Victron MPPT keeps dropping to float way too early — anyone else had this?

Had exactly this with my 100/30 last summer when I was setting up the shepherd's hut build. Drove me absolutely mad for about two weeks before I figured out what was going on.

Turned out my absorption time was set too short and the tail current threshold was doing most of the damage — Victron's defaults aren't always suited to lithium, especially if you're running Fogstar cells like I am. The MPPT was seeing the current taper off and just deciding it was done, nowhere near actually full.

A few things worth checking:

• Tail current setting in VictronConnect — if it's too high (say 4% or more) it'll kick to float prematurely
• Battery capacity entered correctly? If it's undersized in the config the calculations go wonky
• Absorption voltage — worth double-checking it matches your battery spec sheet, not just the Victron preset
• Temperature compensation — if you've got a temp sensor attached and it's reading odd, that can mess with the charge curve

What battery chemistry are you running? Makes a big difference to how you'd approach this. Lithium behaves very differently to AGM and the out-of-the-box profile really isn't optimised for it in my experience.

Also — are you monitoring via VRM or just the app locally? If you're on VRM you can pull historical data and actually see where in the cycle it's bailing out, which helps massively with diagnosis rather than just guessing.

Would be curious what others have found too — feel free to share your setups and whether this is a recurring issue.

Classic Victron trap — the tail current threshold is quietly judging your battery like a disappointed parent, and if your absorption time is set too short it'll bail out the moment current dips even slightly.

Check your MPPT → Battery → Tail current setting; default is often 2A which is laughably optimistic for larger banks. Also worth verifying your battery capacity setting is actually correct — Victron calculates adaptive absorption time from it, so if you've told it you've got a 100Ah bank when you've actually got 200Ah (looking at my own Fogstar Drift 200Ah setup here), it'll cut corners ruthlessly.

@Gill1982 also worth enabling BatteryLife off if you're doing proper charging cycles — counterintuitively that feature throttles things back when it thinks it's being kind.

@Gill1982 worth checking your absorption time setting too — if you're on the adaptive algorithm, Victron calculates it based on time spent in bulk. Short bulk phase (cloudy morning, partially shaded array) means it grants almost no absorption time and boots straight to float.

On my narrowboat setup I switched to fixed absorption time — 2 hours for my 200Ah Fogstar Drift LiFePO4 — and the erratic behaviour stopped immediately. The adaptive algorithm is designed around lead-acid charge curves and genuinely doesn't play well with lithium unless you've dialled in the tail current correctly.

Also double-check your battery voltage sense — if the MPPT is measuring at the controller terminals rather than the battery terminals, voltage drop across your cabling fools it into thinking the battery is fuller than it actually is. Remote voltage sensing via the VE.Smart network sorts this properly.

@Gill1982 this caught me out on my narrowboat setup too. One thing nobody's mentioned yet — check your battery temperature. If you've got a Smart battery sense or temp sensor fitted, Victron will compress the absorption window significantly in warmer conditions. Mid-summer on the boat, my 150/35 was cutting to float after barely 40 minutes because the sensor was reading high bilge temps. Completely legitimate behaviour from the controller, but baffling until you dig into the VictronConnect history graphs. The absorption voltage was also being knocked down by the temp compensation simultaneously, so the tail current threshold was hit almost immediately. Worth logging a full charge cycle via Bluetooth and scrutinising the voltage and current curves together rather than just checking individual settings in isolation.

@Gill1982 one more thing to check that nobody's picked up on yet — your battery voltage sense. If the MPPT is relying on its own terminal voltage rather than a remote sense cable run directly to the battery, voltage drop across your cabling will fool it into thinking the battery is fuller than it is. Even a few millivolts of drop can trigger early float.

On my garden office setup I had 4mm² cable over about 3 metres and it was causing exactly this. Added the remote sense wires (just 0.75mm² is fine, they carry virtually no current) and the behaviour corrected itself immediately.

Worth checking your cable cross-section against the Victron cable sizing guidelines too — undersized wiring is the silent killer here.

@VictronMaster makes a good point on voltage sense — I ran external sense cables on my 150/35 and it made a noticeable difference.

One thing nobody's touched on yet: tail current threshold. If your Victron is set with a high tail current percentage, it'll exit absorption and drop to float early because it thinks the battery is full when it isn't. Worth checking in VictronConnect under the battery settings.

On my LiFePO4 setup (Fogstar Drift cells) I had to drop the tail current right down to around 1-2% of battery capacity before the absorption phase started behaving properly. Default settings are often tuned more for AGM/lead-acid and can behave oddly with lithium.

Connect via Bluetooth, go into expert mode, and have a poke around — it's not obvious from the standard view.