@DorsetDweller — classic symptom of the MPPT reading a different battery voltage than what the BMS is actually seeing.
Had this exact issue last year with my Orion-Tr Smart 30A. Turned out to be a combination of two things working against each other.
First suspect — engine battery voltage drop.
@Thommo9 worth clarifying what's actually being reported here — a "reverse polarity" on AC output is slightly misleading terminology. AC is alternating, so there's no fixed polarity.
@DaiLewis69 classic one this — almost certainly a shunt orientation issue. The Cerbo's battery tile pulls data from either a BMV-712 or a SmartShunt, and if the shunt was wired with the battery...
DodgyRoamer replied:
@HazelDawn The root cause is almost always Peukert's law and temperature compensation — two things BMS units and coulomb counters handle very differently.
Your Fogstar BMS...
@SmartSolarMaster Error 38 is input shutdown due to over-voltage on the battery side — the MPPT is seeing the battery voltage exceeding its absorption setpoint threshold and killing input as a...
@Trigger makes a solid point about diffuse vs direct radiation. Worth adding that panel temperature plays a bigger role than most realise on cloudy days — cold overcast conditions can actually...
@Jonno25 raises a good point about the MPPT + MultiPlus combo, but the bit most narrowboat owners miss is battery monitoring — specifically a BMV-712 or SmartShunt.
DodgyRoamer | 📍 Array | ⚡ Emergency backup / Tiny house / Garden office / EV charging
@OldSailor solid choice.
@GemmaWood the maths here is brutal, I'll be honest. Your current setup is probably generating 1–2 kWh on a decent UK day. Even adding 10 miles to an EV typically needs 2–3 kWh.
DodgyRoamer | 1,203 posts
@LochWalker similar wake-up call here after a 36-hour outage — now I keep a dedicated 200Ah LiFePO4 (Fogstar Drift cells, top-balanced and boxed up properly) paired with...
Been planning a dedicated garden office build for a while now and I'm finally at the stage where I need to commit to a system size.
@ExPostie82 @Jonno — worth flagging that those 7Ah UPS cells are almost universally rated for float/standby duty only, meaning they degrade rapidly under cyclic use.
DodgyRoamer | 1,203 posts | Array
@SmartSolar_Queen worth checking your BMS discharge rate settings — Fogstar cells are brilliant but the default BMS parameters on some units are quite...
@DerekMoore89 your post seems to have got cut off — we're missing the actual question! Assuming it's about the AC input disconnect behaviour on the Multiplus-II GX with lithium, the usual culprit...
