Victron MPPT keeping dropping to float too early — 200Ah lithium not even close to full

Having the exact same issue with my static at the moment and it's driving me daft.

Running a Victron SmartSolar 100/30 with a 200Ah Fogstar Drift lithium and it keeps calling float at around 80-85% SoC according to the battery's own BMS app. The voltage is hitting the absorption threshold fine, but it's clearly not actually full.

A few things I've been looking at:

• Absorption time — mine was set to the default 2 hours but I've bumped it up to 4 to see if that helps
• Tail current — apparently on lithium this is the more reliable way to end absorption rather than time-based. Have you set a tail current in VictronConnect?
• Battery voltage calibration — is your Victron actually reading the same voltage as the battery itself? Even a 0.1–0.2V discrepancy can cause early float

What lithium battery are you running and have you got a BMS that communicates with the Victron via VE.Direct or Bluetooth? If not, you're essentially flying blind and the MPPT is just guessing.

I've also wondered whether the relatively low charge current matters here — on smaller solar days when it's overcast, does it drop to float even sooner for you?

Would be really useful to hear what settings others are using for 200Ah lithium specifically. There seems to be quite a bit of conflicting advice floating around (no pun intended) and I'd rather get it right than just leave it cycling in and out of float all afternoon.

@YorkshireCamper the classic premature float issue — almost certainly your tail current setting is too high.

In VictronConnect, check your charge algorithm settings. The MPPT exits absorption and drops to float when the charge current falls below the tail current threshold. Lithium needs this set quite low — I run mine at 1-2% of battery capacity, so roughly 2-4A for a 200Ah bank.

Also worth checking:

• Absorption voltage — should be around 14.2-14.4V for most Fogstar Drifts
• Absorption time — set a fixed period rather than adaptive if you're still dialling things in

The Drift's BMS will protect it, but you want the MPPT doing the heavy lifting correctly first. Check the Fogstar documentation for their recommended charge parameters — they publish them and Victron's presets don't always match third-party lithium chemistry perfectly.

@YorkshireCamper had almost the identical situation with my shepherd's hut setup last spring — Fogstar Drift behaving exactly as you describe, the controller essentially giving up early like it had lost patience.

What finally cracked it for me was checking the absorption voltage alongside the tail current. I had mine sitting at 14.2V when Fogstar actually recommend closer to 14.6V for a proper full charge. The controller was hitting the voltage threshold, deciding it had done enough, and bailing out.

Worth cross-referencing your settings against Fogstar's actual spec sheet rather than Victron's lithium defaults — they don't always match. The SmartSolar app makes it easy enough to adjust under Battery > Expert Mode.

Once I corrected both absorption voltage and dropped tail current to around 2%, the Drift started hitting genuine 100% consistently.

@YorkshireCamper also worth double-checking your absorption voltage — if it's set below 14.2V the MPPT never truly saturates the cells before the tail current threshold triggers, so you're basically chasing your own tail (no pun intended). Fogstar Drift wants 14.6V absorption from memory. Also, are you monitoring via the VictronConnect app? The charge history graph will show you exactly where it's bailing out — saved me hours of head-scratching on my motorhome build when my 100/20 was doing something similar. Nine times out of ten it's a voltage setting mismatch between what the controller thinks lithium needs and what your specific cells actually want. Pull the BMS data too if your Drift supports it — sometimes the BMS is quietly throttling current and the MPPT sees a low tail current and goes "job done, float time."

@YorkshireCamper this one caught me out badly when I first set up my garden office system. What nobody mentioned to me at the time was the wiring resistance throwing the whole picture off.

If your cable run from controller to battery is anything over a couple of metres, the MPPT is reading voltage at the controller, not at the battery terminals. It hits its target number, thinks job done, calls float — but the battery itself never actually saw that voltage properly.

Victron's answer is VE.Smart Networking — pair your SmartSolar with a SmartShunt or a BMV-712 via Bluetooth and it uses the battery's actual terminal voltage instead. Transformed my setup completely. The controller suddenly had an honest picture and absorption started lasting the correct duration.

Worth checking before you dive too deep into the tail current rabbit hole.