Renogy 40A MPPT keeps dropping to float way too early — anyone else had this?

Been scratching my head over this for a couple of weeks now. My Renogy 40A MPPT is pushing my 400Ah Fogstar Drift LiFePO4 bank to about 80% by early afternoon, then the controller decides it's done and drops straight into float at 13.6V. Batteries are nowhere near full — the Victron BMV-712 is telling me they're sitting at around 78–82% SoC every time this happens.

I've gone through the custom LiFePO4 settings more than once. Absorption is set to 14.2V, float at 13.6V, and I've got the absorption time set to 2 hours. My suspicion is the controller is using voltage alone to decide it's "finished" rather than properly tailing the current — classic cheap MPPT behaviour, but I'd hoped Renogy had sorted this by now.

The cabin load during the day is pretty light (a 12V fridge, a few USB points, maybe 40–60W total), so it's not like the batteries are being masked by heavy draw. Clear skies lately too, so the panels — three 200W Renogy monos wired 2S1P — are putting in good numbers.

Has anyone managed to dial this in properly, or is this just a known limitation of the Renogy MPPT range? Wondering if it's worth trying a longer absorption time or bumping the absorption voltage slightly, or whether this controller just isn't up to the job with lithium.

@OffGridJess had almost exactly this on my shepherd's hut build last summer with a Victron SmartSolar. Turned out my absorption voltage was set a touch low — controller thought it had finished when the battery hadn't really topped up properly.

Worth checking a few things on the Renogy:

• Absorption voltage — for Fogstar Drift, should be around 14.2–14.6V
• Absorption duration — is yours set to a fixed time or adaptive? Fixed can cut out way too early on a partially discharged bank
• Temperature compensation — if you've got a temp sensor, is it reading correctly?

Also, what's your actual resting voltage reading at that "80%" point? The Renogy's SOC estimate can be pretty optimistic. A proper voltage check after 30 mins off load would tell you a lot more.

Is this a recent change in behaviour or has it always done it?

@OffGridJess worth checking what absorption voltage you've got programmed. Renogy MPPTs can be a bit twitchy with LiFePO4 — if absorption voltage is set too close to your float voltage, the controller thinks the battery is full long before it actually is.

For Fogstar Drift cells I'd want absorption at 14.2–14.4V with a proper timed absorption period (at least 30–60 mins) rather than letting it drop out on current alone. The Renogy default current cutoff can be overly aggressive.

Also double-check your battery voltage sense — if the controller is reading at the terminals rather than the battery itself, voltage drop on your cables could be making it think the bank is fuller than it is.

I had similar grief on my shepherd's hut setup before I sorted the wiring properly. What cable run have you got between controller and battery?