12 V Battery Charging System Fault

[DoctorWolf]

Hi all

Apologies if this has been covered elsewhere. I was driving my MG4 Trophy LR (73 plate) and noticed that the red battery light was on and the charge read 11.9V . It slowly discharged before I then got the 12V battery charge system fault warning. It kept going down until it reached 10V and then suddenly it was back to 13.7V and warnings cleared. I was passing a garage and they checked the battery. It was was at about 12.5V and had 65% health. As I restarted, the red battery warning light came back on. I was near home and managed to get there just as the battery appeared to discharge and multiple warnings came on before it eventually shut down. I was able to lock the car and then power on and the charge level said 13.7V again so I could actually get it in through my gates and off the road. Powered off and then on again at home and red battery light back on. Any thoughts and advice greatly appreciated. Worried about trying to get the car anywhere to check it out without it shutting down.

Thanks!

 

[Martinonline]

This is my experience of your problem from 2024, though not as bad as yours:

My Trophy showed a low-voltage battery charging problem when I went to drive it yesterday. The car was showing 11.9V. I've had this before and then after a quick power on and granny-charge the error disappeared. However this time I could not cancel it, a quick drive saw the voltage drop to 11.7V. I am away from home but tried to charge with the hotel's 16amp EVSE. Car would not connect and usual fixes failed. Drove to a 50kW charging station and tried DC but again no joy. (100% level set, no schedules.)

Back at the hotel the battery level was 11.5V. So I unpacked my Noco battery booster and connected it to the battery (with clicking (relay) noises coming from a unit to the right of the battery). The battery reached 14.2V and after I disconnected the booster went to 14.3V. I shut down the car and left it overnight during which time the battery went down to 12.5V.

This morning I powered up the car and it showed no faults and the expected 14.2V charging level. I connected it to the hotel's EVSE and it started charging with no issues.

I don't know if the fault would have corrected itself without the booster but I am just grateful to those posters who recommended carrying one.

Your voltage when the car is powered up seems to be on the low side and perhaps this is associated with a problem that is restricting power to the low power circuits leading to a drain from the 12V battery. We know that the car is supposed to keep the 12V battery topped up but this is the first time its action has been seen 'in the wild'.

Do you have a battery booster? Have you ever seen the battery voltage higher than 13.7V?

You could try disconnecting the battery, leaving it for 10 minutes then re-connecting to see if this 'hardware' reset works. See the Wiki on the actions required to be undertaken after a disconnect.

 

[OriginalBigAl]

Do you , or a neighbour/ friend have a Multimeter so you can keep an eye on the real battery voltage . It may be a problem with the 12 v charging system or how its just reported. Check the the battery terminals are clean and dry and the clamps are tightened up , for starters.

 

[DoctorWolf]

This is my experience of your problem from 2024, though not as bad as yours:

My Trophy showed a low-voltage battery charging problem when I went to drive it yesterday. The car was showing 11.9V. I've had this before and then after a quick power on and granny-charge the error disappeared. However this time I could not cancel it, a quick drive saw the voltage drop to 11.7V. I am away from home but tried to charge with the hotel's 16amp EVSE. Car would not connect and usual fixes failed. Drove to a 50kW charging station and tried DC but again no joy. (100% level set, no schedules.)

Back at the hotel the battery level was 11.5V. So I unpacked my Noco battery booster and connected it to the battery (with clicking (relay) noises coming from a unit to the right of the battery). The battery reached 14.2V and after I disconnected the booster went to 14.3V. I shut down the car and left it overnight during which time the battery went down to 12.5V.

This morning I powered up the car and it showed no faults and the expected 14.2V charging level. I connected it to the hotel's EVSE and it started charging with no issues.

I don't know if the fault would have corrected itself without the booster but I am just grateful to those posters who recommended carrying one.

Your voltage when the car is powered up seems to be on the low side and perhaps this is associated with a problem that is restricting power to the low power circuits leading to a drain from the 12V battery. We know that the car is supposed to keep the 12V battery topped up but this is the first time its action has been seen 'in the wild'.

Do you have a battery booster? Have you ever seen the battery voltage higher than 13.7V?

You could try disconnecting the battery, leaving it for 10 minutes then re-connecting to see if this 'hardware' reset works. See the Wiki on the actions required to be undertaken after a disconnect.

Hi and thanks for your reply. I don't have a battery booster and until today. the charge was often in the 14s. I haven't tried the disconnection yet, although when the battery was checked it would have been disconnected then.

 

[DoctorWolf]

Do you , or a neighbour/ friend have a Multimeter so you can keep an eye on the real battery voltage . It may be a problem with the 12 v charging system or how its just reported. Check the the battery terminals are clean and dry and the clamps are tightened up , for starters.

Cheers. I think I do have one so I can check. I'll look at the terminals although the mechanic didn't comment on any problems there when he checked the battery.

 

[DoctorWolf]

Hi. As an update, I can't get the car booked into the local dealer until 10th April for a diagnostic. The car did charge the following day on a schedule and at the moment the errors have cleared and the resting charge is showing 14.7 or 14.8V. Seems to keep flicking between the two and I've no idea if this is normal. MG suggested getting the battery checked at Halfords and changing it if needed and that could fix it. Halfords is a bit of a drive away and no other garage locally offers that service. I haven't tried driving it any distance yet, so I don't know if it will hold charge when driving. Obviously with it shutting down I'm a bit apprehensive.

 

[DoctorWolf]

And now have just done a short 14 mile trip where you wouldn't have known there was any issues at all. All behaving as expected.

