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

Had this exact headache with my 100/20 last summer. Drove me absolutely mental for about three weeks before I figured it out.

Nine times out of ten it's the absorption voltage set too low or the absorption time cutting out early because the tail current threshold is being hit prematurely. Victron's default tail current setting (by default it's a percentage of battery capacity) can be overly optimistic if your battery's not quite what it says on the tin — looking at you, certain budget cells that shall remain nameless.

Few things worth checking in VictronConnect:

• Absorption voltage — are you running lithium or lead? Makes a massive difference. LiFePO4 should be around 14.2-14.6V depending on your BMS
• Tail current setting — try bumping it down slightly so it holds absorption longer before deciding the battery is "full"
• Battery temperature — if you've got a temp sensor fitted, a cold battery can trigger early float because the voltage reads artificially high

Also worth checking if your battery cable runs are properly sized. Voltage drop between the controller and battery can fool the MPPT into thinking absorption is done when the battery's nowhere near full. Classic rookie mistake, done it myself.

What battery chemistry are you running and what are your current absorption settings? That'd narrow it down considerably. Others on here have had good results tweaking the adaptive absorption settings — @GarethOffGrid sorted something similar last year IIRC.

Don't just factory reset and hope for the best, that never fixes anything, it just gives you a false sense of progress. 😅

@CamperSam mine did the exact same thing in the shepherd's hut — turned out the tail current setting was far too generous, so the Victron thought the battery was full when it had barely had a proper breakfast.

Worth checking your tail current % in VictronConnect — if it's set too high relative to your battery capacity, it'll bail out of absorption embarrassingly early like a bloke leaving a first date.

Fogstar Drift cells sorted me out once I actually dialled in the correct profile; the default Victron AGM settings are about as useful as a chocolate fireguard for lithium.

Great thread — this caught me out on my narrowboat last year too.

One thing nobody's mentioned yet: check your battery temperature. The Victron MPPT will automatically reduce absorption voltage if it detects high temperatures (or if you've got a temperature sensor giving dodgy readings). I had a BMV-712 temp sensor that had worked itself loose, and the controller was essentially getting phantom high-temperature readings and cutting absorption short every single time.

Also worth checking in VictronConnect — look at the history tab and see what your actual absorption time is logging as. If it's consistently very short, you'll get a clearer picture of whether it's a voltage threshold issue like @CamperSam mentioned, or a tail current thing like @OffGridHamish found. Saves a lot of guesswork. The app is brilliant for diagnosing this sort of thing remotely too.

@BoatShaun — fellow boater here, so I'm guessing you were about to mention battery temperature compensation? On the water, ambient swings between seasons are massive and the Victron's default charge voltages are calibrated for 25°C. Without a BMV temperature sensor feeding data back to the MPPT via VE.Smart Networking, you're essentially flying blind in winter.

I run a 702 BMV alongside my 75/15 on the cruiser and the difference after enabling VE.Smart was immediately obvious — absorption time extended properly and the batteries (Fogstar 100Ah LiFePO4) actually reached a genuine full state of charge.

Worth checking in VictronConnect that VE.Smart Networking is actually active and showing a live temperature reading, not just paired. Easy to miss that distinction.

Good shout from everyone here. One thing I'd add that hasn't come up yet — check your battery wiring and connections before you start tweaking software settings. I spent ages adjusting absorption voltages on my setup before realising a slightly loose negative terminal on the battery was causing voltage drop at the controller's sense point. The MPPT was "seeing" a higher voltage than the battery actually held, so it bailed out of absorption early thinking the job was done.

If your controller has a separate voltage sense wire (some do, some don't), make sure it's connected directly at the battery terminals rather than the busbar. Makes a surprising difference. @CamperSam and @OffGridHamish — worth ruling out before anything else really, saves a lot of head-scratching!

@StuCross makes a crucial point about connections — caught me out badly on my 32-footer last spring. But I'll add something slightly different: check your tail current setting in the Victron Connect app.

Most people never touch it, so it sits at the default 2A. If your battery bank is on the larger side, the MPPT interprets "charging current has dropped to 2A" as "absorption complete" and boots it into float way too early. I've got a 200Ah Fogstar Drift lithium setup and bumped mine to around 4% of battery capacity — made an immediate difference.

Worth downloading VictronConnect logs too. The graphs tell the whole story at a glance — you can literally see where it's bailing out prematurely rather than guessing blind.