Victron SmartSolar 100/20 keeps going into absorption way too early — anyone else?

Picked up a Victron SmartSolar MPPT 100/20 for the garden office build and it's driving me absolutely mental. Running two 200W Renogy panels into a 200Ah Fogstar Drift LiFePO4, and the thing is jumping into absorption at barely 80% state of charge. I've got the battery profile set to the Fogstar-recommended settings (14.2V absorption, 13.6V float) but it's acting like the battery is full when it clearly isn't.

Checked the wiring, connections are solid, no obvious voltage drop issues. VictronConnect is showing the panel voltage and current looking sensible, but the transition from bulk to absorption is happening way too fast — we're talking 20 minutes on a decent sunny day when it should be pushing bulk for a couple of hours at least.

Wondering if it's a dodgy temperature sensor reading or whether I've cocked up a setting somewhere in the charge algorithm. Has anyone had the 100/20 do this, particularly with Fogstar batteries? Worth noting I'm also using this same setup as emergency backup so I actually need it to be properly charged, not just pretending to be.

@BitsAndBobs had almost the exact same issue on my static caravan setup last summer. Worth checking a few things:

• What's your battery voltage set to in VictronConnect? LiFePO4 absorption should typically be around 14.2V, not the default lead-acid profile
• Is the Fogstar Drift reporting its actual SOC via any BMS comms, or is the MPPT just guessing from voltage?
• Tail current setting — if it's too high, it'll flip into absorption prematurely thinking the battery is nearly full

The Fogstar Drift's resting voltage can sit quite high even at 60-70% SOC, which I suspect is confusing the controller into thinking it's further along than it actually is. Have you set a user-defined battery profile in VictronConnect rather than using one of the presets?

Nobby78 | Posts: 847 | Location: Peak District

@BitsAndBobs one thing nobody's mentioned yet — are you using the VE.Direct cable or Bluetooth to monitor it? I only ask because I had a maddening situation where my readings looked completely wrong until I realised the controller wasn't actually seeing the battery voltage properly due to a loose connection on the negative busbar. The MPPT was effectively guessing the battery state based on dodgy voltage readings and triggering absorption prematurely as a result.

Also worth double-checking your battery absorption voltage setting matches what Fogstar actually recommend for that specific Drift model — I've seen folk use generic LiFePO4 figures that are slightly off. Their support team are pretty responsive if you drop them an email.

What are your current absorption voltage and tail current settings set to?

LisaHunt | Posts: 312 | Location: Norfolk

@BitsAndBobs I had this exact headache with my Fogstar Drift last year! One thing worth checking — have you set the battery capacity correctly in the VictronConnect app? If it's set too low, the algorithm thinks the battery is fuller than it actually is and rushes into absorption prematurely.

Also, what absorption voltage are you running? Fogstar actually recommend 3.45-3.5V per cell (so 13.8-14.0V for a 12V pack) with a fairly short absorption time. I'd also double-check your tail current setting — dropping it to around 2-4% of battery capacity made a massive difference for me.

The Victron/LiFePO4 combination is brilliant once it's dialled in properly, just takes a bit of faffing to get there! 😅

KeithYoung | Posts: 1,204 | Location: Yorkshire Dales

@BitsAndBobs one thing worth double-checking is your battery cable sizing and length between the controller and the battery. Even a small amount of voltage drop across undersized cables can make the MPPT "think" the battery is higher than it actually is, causing it to trip into absorption prematurely. The 100/20 is measuring voltage at its own terminals, not at the battery terminals.

If you haven't already, try enabling the battery voltage sense feature via VictronConnect — the SmartSolar has Bluetooth built in so it can compensate somewhat, though ideally you'd use a BMV or SmartShunt with the VE.Smart networking to give it accurate readings directly from the battery. Makes a massive difference in my experience. What cable gauge are you currently running?

SilverTrekker | Posts: 634 | Location: Shropshire

@BitsAndBobs one thing I haven't seen mentioned yet — check your cable sizing between the panels and controller. Undersized or lengthy cable runs create voltage drop that can fool the MPPT into thinking the battery is fuller than it actually is. I had almost identical symptoms on my off-grid shed setup until I realised I'd used 4mm² cable over a 6-metre run when I really needed 6mm². Swapped it out and the absorption behaviour completely normalised. Also worth confirming your battery's internal BMS isn't occasionally throttling charge acceptance and sending confused voltage signals back to the controller — Fogstar Drifts are generally solid but it's worth ruling out.

Midge55 | Posts: 487 | Location: Derbyshire

@BitsAndBobs worth looking at your BMS communication setup. If the Fogstar's BMS isn't talking to the SmartSolar via VE.Direct or at least having its charge parameters respected properly, the controller can get confused about actual battery state. The 100/20 is also quite sensitive to temperature — do you have the optional temperature sensor fitted? Without it, the controller makes assumptions that can throw off the absorption trigger, especially in a garden office where temps might swing quite a bit. Also, have you connected it to the VictronConnect app yet? The live data view is brilliant for spotting exactly what voltage the controller thinks it's seeing versus what the battery's actually at. Sometimes there's a small resistance issue in the wiring causing a voltage discrepancy that makes it think the battery is fuller than it is.