 

[ReintjeWA]

Hi. As an update, I can't get the car booked into the local dealer until 10th April for a diagnostic. The car did charge the following day on a schedule and at the moment the errors have cleared and the resting charge is showing 14.7 or 14.8V. Seems to keep flicking between the two and I've no idea if this is normal. MG suggested getting the battery checked at Halfords and changing it if needed and that could fix it. Halfords is a bit of a drive away and no other garage locally offers that service. I haven't tried driving it any distance yet, so I don't know if it will hold charge when driving. Obviously with it shutting down I'm a bit apprehensive.

Is there a road side assist that could come and check your battery at home? Not sure if the MG road side assist does that to be honest.

And maybe there’s a tyre place nearer to you? They sometimes do battery checks, bc they’re fairly easy to swap out and good little income maker,

 

[Cabansail]

May be an idea to pop off the terminals and check for any corrosion. Look at the posts and inside the connectors. Clean them anyway and refit them. It is a very common cause of problems with Lead Acid Batteries.

No need to test the Starter Motor, as it will not have a problem. ;-)

 

[MickeySw]

I was driving my MG4 Trophy LR (73 plate) and noticed that the red battery light was on and the charge read 11.9V . It slowly discharged before I then got the 12V battery charge system fault warning. It kept going down until it reached 10V and then suddenly it was back to 13.7V and warnings cleared.

When the car is READY all low voltage is provided by the DC/DC converter (around 14V give or take).

If you get a 12V battery charge fault at that stage suspect the CCU rather than the aux.

 

[Martinonline]

The car does not have to be in READY mode, just powered up or charging to use the DC-DC converter.

@DoctorWolf, to see a truer state of the battery the car should be powered off/not charging and you must use the iSmart App / Status page and refresh it a number if times. Or use a multimeter with the car powered off.

Your problem may have been a glitch like mine was and you may not see the problem again, though it is worrying that it occurred in the first place.

 

[DoctorWolf]

The car does not have to be in READY mode, just powered up or charging to use the DC-DC converter.

@DoctorWolf, to see a truer state of the battery the car should be powered off/not charging and you must use the iSmart App / Status page and refresh it a number if times. Or use a multimeter with the car powered off.

Your problem may have been a glitch like mine was and you may not see the problem again, though it is worrying that it occurred in the first place.

Thanks. Last night after being powered off for a few hours after driving the app showed 12.8V which by all account is fine for a car that hasn't been powered for a number of hours. Just refreshed the app and it is saying 14.8V. Going for another test drive soon.

 

[Martinonline]

Was your car connected to your EVSE when the iSmart reading was 14.8?

 

[DoctorWolf]

Was your car connected to your EVSE when the iSmart reading was 14.8?

Hi. No it wasn't. Just been for another drive and everything again seems to be a it should be. Running at 14.7V with no apparent issues.

 

[Martinonline]

Please add how you measured the voltage so it can be seen if the value you provide comes from the battery or the DC-DC converter. To me, the 14.7V seems too high for a nearly 3-year old battery rated at 65% health. My 23-plate LV battery usually shows about 12.3V when the car is not powered up nor connected for charging when queried via the iSmart App.

 

[DoctorWolf]

Please add how you measured the voltage so it can be seen if the value you provide comes from the battery or the DC-DC converter. To me, the 14.7V seems too high for a nearly 3-year old battery rated at 65% health. My 23-plate LV battery usually shows about 12.3V when the car is not powered up nor connected for charging when queried via the iSmart App.

The 14.7 comes from the display when driving. The earlier 14.8V was the iSmart app. As I said, it was 12.8V last night when powered off. I assumed the converter kicked in overnight to boost the 12V?

 

[Martinonline]

Thanks for your replies, you are providing some very interesting information that has, as far as I know, been reported before.

As @MickeySw said above the DC-DC Converter when active provides a voltage of around 14V. The car should activate it when not powered up/connected for charging to ensure the LV battery level is maintained as you have seen yourself.

Hopefully the issue was just a glitch and will not occur again. However, that will not help you with the dealer if the issue cannot be repeated.

Please tell us the results of your visit to the dealer.

 

[DoctorWolf]

Thanks for your replies, you are providing some very interesting information that has, as far as I know, been reported before.

As @MickeySw said above the DC-DC Converter when active provides a voltage of around 14V. The car should activate it when not powered up/connected for charging to ensure the LV battery level is maintained as you have seen yourself.

Hopefully the issue was just a glitch and will not occur again. However, that will not help you with the dealer if the issue cannot be repeated.

Please tell us the results of your visit to the dealer.

Will do. Yes, I'm hopeful it was a glitch but I am keeping the dealer appointment so they can have a proper check of the car. Thanks for all your input.

 

[Martinonline]

One last thing. I suggest you buy a battery booster just in case. Many of us here have one along with a 10mm spanner and, some, a window-breaker.

You can be a good samaritan with the booster because it can start ICE cars.

 

[MickeySw]

Will do. Yes, I'm hopeful it was a glitch but I am keeping the dealer appointment so they can have a proper check of the car. Thanks for all your input.

I’m not convinced it’s a glitch:

14.7/8V for a running DC/DC seems high (not sure if that’s normal for a MG4)?

A 12V stater battery is never going to show 14.8V unless it’s charging. Maybe accessing the MG4 via iSmart enables the DC/DC, our MG5 certainly doesn’t?

If the aux was @ 12.8V when the car was powered off, there should be no need to boost it overnight?