Reduced Power - MG UK Deny Fault.

WAC 768

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Long story but please read this could be applicable to all MG ZS EV….

Within the first 30 days of ownership my new car, I noticed that at low SOC <25% the power of the car would not go above 30% (+ 2C) when going from 10mph and up a very small incline. I immediately informed my dealer who at the request of MG did a software reload / update. After this, the problem was still present and after a further 3 BMS update (batteries fully balanced) it remained but with varying levels of reduced power, it was getting warmer.…...

By this time, I had owned the car for around 3 months and to support my dealer, I had also raised this to MG UK stating the car was not performing ‘as designed’. Initially, MG UK customer services replied promptly to my emails, but that was only for a short period of time and the more I stated the car was not ‘Fit for purpose’ the communication from them stopped. Eventually though after my dealer had exhausted everything, they could do under the advice of MG UK and asking them to confirm, I finally received this.

“I wanted to touch base with you as I am now aware of the outcome of the technical discussions between the MG technical team and your dealership. The outcome of all testing and discussions is that there are ‘no faults’ and the vehicle is performing as expected. The questions you raised have been answered with the information that we hold and has been verified by our technical team in the process. I am aware that you may be disappointed with the outcome, but I am wanted to write to advise that the request for rejection is not supported by your dealer and MG agree with this.”

Now clearly, MG UK clearly believes that a ‘Fault’ code must be generated if there is something wrong otherwise everything is OK. Having worked in the automation industry for many years I know this is not always the case, and I don’t believe the recent BMS problem shown any fault’s and moreover, it was the reduced range that triggered this update?

MG UK has stated this reduced power output is a “design characteristic” so that you have enough charge to get you to a charging point, but I disagree and here’s why and which I have shared these tests with MG UK but they never responded….

Scenario 1

First battery charge warning appears @ 25% and car is parked overnight within a 5 - 10 miles of this coming on.
Next morning the maximum power the car will provide is between 30 – 60% (Very much outside Temperature dependant) – SOC at charging point 18%

Scenario 2

First battery change warning appears @ 25% after a constant drive of circa 100 mile and driven down even to as low as 8% SOC, 100% power is available.

Now if the power is restricted by design, then clearly, I you would see the same in Scenario 2?

Most of you may never notice this if you charge at home every night or use rapid chargers after a long journey, but for me and like many others I only use public chargers and leave my car overnight on low SOC. It is simply unacceptable of not knowing how much power the car will provide without warning and more importantly very dangerous.

So, to summarise and what I have learned from this is that MG UK don’t really care on what the customer says, and I should have only allowed them to try and fix the car once and rejected it then. On the plus side my dealer has been excellent throughout, but their hands are tied by MG UK.

Hope your cars don’t have the same, but if they do and many of you complain perhaps this will put pressure on MG UK to fix this.
 
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Have you actually spoken to MG as opposed to written exchange?
 

Cocijo

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Having reduced power when battery is at the low end is usualy a sympton of battery cell imbalance. It’s done by the bms to protect the pack because providing full power would stress cells with a lower top end than the rest. I know that this won’t help but I thought that I would mention it just for completeness.
A couple of quick comments.
Has your dealer experienced and accepted that your car does not perform as it should when at a lower state of charge? By this I mean a documented test drive where it is noted? If not then this needs to happen.
If MG then do not accept that a car that has been tested and confirmed as faulty compared to how these cars should perform, by one of their dealers they are on thin ice.
I appreciate that you say that the dealer is ok - if the dealer accepts there is an issue then that is the first route I would take. The dealer then should be able to confirm that your claim is correct.
 

Cocijo

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Good point about battery %. This looks on the face of it like a bms balancing issue. I’m not sure about warning coming on at 25% either. Normally get a low HV battery warning when on last notch (Whatever % that is). Need also to check what fully charged HV pack voltage is showing (i’ve assumed that all the basics have been done, but this needs confirming).
 

Lovemyev

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If I remember correctly, the first yellow low range battery warning ⚠️ comes on around the 30 - 35 miles mark on my car.
Pre BMS update, it came on anything around the late thirties to forty mile mark.
It appears to display a little latter after the new update.
Never seen the red warning ⚠️ yet !.
I would imagine it would be displayed at around 15 miles ish remaining ?.
 

Kithmo

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Quoted in a previous post, 25% is on the border of 1 to 2 bars:

For the benefit of ZS EV new owners here's the state of charge as per the 8-bar gauge:
8 : 91-100%
7 : 80-91%
6 : 69-80%
5 : 58-69%
4 : 47-58%
3 : 36-47%
2 : 25-36%
1 : <25% (HV battery light comes on)
HV battery light starts flashing at 10% and will warn about aggressive driving. I believe it stops flashing and disappears at 5%.
The GoM turns to --- at 10 miles
Stick this in your phone notes for future reference.
 

Lovemyev

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Quoted in a previous post, 25% is on the border of 1 to 2 bars:

For the benefit of ZS EV new owners here's the state of charge as per the 8-bar gauge:
8 : 91-100%
7 : 80-91%
6 : 69-80%
5 : 58-69%
4 : 47-58%
3 : 36-47%
2 : 25-36%
1 : <25% (HV battery light comes on)
HV battery light starts flashing at 10% and will warn about aggressive driving. I believe it stops flashing and disappears at 5%.
The GoM turns to --- at 10 miles
Stick this in your phone notes for future reference.
Nice work !.
My orange low range warning ⚠️ message usual displays around the 35 ish remaining mileage on the GOM.
Time to pull over and start considering looking for a charger, if home is not close by.
Pre BMS update it would display a little bit sooner at 38 - 40 miles on the GOM.
Pretty much as soon as you hit the two white marks on the GOM.
A little latter and slightly lower remaining range after the latest BMS update.
 
