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

Had almost identical behaviour with my cabin setup last spring — Victron SmartSolar 100/30 was hammering into float after maybe 90 minutes of bulk, with 200Ah Fogstar Drift cells sitting at around 45% SOC according to the battery monitor.

Turned out to be a combination of two things:

1. Tail current set too aggressively
The MPPT was interpreting a relatively small absorption current as "battery full" and rolling into float early. In VictronConnect, check your tail current percentage under the absorption settings. I had mine at 2% which sounds sensible but with lithium it can trigger prematurely, especially if your panels are undersized relative to battery capacity and the current naturally drops off in partial shade or late afternoon.

2. Absorption voltage set slightly low
If you're using a generic lithium profile rather than dialling in the actual values from your BMS or cell spec sheet, the voltage ceiling can be conservative. Worth cross-referencing with your battery manufacturer's recommended charge parameters.

A few questions worth answering before going further:

• What's your panel wattage vs battery capacity ratio?
• Are you using a Victron BMV or SmartShunt alongside the MPPT, and is it networked via VE.Smart?
• What lithium chemistry — LFP or something else?

If you're not running VE.Smart networking between the BMV and the MPPT, that's likely a big part of it. Once I linked mine up, the MPPT used actual measured current from the shunt rather than estimated values, and the premature float issue completely disappeared.

Worth pulling the history data from VictronConnect too — the charge cycle graphs will show exactly where it's transitioning.

@WezWhite72 classic symptom of the tail current threshold being set too aggressively. Victron defaults the float transition at 2A tail current, which on a 200Ah lithium is far too early — you're basically telling it "done" at 1% C.

In VictronConnect, try bumping your absorption time to a fixed period (I use 2 hours on my static van setup) rather than relying on tail current alone. Also worth checking your battery capacity is entered correctly — if it thinks you've got 100Ah it'll cut even sooner.

Also double-check absorption voltage is actually reaching the BMS target. Had mine sitting at 14.2V when the Fogstar Drift wanted 14.6V, so it never properly saturated before dropping out.

What's your current absorption voltage set to?

@RetiredPlumber is spot on about tail current, but worth checking your battery voltage sense configuration first — if the SmartSolar is doing voltage sensing at the controller terminals rather than the battery terminals, you'll get a false voltage reading inflated by cable voltage drop, which tricks the unit into thinking absorption is complete well before the cells are actually charged.

On my static caravan setup I was seeing exactly this until I ran a dedicated sense wire pair directly to the battery terminals. Dropped maybe 0.3V of phantom drop out of the equation immediately.

Also double-check your absorption voltage is set correctly for Fogstar Drift cells specifically — they want 3.45–3.55V per cell, so 13.8–14.2V for a 12V bank. If it's sitting lower than that, the controller hits tail current sooner than it should.

Both @RetiredPlumber and @VitoCamper are covering the obvious culprits, so let me add the one that bit me personally with my cabin setup last year — absorption time limits.

In VictronConnect, check Expert Mode → Absorption Time Limit. Factory default is often 1 hour, which on a partially-discharged 200Ah lithium is laughably short. The MPPT hits that timer ceiling and surrenders to float regardless of actual SOC.

Specifically for Fogstar Drift cells, I'd recommend:

• Absorption voltage: 28.4V (14.2V × 2 for 24V)
• Tail current: 2–4% of capacity (~4–8A for 200Ah)
• Absorption time limit: disable it entirely or set 4+ hours

Also worth cross-referencing your BMS data against what Victron thinks is happening — the two often disagree hilariously, and the truth usually lives somewhere between them.

Had this exact drama with my garden office setup last year. One thing nobody's mentioned yet — check your absorption voltage setting. If it's too low (common Victron default is around 14.2V for lithium), the MPPT decides absorption is "done" far too quickly because voltage climbs fast with lithium's flat curve.

Bump absorption to 14.4V and suddenly you get proper bulk/absorption phases.

Also worth checking in VictronConnect — look at the history tab and see how many Ah were actually pushed in during bulk. That'll tell you immediately if it's genuinely short-cycling or just your battery monitor lying to you about SoC.

Fogstar Drift cells specifically — @WezWhite72 make sure your battery profile matches Fogstar's actual spec sheet, not just generic LiFePO4 defaults. Made a massive difference on mine.

@QuietTrekker raises a good shout on absorption voltage, but nobody's mentioned the BMS communication angle yet — if your Fogstar Drift's BMS is whispering "I'm full, ta very much" to the Victron via VE.Smart networking before the cells actually are, the MPPT believes it like a golden retriever believes you've thrown the ball. Had this exact farce on my boat last summer — turned out the BMS temperature compensation was lying through its teeth and triggering an early float every single time. Check whether you've got any smart networking active in the VictronConnect app and temporarily disable it to see if the MPPT behaves itself on standalone voltage sensing alone.