R40 BMS Update (range and balancing issues)

I thought this might be a cood place to collect information and provide a central location to put the knowledge about this new issue together.

thanks to @Mike and Bartzzz and others for the info

There is a BMS update that has been provided to EU users in cold climates that allows the battery heater to be switched on on the infotainment screen. The idea behind this is that you can manually warm up the battery so that rapid charge is faster.

The F183 part number for this software is 1016620R40 ( this can be read with OVMS).

The issue we have seen with this update is as follows.

  1. The BMS does not appear to perform the balancing stage correctly. ( Balancing occurs usually at the end of a charge session, to bring all the cells up to the same voltage, typically 4.2V on the ZS)
  2. As the battery is not balanced, the cells become more and more unbalanced over time, this can be seen really clearly when the car is fully charged as instead of 454 V or so, the car only reads 440V or less.
  3. as the battery is not balanced, the range is severely affected. The BMS may say the range is ok at full charge but will rapidly fall when driving, resulting is only about 80 or so mile range.

Additionally the update has changed a lot of the PIDs that OVMS uses, including the range display. this one is really strange, as it will say the car has around 10 mi range at 100%.

Maybe needless to reply, but I will share more details from NL. Last month MG invited us somewhere in December 2020 for a new update, not sure what it was but I took a picture when everything was over.

Then day by day car started to behave weird. Showing range which it could not match. For example, being at “eco” it would display 270km and would show “-- --” of range available after 150-160 actual km.

After weeks of this, the car reports fully charged at 438V.

Called to MG, scheduled an appointment with them to fix this next week, they said there is another update to fix this, will report back here in February :wink:

Let me know what you think guys.

PS: forgot to mention when the car believes it is empty (reporting “-- --”) it accepted only 35Kw on the charge test…

Same issue here with our car.

Timeline: Since this summer MG offered a BMS update that would prevent the car from falling asleep when your charger would turn off and back on later (for example, this might happens with some chargers to balance out with your house if other appliances use power).

Somewhere by October, MG found that in some cases the main fuse would burn and they found out the cause. They released a new BMS update which should have fixed both 1) the fall asleep issue and 2) prevent burning the main fuse. This update was installed on cars between November and December.

The update to prevent the fuse from burning seems to have achieved this by stopping the charging process when the first battery cell is fully charged. Because not all cells charge and decharge at the same rate, at some point most cells only get half charged before the first one is fully charged, and the charging process would stop. This resulted in cars only being able to drive 50 to 100 km instead of the normal 250 km. The range displayed on a fully charged car however would still show 250 km, but quickly drop to a more actual estimate. You can observe the issue when you fully charge your car and then use the panel behind the steering wheel to see the available voltage (note; turn off all power consumers such as the heater). It should be around 455 V, but in a lot of cases it will show less than 430 V.

A third BMS update was released this month (January) and should revert some of the changes from the second update. The Van Mossel dealership claims that after three weeks of normal use of the car, the battery should be in its original state if you allow it to balance the cells after every charge (which takes 5 to 10 hours). It looks the balancing consumes at least 2,6 kW per hour. Other people on the Dutch MG Facebook page have pointed out that probably at least 60 full cycles of uncharging and charging are necessary to get the battery back to 455 V.

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The timeline was different here in the UK. (Sorry for long post!)

There have been 4 BMS updates that I’m aware of.

November 2019, enabled timed charging of sorts - mainly only Ohme, Rolec didn’t work without a work around - trickle charge.

July 2020 that allowed the Rolec & other chargers to smart charge, plus would work if the car wasn’t locked. It also introduced an equalisation issue where the car could take many hours to balance the cells.

October 2020 This was an update to sort out the long equalisation times but introduced other issues, such that the cells would not balance at lower state of charges and resulted in range issues and also lack of power and regenerative braking especially when cold.
This update only really came to light when the temperatures dropped and we all found that our range had decreased dramatically, with some users only able to add 20+ kWh to an almost empty battery.
I understand this update was also applied to European cars at the same time as the battery heater update, so owners thought that the heater update was causing the drain, but I suspect because that the heater update isn’t causing the drain, but it is the duff BMS.

January 2021 This is the latest update which is supposed to rectify the issues introduced previously. We understand that it may take a few cycles with equalisation to get back to what it was like after the November 2019 update! The following is from the Israeli MG importer:

As a result of an inaccurate algorithm in the software installed in the vehicles on specific dates, an imbalance was created between the battery cells.
In practice - the battery management computer detects and stops charging when the first cell reaches maximum charge and sets this mode to about 100% charge
As a result, over time and depending on the nature of use and charging, a greater imbalance is created between the cells that the battery management computer did not correctly detect and did not reduce, which actually resulted in a reduction in the amount of energy the battery contains after a full charge and a reduction in the possible range of travel.
The update received from the manufacturer, and tested by us, updates the software of the battery management computer (BMS) and performs a slow procedure of balancing cells while using the vehicle and at the end of charging until reaching an optimal state.
The length of time it will take to reach the ideal balance varies and depends on the condition of the cell balance before the update, the nature and duration of use of the vehicle and charging each day. It is therefore necessary to coordinate an orderly appointment and examine the alternatives to the solution until the end of the treatment.”

I understand to get the car back to normal will take 50-100 hours of equalisation charging per10V out. 445-455V.

If your car is out by 20V, that will take at a guess 100-200 hours of equalisation charging to get back to 455V.

Equalisation charging is the bit after the car has finished charging to 100%, when the MG badge glows constantly.

The BMS i understand can only move around 1-2mV per cell per hour.

