Reduced Power - MG UK Deny Fault.

JodyS21

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Is it possible with an OBD App to measure the individual cell voltage under full power at low SOC?
My car does not have this problem, what power are others getting with a low SOC?
You can see each cell packs min and max voltage, 9 packs each containing 12 cells.
 

Cocijo

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I’m sorry if i’ve missed the point here - and stand to be corrected - when getting in a car with the low HV battery warning light illuminated and a low HV battery that has stood overnight, are you not alerted to the fact that power may be limited by the very fact the battery is so low and that you need to charge asap? To jump in, ignore the warning and then be surprised and say it’s dangerous because power is limited and you’ve had no warning seems illogical. I had to leave my car overnight with a low battery and not charge a few months ago. Next morning I took it very steady with the warning light in and 20miles range predicted to get to a charger - the last thing on my mind was full power!
 

JodyS21

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The last time the dealer had the car the imbalance result was 0.006mv across the pack and it was the best they had ever seen, so not a BMS problem.
I know the dealer has never tested my car but they did their own test in another MG ZS EV and the result was the same and this does suggest it's more a generic problem.
Just a thought on this. Was the good (low) imbalance reading taken when the car was at the low soc & the problems were experienced?
As an imbalance reading taken at full charge, would be very different to what it would be at other moments in time/other states of charge.
 

WAC 768

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I’m sorry if i’ve missed the point here - and stand to be corrected - when getting in a car with the low HV battery warning light illuminated and a low HV battery that has stood overnight, are you not alerted to the fact that power may be limited by the very fact the battery is so low and that you need to charge asap? To jump in, ignore the warning and then be surprised and say it’s dangerous because power is limited and you’ve had no warning seems illogical. I had to leave my car overnight with a low battery and not charge a few months ago. Next morning I took it very steady with the warning light in and 20miles range predicted to get to a charger - the last thing on my mind was full power!
Do you consider 25% remaining to be low SOC that's a 1/4 of the overall battery, so effectively from your comments you are happy with a car that performs perfectly fine for 75% that's equivilant to a 33KW battery... I agree why would you need full power when the first warnings come on? Well should you ever never to need pull out in winter only to find there's only 30% power with a car coming the other way then i think you would change your mind!!
Lastly, the car at 8% SOC and after i had driven the car for a few hundred miles gave 100% power, so why not at 25% when the same warning was present?
 

BarryH

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I think there are some very good observations above from both @Cocijo and @JodiS21. I would ad that battery chemistry at the end of c100 miles will be very different to that of the battery having rested overnight. The internal temperature has an impact on the ability of the battery to deliver a high current. I'd also want to think about the Balancing of the battery. The car can't be charged at home overnight as I understand it. I know that there is a small difference in cell voltages but in my experience, even if I should need only 3 to 4 hours to get full charge it does make a difference to leave the car on a 6-hour charge once a month. The extended charge at around 300watts after the 7kW charge to full really pays dividends. Maybe if you're not able to charge at home you're not able to let it fully balance out across the pack. I the whole pack doesn't fully balance then the cell with the lowest share is the limiting factor when you demand power at low SoC
 

MEH

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I had an Astra years ago, 6,000 miles from new had to have a new engine due to the distributer seizing to the distributer shaft due to water leaking on it and the distributer damaged the mounting beyond repair (according to the dealer), this despite being recalled a few days earlier for a distributer modification

Make your own mind up as who is pulling the wool over whose eyes
 

Popsie5859

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I enjoy reading all these posts especially as I am a brand new electric vehicle owner, please could someone just explain the abbreviations being used 'SOC GOM BMS' I do know about OBDI so that the posts become more meaningful for me thanks.
 

SRMiller07

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Hi, Newbie here. Did you every get to the bottom of this as I seem to be having a very similar experience with my ZS where it will limit/reduce power without any warning. It usually happens when down to 2 bars but this morning when I did the school run it was at 3 bar and still limited power. It's a bit worrying, and have been caught out on this, as I don't find out I have limited power until I pull out onto the main road which is a 60 and slightly up hill. I then find out I have a max 60-65% power.
I try to leave the car plugged in over night once a month to balance but can't believe that it doesn't tell you when it drops. Even happens when I have been driving for a while, get to 2 bar and then drops power. This is usually a few miles before the low battery light comes on.
We have a Zoe and that doesn't limit the power at all when either cold or low (granted I'm usually driving slow anyway when it's flashing red ;-) )
I don't know if I should take this up with the dealership or what?
Cheers
 

BarryH

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Hi, Newbie here. Did you every get to the bottom of this as I seem to be having a very similar experience with my ZS where it will limit/reduce power without any warning. It usually happens when down to 2 bars but this morning when I did the school run it was at 3 bar and still limited power. It's a bit worrying, and have been caught out on this, as I don't find out I have limited power until I pull out onto the main road which is a 60 and slightly up hill. I then find out I have a max 60-65% power.
I try to leave the car plugged in over night once a month to balance but can't believe that it doesn't tell you when it drops. Even happens when I have been driving for a while, get to 2 bar and then drops power. This is usually a few miles before the low battery light comes on.
We have a Zoe and that doesn't limit the power at all when either cold or low (granted I'm usually driving slow anyway when it's flashing red ;-) )
I don't know if I should take this up with the dealership or what?
Cheers
Simple answer is to take it to your dealership. You could possibly get low power when the battery is in a low SOC and it is very cold until you get some heat into the battery but not with 2 to 3 bars! I have only ever had the warning saying to effect don't hammer it your battery is too low for full power or words to that effect warning that I shouldn't accelerate hard on two occasions when I've been on 1 flashing bar. Then when I pulled into a rapid shortly after I had 5% or less.
 

