge cycle.