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

Had this exact same faff with my 100/30 last summer. Drove me absolutely mental for about two weeks before I figured it out 😅

Turned out my battery voltage sense was reading slightly high because of a dodgy connection on the sense wires — controller thought the bank was fuller than it actually was and kept cutting to float early. Cleaned up the terminals, re-did the sense wires properly and it sorted itself out overnight.

Few things worth checking:

• Battery type settings — easy to accidentally have it on a generic profile when your lithium/AGM needs something more specific
• Absorption time — if you've got "adaptive absorption" on, it'll shorten the time based on previous cycles, which can catch people out
• Temperature compensation — if you're running a temp sensor, a knackered one can skew the readings badly
• State of charge history — sometimes the MPPT just needs a proper full charge cycle to re-calibrate itself

What batteries are you running? If they're Fogstar or similar lithium, worth double-checking your charge profile against their recommended settings. Some of the cheaper cells are quite fussy about absorption voltage tolerance.

Also — are you using VictronConnect to monitor it in real time? The history graphs are genuinely useful for spotting patterns. Mine showed the voltage spiking right before float kicked in, which is what pointed me toward the sense wire issue.

Might also be worth checking if there's a firmware update pending. Victron pushed a few MPPT fixes last year that sorted some odd float behaviour people were seeing.

What does your absorption voltage look like when it drops? That'd help narrow it down.

HappyWelder replied:

@LK_Solar — classic one this! I'd add that loose or corroded connections between the MPPT and battery can cause the same symptom. The controller sees a slight voltage spike under light load and thinks "job done, time to float."

Worth checking your cable terminations are properly crimped and torque-tight on the terminals. Had exactly this on my 150/35 — looked fine visually but there was enough resistance in a dodgy lug to throw the voltage reading off by nearly 0.3V. That was enough to kick it into float an hour or two earlier than it should've.

Also double-check your absorption time settings in VictronConnect if you're running AGM. Sometimes the defaults are surprisingly conservative straight out of the box.

@LK_Solar and @HappyWelder have both touched on the voltage sensing and connection side, but there's another culprit worth checking — the absorption time algorithm on the Victron MPPT is adaptive by default.

If your battery hasn't been deeply discharged, the controller deliberately shortens absorption. That's intentional behaviour, not a fault. People mistake this for premature float constantly.

In VictronConnect, check your absorption time under the charger settings — if it's set to "adaptive + BatteryLife" you may want to switch to a fixed absorption time appropriate for your battery chemistry. For LiFePO4 particularly, a fixed 2-hour absorption (or even shorter) with a lower tail-current trigger is often far more appropriate than letting the algorithm decide.

On my narrowboat setup with a 200Ah Fogstar Drift, I run fixed absorption at 14.2V for 1 hour — works a treat.

Something worth adding to what @LK_Solar and @HappyWelder have covered — check your absorption time settings if you're running the fixed algorithm rather than adaptive. On the Victron MPPT app, default absorption time can be surprisingly short depending on how you've set it up, and if the battery hasn't actually reached full charge the controller just shunts to float anyway once time's elapsed.

Also worth checking: what battery type have you got it set to? I had mine on AGM preset for a while before I switched to Fogstar lithium — the voltage thresholds are quite different and getting them wrong means float kicks in prematurely even when SOC is still low. Use the custom settings and match your battery manufacturer's recommended charge profile exactly rather than relying on presets.

VictronConnect makes all of this pretty straightforward to diagnose via the history tab.

WattCharlie replied:

Good thread. One thing nobody's mentioned yet — check your battery temperature compensation settings in VictronConnect. If you've got a BMV or SmartShunt with a temp sensor, and the battery is reading warmer than ambient (common in a confined space like a shepherd's hut install), the MPPT will deliberately lower the charge voltage thresholds. Looks exactly like premature float.

Caught me out badly on my hut setup last spring. Batteries sitting in a warm cabinet, MPPT thought they were "full" at what looked like an oddly low voltage. Disabled temp compensation temporarily to confirm the diagnosis, then repositioned the sensor away from the battery terminals.

Worth cross-referencing your VRM portal history too if you're on a networked setup — the absorption duration logs will tell you pretty quickly if something systematic is happening.