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

Had this exact issue with my setup in the tiny house last summer and it drove me absolutely mad trying to figure out what was happening.

Mine turned out to be the battery temperature sensor giving daft readings — the MPPT was seeing inflated temps and throttling back to float to protect the batteries. Worth checking if yours has a temp sensor attached and whether it's reading correctly in the VictronConnect app.

A few things worth looking at:

• Absorption time settings — is yours set to a fixed time or adaptive? Adaptive can sometimes be overly cautious with smaller battery banks
• Battery voltage calibration — are your cables causing enough voltage drop that the controller thinks the bank is fuller than it actually is?
• Which firmware version are you running? There were some known quirks in earlier versions that affected the charge algorithm

What battery chemistry are you using? LiFePO4 behaves very differently to AGM and the default Victron profiles don't always suit every Fogstar or similar cell setup out of the box. I ended up creating a custom charge profile and it sorted things considerably.

Also — what's your actual state of charge when it drops to float? Are you monitoring through a BMV or Cerbo, or just going off the controller itself? The controller alone can be misleading without proper battery monitoring in the loop.

Would be curious whether others have seen this tied to specific panel configurations too — I've wondered if partial shading triggers something weird in the algorithm.

Chris1991 | 47 posts

@ZoeRoss sounds frustrating! I had something similar last year with my 100/30 on a 200Ah leisure battery bank. Turned out my absorption voltage was set slightly too low in VictronConnect — the battery was technically hitting the target voltage but nowhere near fully charged because of voltage drop across a dodgy connection.

Worth checking your wiring between the controller and battery terminals with a multimeter whilst it's under load. Even a small resistance can fool the MPPT into thinking absorption is complete when it really isn't.

Also double-check your absorption time settings — if you're on the fixed time option rather than adaptive, it might just be timing out too quickly. I switched mine to adaptive and it made a noticeable difference straightaway.

What battery chemistry are you running?

SussexVanLifer | 312 posts

Worth checking your absorption voltage setting against what your battery manufacturer actually specifies. Classic mistake I made early on with my van build — Victron defaults are conservative and if your absorption voltage is set too low, the controller thinks it's done the job when the battery's still half hungry.

Also, what's your absorption time set to? In VictronConnect you can switch from adaptive to fixed absorption time. Adaptive can be fooled on partially discharged batteries.

Mine's a 100/50 running Fogstar Drift lithium — took me an embarrassing amount of faffing before I realised I'd left it on the AGM preset. Check your battery profile in VictronConnect before anything else. Takes two minutes and rules out the obvious culprit.

What battery chemistry are you running?

Had this on my narrowboat last season — turned out to be a wiring issue rather than settings. Voltage drop between the batteries and the MPPT meant it was reading a higher voltage at the controller than what was actually at the battery terminals. So it thought absorption was done when it wasn't.

Worth checking with a multimeter — measure voltage directly at the battery terminals and compare to what VictronConnect is showing. If there's more than about 0.2V difference you've likely got resistance somewhere in the cabling or a dodgy connection.

Also make sure you've got the battery voltage sense wires connected if your model supports it — that fixed it properly for me. The 100/50 I run now reads bang on once I sorted that out.

SophieFisher | 203 posts

@Jason1970 raises something worth dwelling on — voltage drop is exactly the sort of thing that looks like a settings problem but isn't.

On my narrowboat I spent the better part of a fortnight convinced my Victron SmartSolar 100/50 was misbehaving, only to discover the sense wires weren't connected to the battery terminals at all. The controller was reading voltage at the busbars, which under any meaningful charge current was consistently optimistic by nearly 0.4V — enough to trigger float prematurely every single time.

The fix took about twenty minutes once I'd actually diagnosed it properly.

If your MPPT has dedicated battery voltage sense terminals (the 100/50 does), use them. Run them directly to the battery posts, not the busbar. It's in the manual, admittedly not prominently.