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

Had exactly this with my shepherd's hut setup last summer. Victron SmartSolar 100/30, Fogstar Drift lithium battery — kept hitting float after barely two hours of bulk charging. Drove me absolutely mad.

Turned out the culprit was the battery voltage rising faster than expected because I'd undersized my load during charging. The MPPT was seeing the voltage tick up, thinking "job done, lads," and switching to float prematurely. Classic case of the controller being technically correct but practically useless.

Few things worth checking in your situation:

• Tail current setting — if it's too high, the controller thinks absorption is complete before the battery's actually full. Drop it down to around 2-4% of your battery capacity.
• Absorption voltage — worth confirming it matches your battery manufacturer's spec exactly. Even 0.2V off can cause early dropout.
• Temperature compensation — if you've got the SmartSolar pulling temperature data from somewhere inaccurate, it'll skew everything.

The VictronConnect app is genuinely brilliant for diagnosing this. Pull up the history graph and you'll see exactly how long it's spending in each charging stage. Mine showed embarrassingly short absorption periods before I sorted it.

Also — are you running a shared battery sense wire or relying on the MPPT's internal voltage reading? Internal readings can be optimistic depending on cable runs.

What battery chemistry are you working with? Lead-acid and lithium behave quite differently here and the fix changes accordingly. Curious whether others have hit this with different controller models too — I know the Renogy Wanderer crowd sometimes reports similar quirks.

@CopperWelder welcome to the forum, great first post — nothing like a charging gremlin to get you stuck in! 🔧

Classic gotcha with Victron + lithium — the default battery profile is almost certainly set up for AGM. Nip into the VictronConnect app and check your absorption voltage and tail current settings. If tail current is too high, it'll boot you to float embarrassingly early.

Had the exact same head-scratch on my boat last year. Switched to a custom lithium profile, dropped the tail current right down (I use 2% of battery capacity) and bulk charging went from a sprint to a proper marathon.

Your Fogstar Drift should have recommended charge parameters in the manual — worth cross-referencing those directly into the custom profile. Game changer! 🚀

@CopperWelder check your absorption voltage threshold — if it's set too low, the Victron thinks it's done when it's barely started, bless its little heart.

@CopperWelder had this exact issue on my narrowboat last year — turned out to be the tail current setting rather than absorption voltage.

Fogstar Drift cells have a relatively low internal resistance, so they accept charge efficiently and the tail current drops off fast. If your Victron's tail current is set at the default ~4% of battery capacity, it'll terminate absorption prematurely with lithium.

In VictronConnect, go to Expert Mode and drop the tail current to 1–2% of your battery's Ah rating. Also worth checking:

• Absorption time set to adaptive not fixed
• Ensure BMS charge enable signal isn't cutting in early
• Check battery temperature — cold cells behave oddly

The Fogstar Drift documentation does specify recommended charge parameters; worth cross-referencing with what you've actually programmed into the MPPT. Nine times out of ten it's a configuration mismatch rather than a fault.

@LH_Marine that's interesting about tail current — what did you end up setting yours to?

I'm dealing with something adjacent on my garden office setup (SmartSolar 150/35, Fogstar Drift 200Ah). Mine was dropping to float early and I eventually traced it back to the battery temperature sensor not being connected properly, so the MPPT was applying default temperature compensation and skewing the voltage thresholds completely.

Worth checking in VictronConnect whether your temperature readings look sane — if it's showing ambient as something wildly off, that could be throwing everything out. Also, are you using the battery's own BMS comms or just standalone voltage sensing? My Fogstar behaves very differently depending on which way I configure it.

Has @CopperWelder checked whether the issue is worse on overcast days versus full sun? That helped me narrow down whether it was a voltage or current-based trigger causing the early cutoff.

@LH_Marine is on the right track with tail current, but worth checking one more thing first — the battery temperature compensation setting.

If you're running Fogstar Drift cells (or any LiFePO4), temperature compensation should be disabled entirely. I had a similar issue in my static caravan setup where the controller was compensating downward on cooler mornings, which compressed the absorption window dramatically.

In VictronConnect, go to the battery settings and set temp compensation to 0 mV/°C.

Also double-check your absorption time is set to a fixed minimum rather than adaptive — adaptive can be very aggressive on partially discharged lithium banks where the resting voltage is already fairly high when charging begins.

Both settings together sorted mine completely. The Victron documentation on this is actually decent if you search "LiFePO4 settings guide" on their site.