So, my takeaway is it’s going to take a while to sort out.

I understand that MG tested this by leaving it in ‘Ready’ overnight, then charged during the day and repeated a few times. Not many of us can do that though… if we want the car to be there in the morning!!!

What you can do to help speed this up is to charge to 100%, then discharge down to 93% and then charge up and balance. You don’t need to discharge the battery completely

There has also been a High Voltage Fuse update where the DCDC & TC modules were updated because a few vehicles just completely stopped for no apparent reason whatsoever, this was in August 2020.

There was a batter heater update for the Scandinavian countries to help alleviate coldgate where the car would only charge very slowly at a rapid charge due to the cold battery and ambient temperature, but this has very strict criteria to work where the minimum cell temperature has to be below 10C, the state of charge between 20% & 55% and the battery coolant has to be below 35C. It turns off as soon at the battery coolant temperature reaches 25C. When working you would expect to see around 20A current draw. The automatic heater still works when plugged into a rapid charger even if the heater is turned off.

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This rule is making me very confusing, it suggests that this is a kind of combination update for the main fuse and BMS. As far as I know this are two seperate updates: one for the main fuse and one (the R40) for the BMS. I have got the main fuse update at the end of october but I don’t have the R40 for BMS.

The R40 should stop charging when the first cell (actually block of many parallel cells) is reaching 100% let’s say 4,2V. After there are maybe many blocks (107 pcs) which are having 4,196 to 4,198V or so.
This is what balancing is for, just making the other blocks also 4,2V.
When I read the story about the R40 it seems that there is no balancing at all after the first block has reached 100%, what other reason there could be that the battery is so extremely out of balance?
This is really a very bad thing which better should not have been happened.
Thanks God, I don’t have that R40. I charge only about 60 times a year.
What should I have done?? Endless repeat charging from 93 - 100%?
This would have been very bad for the battery.
I think the battery has been harmed anyway by causing this unbalance, but I don’t hope so for those people who are having problems now :worried:
Good luck and I hope everything wil become fine again as it was before.

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From a consumer point of view, this is how it was presented. We were summoned for a “mandatory” recall and ended up leaving the dealership with both updates installed even though we did not explicitly ask for the BMS update.

Our car currently shows 438 V when fully charged. After 4 full cycles of first unloading to 90% and then balancing over the weekend (which for some reason consumes 2,6 kW for about 5 to 10 hours while balancing is in progress) this number is still the same.

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If the voltage is not rising then the “balancing” is probably not working.

2.6 kW for 10 hours (26 kWh) or 2.6 kWh for 10 hours?

I estimate the car needs about 120 W to operate the ECUs, if it is charging the 12V as well that could be another 50 odd Watts. I wonder if it is running the balancing burn off or perhaps a battery heater, or maybe the EVCC is just a bit wasteful.

Also regarding the fuse update. I would assume it was an update to the PLC (power junction box) but it may be the BMS also was updated to restrict the max current derate to limit the current out of the pack.

It is using 2.6 kW for 10 hours (26 kWh) after charging completes, but some balancing cycles it only consumes between 100 and 300 W per hour. The temperatures were below zero some portion of the weekend so maybe it was actively heating the battery?

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I understand the battery heater takes about 20A when being used.

20A at 12v is in between that 100 and 300w :slight_smile: But that would be a very small heater.
But 20A on 450v is 9000W and that sounds very high for a heater. It could heat up a medium sized house in winter conditions with that :slight_smile:

Seeing as the heater only works between 20% & 55%, the voltage will be around 390V, which is still a decent 7.8kW, but sounds about right to heat up the battery.

Hi guys, today our car was brought to the dealer and I thought I will share here all details so we can track how the healing process will progress (if it will) throughout time!. So this morning the car assured me that there is 250 km of range.

Forgot to mention that s of yesterday the car started begging for slow charge, however it is only being slow charged.

We have a smart charger so I know exactly what was going on.

As you see the charge power is limited to 2300 watts, which is done on purpose (during this phase MG logo blinks). Then the car slowdowns to 385Watt (MG logo lit up all the time).

So get back to this morning, when the car was sure it is “full” battery voltage was 433V, it dropped further since the previous measurement of 5 days ago.


Having ridden 35km the car reported the drop in range of 67km (from 250 to 183) provided the max speed was set to 90km.

Today Van Mossel (MG dealer NL) applies magic fix patches, will keep you posted on the progress!!!

Healing Report - Day 1.
Guys I am unable to create new posts until someone replies. So I have to carry on in this post with updates.
I have changed that charging speed last night to see the effect on the patch on balancing. As you will see the car spent approximately as twice as more on balancing today (solid circle from the right).

Note, Voltage increased from 433 to 434 :smiley:

As you also see the car reports 10km more than yesterday or whatever reason…

What is also interesting is that when I powered the car this morning it reported 98% of the charge and started balancing again, I could not carry on (the car had to be used) and had to disconnect the charger, but you see the balancing attempt on the chart.

PS: Van Mossel said yesterday it will take a week to fix the issue…

To be continued…

I have now done 4 equalisation charges.
Started 433V. Every charge it has gone up by 1V.
According to OVMS it was 447V this morning, but by the time I switched on it was showing 446V in car.
Cell imbalance has gone from 114mV on Tuesday morning to 85mV this morning.
I will do yet another charge tonight.

Silly question, where can you find the cell balance and would it ideally be 0mV?


In an ideal world yes it would be 0mV. However we don’t live in a perfect world… in my previous 24 kWh Leaf it was down to 6mV at one point…