Sudden SOC drop on my Mk2 ZS EV SR

[Tim29]

Hello guys. So this happened to me twice since last week, left the car with 51% remaining yesterday, came to drive today and it shows 37%. The first time occurs the same scenario, 50% SOC remaining then surprised to see 36%, that’s 14% lost parked overnight.
At first I thought it must have something to do with the cold weather but today wasn’t that cold.
Strangely, when I charge the car at home (7kw) after it reached 85%, it suddenly went to 100% after few mins, it seems the missing 14% has been added.. It’s really bizarre.. twice in a row. Anyone had the same experience or can exaplain this odd behavior? Thanks guys. The car is great all around though?

 

[N2STY]

Been talk about on here quite a bit probably the biggest down side of LFP batterys the BMS just can't cope with that flat Curve

 

[N2STY]

Basically the standard range voltage is the same at 30% as it is at 70% so the car is just guessing state of charge so needs a 100% starting point to be more accurate View attachment 12093where the long range the voltage slowly drops which gives more accurate state of chargeView attachment 12094

@MTS this explains it well

 

[Tim29]

Thanks @N2STY. So in conclusion, with the LFP battery this is completely normal?

 

[N2STY]

It's not so much the battery but the BMS has trouble seeing charge state if say the battery was warm when you parked up then cooled down a bit and losers 0.1v BMS has new reference voltage ie 30 ish %
30 to 70 looks the same on BMS that's why it likes a starting point of 100%

 

[Coulomb]

While providing a good estimation of SoC with an LFP battery is harder, it seems that MG/SAIC could do a much better job than they do at present. It's basically coulomb counting, plus a long term estimation of SoH. Not rocket science.