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

Had exactly this on my narrowboat last summer — drove me absolutely mad for weeks before I figured it out.

The culprit for me was the tail current setting. By default Victron sets it at 1A, which sounds reasonable until you realise that on a large battery bank, that's nothing. My 200Ah Fogstar lithium would hit that threshold while still sat at 95% and the controller would smugly declare it fully charged and drop to float.

Worth checking a few things:

• Tail current — bump it up to around 2-4% of your battery capacity
• Absorption time — if you're running adaptive absorption, it can cut short on partially discharged days
• Battery temperature — colder mornings can skew the voltage readings and trick the MPPT

On the boat I ended up going into VictronConnect and tweaking the absorption voltage up slightly and setting a fixed absorption time rather than adaptive. Transformed how the system behaved, especially through those grey October days when the panels were working hard just to scrape anything useful.

One other thing — are your battery cables properly sized? Undersized cables create enough voltage drop that the controller thinks the battery is fuller than it actually is. Classic gotcha that catches people out.

What size bank are you running and which MPPT model? The SmartSolar 100/30 and the larger 150/100 behave a bit differently in my experience. Would help narrow it down.

@KangooAdventure interesting — on my static setup I had a similar symptom but different root cause.

Worth checking your battery voltage sense as well. If the MPPT is only sensing at its own terminals rather than at the battery, voltage drop across your cabling will fool it into thinking the battery is fuller than it actually is. It'll hit the absorption voltage threshold early and start the float countdown prematurely.

Victron's own guidance suggests enabling the remote voltage sense wire if your cable run is more than a metre or two. Made a measurable difference on my install once I sorted it.

Also — what battery chemistry are you running? LiFePO4 users sometimes inadvertently import absorption profiles designed for AGM, which have completely wrong tail current and voltage parameters out of the box.

Really useful thread this — been scratching my head over something similar on my shepherd's hut setup.

@KangooAdventure did you find the tail current fix held up long-term? I'm wondering whether battery chemistry plays into it too — I swapped from AGM to a Fogstar Drift LiFePO4 last spring and had to completely revisit my absorption/float settings afterwards. The charge profile is just so different.

@DucatoProject curious what your root cause turned out to be on the static setup?

One thing worth checking that nobody's mentioned yet — are your battery temperature readings accurate? I've got a Victron Smart Battery Sense dongle and discovered mine was reading a couple of degrees high, which was causing the controller to back off earlier than it should. Easy fix once you spot it, but took me ages to think of checking it.