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

Had this exact saga with my 100/30 last summer. Spent three weekends convinced the controller was faulty before I finally cracked it.

Turned out my battery voltage sense was reading slightly high — a dodgy connection on the sense wires was adding maybe 0.2V of phantom resistance, which was enough to trick the controller into thinking absorption was done. It would sail into float around midday with the batteries barely at 80%.

A few things worth checking in your situation:

• Absorption voltage setting — what have you got it set to? Factory defaults on Victron can be conservative depending on your battery chemistry
• Tail current threshold — in VictronConnect, check what percentage you've got this set to. If it's too high, it'll cut absorption short thinking the battery is full
• Temperature compensation — if you've got a BMV or battery sensor, a warm battery will show higher voltage and trigger float sooner

My setup is Fogstar Drift lithium paired with a Victron SmartSolar, and once I sorted the sense wires and dialled the tail current down to around 2%, it behaved perfectly. Been solid ever since.

Worth connecting via Bluetooth and watching the live data during a charge cycle — you can actually see the moment it drops and correlate it with what the readings show. That's what finally gave me the answer rather than just guessing blind.

What battery chemistry are you running? That changes the diagnosis quite a bit. Lithium and AGM behave very differently in this situation and the fix is rarely the same.

@KentVanLifer had this exact same head-scratcher with my shepherd's hut setup. Turned out to be a loose negative connection at the busbar causing a slight voltage drop in reverse — controller was seeing a falsely elevated voltage and calling float early.

Worth checking your cable sizing too. Undersized wire between controller and battery creates enough resistance that the MPPT thinks it's hit absorption target before the battery actually has. Victron's own wiring calculator is pretty handy for checking this.

Also — if you're running a BMV-712 alongside it, enabling DVCC in VictronConnect and letting the battery monitor handle voltage sense rather than the MPPT itself sorted it completely for me. Night and day difference.

@KentVanLifer interesting — how far off was the voltage reading vs actual? I'm running a 100/20 on my motorhome with a 200Ah Fogstar Drift LiFePO4 and I've noticed it's been hitting float earlier than expected lately too.

Before I start chasing wiring issues like @HeatherGazer found, I want to rule out the absorption time settings first. In VictronConnect, are you using the adaptive absorption option or fixed? I've got mine on adaptive and wondering if that's part of the problem — the algorithm might be cutting short if it detects what it thinks is a charged battery.

Also, what are your tail current settings set to? I haven't touched mine from default and I'm starting to wonder if that's the culprit rather than anything physical.

Classic Victron rabbit hole — worth also checking your battery temperature compensation settings in VictronConnect, because if the BMS or temp sensor is feeding it slightly wrong data, the controller happily decides your batteries are "full" when they're sitting at 80% SoC.

@BevJackson64 on a motorhome I'd also verify the cable run length between your MPPT and battery — voltage drop across a long or undersized cable means the controller sees a falsely elevated voltage at its sense terminals. Fogstar recommend 6mm² minimum for most 100-series Victron installs at anything over a couple of metres, and it genuinely makes a difference.

Quick sanity check: enable the VRM portal or just watch the history tab in VictronConnect — if your daily Ah absorbed looks suspiciously low versus panel capacity, premature float is almost certainly your culprit.

Good shout from @MarineGeoff on the temp compensation — that caught me out on my boat setup. I'd also add: check your absorption time settings in VictronConnect. If you're running lithium (Fogstar Drift in my case), the default absorption time can be absurdly short, so the controller hits the voltage threshold and bails out early thinking it's done.

Also worth checking your cable runs for voltage drop — if the MPPT is sensing at the controller terminals rather than at the battery, any resistance in the wiring means the battery never actually hits the real target voltage. Victron's own wiring guide recommends keeping that cable run as short as practically possible for exactly this reason.

Running a dedicated sense wire directly to the battery terminals fixed a similar issue for me.

@KentVanLifer had this exact same phantom absorption issue on my cabin setup — ended up being a dodgy crimped connection between the battery terminal and the Victron's sense wire, adding maybe 0.3V of false reading, which is enough to trick the 100/30 into thinking it's done when it's barely started.

Grab a proper multimeter and measure directly at the battery terminals while VictronConnect shows absorption — if those numbers disagree by more than 0.1V you've found your culprit.

Also worth checking: if you're running Fogstar or similar lithium, make sure your absorption time isn't set absurdly short — Victron defaults can be quite conservative and some cheaper BMS units report full charge voltage prematurely.