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

Had exactly this with my 100/30 on the boat last summer. Turned out the tail current setting was the culprit — it was set too high, so the controller was interpreting normal absorption behaviour as "battery full" and bailing out early.

Worth checking a few things in VictronConnect:

• Tail current — default is often 2A, but if your battery bank is small this triggers float prematurely. Try dropping it to 1% of battery capacity
• Absorption time — are you on fixed or adaptive? Adaptive can go short if the previous day's discharge was shallow
• Battery voltage sense — if you're using the controller's internal sense rather than a dedicated BMV or SmartShunt, voltage drop across cabling can fool it into thinking the battery is fuller than it is

My setup is a pair of 200W panels into a 200Ah Fogstar Drift LiFePO4, and once I sorted the tail current and wired in a SmartShunt for proper voltage sense, it stopped cutting to float after 20 minutes. Night and day difference.

Also — are you in a network with other Victron kit via VE.Smart Networking? If the MPPT is receiving voltage from a BMV that's mounted right at the battery terminals versus somewhere with resistance in the circuit, you can get conflicting readings that cause odd behaviour.

What firmware version are you running? There were some quirks in older builds around adaptive absorption. Victron's been pretty good with updates but it's worth checking.

What does your absorption voltage actually reach before it drops? That'd help narrow it down — post your VictronConnect screenshots if you can.

Davo2 | 312 posts

@LakelandNomad yes, tail current catches a lot of people out — worth adding that the battery temperature plays into this as well. If your sensor isn't fitted (or fitted poorly), the controller might be compensating incorrectly and hitting float thresholds sooner than it should.

Also worth checking your absorption voltage setting against your battery manufacturer's actual spec rather than the Victron default. I've seen the defaults sit slightly low for certain AGM brands, which means the battery technically "satisfies" the tail current condition before it's properly full.

If you're running lithium, that's a whole different conversation — the absorption phase behaves quite differently and folk often import profiles that aren't quite right for their specific cells.

What battery chemistry are you running?

Good point from @Davo2 on tail current, but I'd add that people often overlook the absorption voltage setting being incorrect in the first place. If your absorption voltage is set too low, the battery reaches it quickly, tail current drops off rapidly, and the controller assumes it's done — when it hasn't properly charged at all.

Worth double-checking your battery manufacturer's exact specifications rather than relying on Victron's presets. In my experience with Fogstar Drift lithium cells on the motorhome, the default profiles are close but rarely spot-on. A 0.1V discrepancy in absorption voltage makes a measurable difference to actual state of charge.

Also — are you monitoring via Victron Connect? The history tab will show you exactly how long absorption is actually running per cycle, which helps isolate whether this is a voltage or tail current issue.

FrostyTinker | 847 posts

Great thread — one thing nobody's mentioned yet is checking your cable losses between the MPPT and battery. If there's significant voltage drop across dodgy connections or undersized wire, the controller "thinks" the battery has reached absorption voltage earlier than it actually has, which can trigger an premature transition to float. Worth measuring the actual voltage at the battery terminals with a multimeter whilst the controller is running and comparing it to what VictronConnect is reporting. A bigger discrepancy than about 0.1V suggests you've got a wiring issue worth sorting before tweaking any software settings. Fixed this exact problem on a friend's narrowboat last autumn — turned out to be a loose lug on the busbar.

PanelDai | 203 posts

Good shout from @FrostyTinker on cable losses — that's caught me out before too. One thing I'd add that hasn't been mentioned: double-check your battery capacity setting in VictronConnect if you're using the adaptive absorption algorithm. If it thinks you've got a smaller bank than you actually have, it'll calculate a shorter absorption time and bail out early regardless of what your tail current is doing. Had this exact scenario with a 200Ah lithium build last spring — spent ages chasing the wrong settings before spotting the battery capacity field was defaulted to something daft. Worth a two-minute check before diving deeper into the other parameters.