Victron MPPT keeps dropping to float too early — 200Ah lithium not even half full

Having this exact issue with my narrowboat setup — Victron SmartSolar 100/30 connected to a 200Ah Fogstar Drift lithium. It'll sit in bulk for maybe 45 minutes on a decent day, then suddenly decide it's done and drop to float at around 26.8V. Battery monitor is showing 40-something percent. Something clearly isn't right.

I've gone through the settings a few times and I think the tail current is the culprit. Mine was set at the default which I believe is quite high for lithium — it's interpreting the naturally low acceptance current as "battery full" and calling it a day. Does that match what others are seeing?

A few things I'm wondering:

• What tail current percentage are people using for lithium on similar sized banks?
• Is the absorption voltage set correctly — should I be at 28.4V or going lower?
• Could the battery BMS be communicating oddly and confusing the controller, even without a VE.Direct cable connecting them?

I've had a poke around in VictronConnect but there's a lot of options and I don't want to misconfigure something and damage the cells. Currently running the "Lithium Iron Phosphate (LiFePO4)" preset but wondering if manually tweaking is the way to go.

Anyone who's nailed this with a similar boat or off-grid cabin setup, would love to know what charge profile you're actually running. Screenshots of VictronConnect settings would be genuinely helpful if anyone's willing to share.

@NicolaTaylor72 had the same thing with my garden office setup — turned out my absorption voltage was set too low (default profile wasn't right for lithium). Worth checking your charge profile in VictronConnect. Fogstar Drift wants absorption at 14.2V and float at 13.5V, and the absorption time needs to be long enough or it'll tail-current exit too early.

Also check whether you've got the tail current setting enabled — if it's set too aggressively (say 2-3% of battery capacity) it'll declare absorption "done" well before the battery is actually full.

If you're on the default AGM preset that's almost certainly your problem. Create a custom lithium profile from scratch rather than modifying an existing one.

Classic Victron/lithium mismatch issue this. The SmartSolar's default absorption time algorithm is designed around lead-acid behaviour — it calculates absorption duration based on how long bulk took, so a fast-charging lithium fools it into thinking the battery is nearly full.

Beyond what @Louise1984 said about absorption voltage, check your tail current setting in VictronConnect. For a 200Ah Fogstar Drift you want the float transition tail current set around 4-5A (roughly 2% of capacity). If it's sitting at the default, it'll drop to float the moment charge current dips during a cloud shadow.

On my narrowboat I run:

• Absorption: 14.2V
• Float: 13.5V
• Tail current: 4A
• Absorption time: fixed at 1hr rather than adaptive

The fixed absorption time option in VictronConnect is the quickest fix — stops the adaptive algorithm causing grief entirely.