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Cocijo

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Another quick point. I’ve notived that the OP seems to also be saying that the car does provide 100% at a low state of charge at the end of a run (100miles used as an example), but when left overnight with a low soc and not charged up at all the next day 100% power is not available. I would suggest that this is not necessarily a fault but the bms protecting the pack? I don’t think it’s unreasonable if you leave a car overnight with a low soc that the next day the power is limited (bear in mind that the predicted range may also have reduced overnight). 🧐 I’m not sure why you’d want or need 100% power after leaving an overnight low soc and using the car the next day without a charge - the priority would be a charge asap - range not power. Have MG customer services got it right?
 
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Phil Hayward

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My battery was down to 26 mls of range today charge light on, aircon on and in normal drive mode, I got 90% power at 70mph and 100% at 50 mph with full throttle. It was 23 c today.
Car did 3.8 mls per KW, 42 mile journey, average speed 62 mph.
I have had the latest BMS.
Lipos don't like the cold I believe our Iron Phosphate batteries are more tolerant to cold. Regen goes to E with a warm battery only to A at 0 c.
I don't normally go that low, usually do an overnight half charge from 30% to 80%,
 

WAC 768

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Having reduced power when battery is at the low end is usualy a sympton of battery cell imbalance. It’s done by the bms to protect the pack because providing full power would stress cells with a lower top end than the rest. I know that this won’t help but I thought that I would mention it just for completeness.
A couple of quick comments.
Has your dealer experienced and accepted that your car does not perform as it should when at a lower state of charge? By this I mean a documented test drive where it is noted? If not then this needs to happen.
If MG then do not accept that a car that has been tested and confirmed as faulty compared to how these cars should perform, by one of their dealers they are on thin ice.
I appreciate that you say that the dealer is ok - if the dealer accepts there is an issue then that is the first route I would take. The dealer then should be able to confirm that your claim is correct.
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.
 

WAC 768

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Another quick point. I’ve notived that the OP seems to also be saying that the car does provide 100% at a low state of charge at the end of a run (100miles used as an example), but when left overnight with a low soc and not charged up at all the next day 100% power is not available. I would suggest that this is not necessarily a fault but the bms protecting the pack? I don’t think it’s unreasonable if you leave a car overnight with a low soc that the next day the power is limited (bear in mind that the predicted range may also have reduced overnight). 🧐 I’m not sure why you’d want or need 100% power after leaving an overnight low soc and using the car the next day without a charge - the priority would be a charge asap - range not power. Have MG customer services got it right?
I am happy with the reduced power if the car tells's me this is limited, but it doesn't, and if you come to overtake or can only manage 25MPH up the smallest of inclines, then clearly this is not right or at least for me; it's not. Perhaps though people are happy with a car that performs for only 75% of battery as expected when not upto the correct operating temperature, and it's just me, but i am sure when you purchased your MG the dealer didn't mention this....
 

Kithmo

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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.
Maybe this is why they say it's a feature, because they also have a faulty one ? :unsure:
 

JodyS21

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This is not a nice situation to be in, I agree that it is dangerous if it is occurring as you say.
The car should at the very least warn the driver that they will have reduced power.
I've no law background, but cannot believe that legally you could not win a case against them regarding the car not being as advertised/not fit for purpose/dangerous. It will not be an easy fight though unfortunately & will take ££s.

A few points that come to mind:

There are 2 different SOC%, the displayed one and the real one. Do not get them mixed up when you are obtaining/quoting figures.
If you are looking at the real SOC% (via ODB2), then bear in mind that the BMS will not let you go below the low buffer; therefore it may say e.g. 10% but really it'll only allow it to go down to say 4%, in which case you're at more like 6%.

Batteries cannot be drained at such high currents when they are at low states of charge(lower voltage).
I believe the manufacturers should state this as most people don't realise. Plus I think that there should be a legal minimum performance figure necessary for the WLTP test, such that when it hits that as the battery gets low, the test should end and the WLTP range should reflect only the safe usable mileage attainable.
I would expect the problems to occur more so when already at higher speeds and then requesting more power, than if requesting more power from stationary/low speed. I think (I've not tested it properly), that the AMPs only goes up to the extreme end of 250 odd when you're already going quite fast and then planting your foot.
I would suggest that when testing the power limitation, make a note of the AMPs rather than the 0-100 power display, this is the real load being drained from the battery, it may give a better indication of what is actually happening/what the limit it.
Also, remember power is volts x amps. At lower SOC the voltage obtained from the cells is lower, so to get the same power it actually needs to draw a higher current.

Battery imbalance. I do not know if this changes if the car is under load compared to when starting the car.
If you've been driving lots it maybe that the BMS doesn't know what the real imbalance is, so it just carries on as if it's all ok. When starting from cold, maybe then it looks at the imbalance and says I'm going to limit power due to the lowest cell.
Ideally you'd want to capture all the cell voltages in real-time whilst testing the reduced power and see if there is one cell for example that is causing it.

Battery temperature as you know I'm sure also plays a part.
After sitting idle it'll be colder than when you've been using it for a while, so the battery will not be able to output the same power as when the battery is at optimum temperature.
I would imagine that at an exact like for like SOC, you would see a higher power reduction when the battery is "cold"

There has been a few people on here talking about reduced power, everyone seems to have slightly different experiences/it happening at different times to different extents.
I think the BMS versions do have an effect on this as following updates I think people have said it acts differently.
The question is, would 2 identically configured cars behave differently. If they do (and I would have to say they do appear to), then if there is no "fault" it must be down to the battery cells mustn't it? There will be better and worse battery cells, it's the way they are (chemistry etc). How you prove this......and at what point is it not acceptable......very tough.

Good luck and keep us updated on the outcome please.
 

Phil Hayward

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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?
 
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