SRMiller07

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Simple answer is to take it to your dealership. You could possibly get low power when the battery is in a low SOC and it is very cold until you get some heat into the battery but not with 2 to 3 bars! I have only ever had the warning saying to effect don't hammer it your battery is too low for full power or words to that effect warning that I shouldn't accelerate hard on two occasions when I've been on 1 flashing bar. Then when I pulled into a rapid shortly after I had 5% or less.
Hi cheers for this. It does happen more when its cold. Think I'll run it down to the dealer and have a chat with them about it. If it was part of the design then fine but at least notify the driver that the power has been limited (bit like when the battery is full and warns you about the lack of regen).
Thanks again.
 

BarryH

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Hi cheers for this. It does happen more when its cold. Think I'll run it down to the dealer and have a chat with them about it. If it was part of the design then fine but at least notify the driver that the power has been limited (bit like when the battery is full and warns you about the lack of regen).
Thanks again.
How long have you had this car? There was a bad Battery Management System (BMS) update applied to some ZS cars at the tail end of 2020 and that would cause the battery not to balance and progressively limit power and range. Some cars were badly affected and all cars affected ought to have been recalled for a correction but after the correct BMS update, it could take many many daily balancing charges to correct the balance in the car's battery. Balancing, also called "Equalising", only happens on AC charging by 7 kW Charger or Granny Charger and Never on a DC Rapid. The car has to be charged to 100% and then the Equalising kicks in and it will trickle charge at about 300 - 600 watts. This continues until either a certain number of mV difference in cell voltage is reached or 6 hours have elapsed whichever is the sooner. I forget exactly how many millivolts it can correct for but it isn't very much. The process charges only the cells with the lowest voltage to bring them closer to the normal voltage. You see if one cell of the full pack ( ie. 108 cells in ZS Gen 1 pack) is particularly low this cell will limit the lowest the pack can discharge and hence would flag up low Battery and would define the car's range.
 
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Lovemyev

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It has been strongly suggested at the time when this problem first raised it's ugly head, that it was likely that a LOT of the original ( Gen1 ) ZS EV's still being driven, with the "Buggy" BMS software affecting their cars unnoticed, and low usage owners are totally unaware of the problem.
The idea was, that ANY of the affected cars, would get picked up at the point of the regular service intervals.
But we are clearly aware now that some of these affected cars, have simply slipped through the dealer network.
There was no blanket recall made of these cars at the time.
Just a service bulletin sent to all dealers advising of the condition and action applied should a customer present with issues of range loss & reduced power levels at a lower level of remaining range.
Low usage owners where therefore less likely to recognise the symptoms early on, until the cars started displaying some strange behaviour caused by the "Buggy" BMS update.
 

Tim Green

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I have a fairly early car, delivered January 2020. I have never experienced any reduction in power at any SOC, and I have run the battery down to virtually nothing.
I think there must be a fault somewhere if you get reduced power. It could be some protection device operating to prevent a system shutdown. Similar to "get you home" mode in ICE cars.
 

SRMiller07

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Hi, I got the car in September 2020 and when it had it's first service last year I had asked them to update the BMS as my car has to be locked to charge on the AC chargers. They said it was all updated but my car still has to be locked to charge on the 7kw chargers. I have a home charger and when it is plugged in it's left over night so does get a "balance" 1-2 times a month which gets it up to 454-455v. Again I believe with the newest BMs they lowered the top end to get the regen to kick in sooner? I think it's clear that the car is on an iffy BMS so I'll get on to the dealership to get it updated. It's nice to know it's not just in my head! Thanks all and I'll try and remember to post an update if this is resolved!
 
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JohnInFrance

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If you firmly believe that the car is at fault you should engage an independent qualified engineer to liaise with your MG dealer and review the advice they have recieved from MG UK.

The engineer's report will then provide you with a clear direction in which to take your issue - if the engineer agrees with MG then the issue would be closed; in the event the engineer disagrees then you have the basis for leverage with MG to either repair or reject the vehicle or to begin legal proceedings (which should always be a last resort).

Or you could just sell it - with the delivery delays I'm sure that it would go for top-dollar anyhow (is that allowed in the UK if there was a PICG against it?).
 

Kithmo

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Hi, I got the car in September 2020 and when it had it's first service last year I had asked them to update the BMS as my car has to be locked to charge on the AC chargers. They said it was all updated but my car still has to be locked to charge on the 7kw chargers. I have a home charger and when it is plugged in it's left over night so does get a "balance" 1-2 times a month which gets it up to 454-455v. Again I believe with the newest BMs they lowered the top end to get the regen to kick in sooner? I think it's clear that the car is on an iffy BMS so I'll get on to the dealership to get it updated. It's nice to know it's not just in my head! Thanks all and I'll try and remember to post an update if this is resolved!
You haven't got the latest BMS, but definitely haven't got the "iffy" BMS if your voltage when full is 455v, it would be much lower.
I suspect it's a fault with the system software, not the BMS.
 
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