Problem with the 12v battery on my MG4

Hi,
I have a question about your experience with the 12v battery.

The thing is that about a month ago I parked my Mg4 mat home and the next day I could not unlock the car and had to call Mg assistance to help me with this. I suspect that due to some unknown circumstances something like the headlights was on even if I locked the car and everything looked normal.

Now I hav had another instance of a similar problem where I suspect that it is caused by the 12v battery. This time I am on vacation and left my car at my brothers place. I did check from the app after 1 week and it seemed ok but today I was doing another check after a little over 2 weeks of vacation and I could not connect to the car. I think that the 12v battery should not die after only 2 weeks of inactivity. This is bad and I will contact MG service as soon as I come back home.

Sara4711 said:

Hi,
I have a question about your experience with the 12v battery.

The thing is that about a month ago I parked my Mg4 mat home and the next day I could not unlock the car and had to call Mg assistance to help me with this. I suspect that due to some unknown circumstances something like the headlights was on even if I locked the car and everything looked normal.

Now I hav had another instance of a similar problem where I suspect that it is caused by the 12v battery. This time I am on vacation and left my car at my brothers place. I did check from the app after 1 week and it seemed ok but today I was doing another check after a little over 2 weeks of vacation and I could not connect to the car. I think that the 12v battery should not die after only 2 weeks of inactivity. This is bad and I will contact MG service as soon as I come back home.

Click to expand...

Connecting to the car with the iSmart app does use up some 12v charge. So does putting the car in READY, locking and unlocking doors, folding mirrors etc.

If the 12v was full (12.6v+) when you left it, and didn't do any of the above, then it looks like you have a parasitic battery drain. It's fairly easy to diagnose and then fault find, especially for an experienced mechanic.

I think the car stops sending data to the servers after so many days of being off to save the battery. MY old EV6 used to display me a message saying due to inactivity status cheeks have been suspended.

I do feel a bigger 12v battery could be the safer bet going forward. I do like the jump starter's people have suggested but if the door release is electrically controlled & not mechanical, you can't manually open the doors to get to the bonnet release, so then there's no point in having one. If anyone lucky enough to already have their mg4 could correct me if I'm wrong about that it would be appreciated.

jpk said:

I do feel a bigger 12v battery could be the safer bet going forward. I do like the jump starter's people have suggested but if the door release is electrically controlled & not mechanical, you can't manually open the doors to get to the bonnet release, so then there's no point in having one. If anyone lucky enough to already have their mg4 could correct me if I'm wrong about that it would be appreciated.

Click to expand...

Page 154 of the owners manual shows the "mechanical" key to unlock driver's door.

Pebble said:

Page 154 of the owners manual shows the "mechanical" key to unlock driver's door.

Click to expand...

Thanks for that I'd wrongly assumed the car relied upon the battery

Have to admit that in preparation for the arrival of my vehicle I took delivery of one of these today, not that I think the battery is going to be an issue....honest


Amazon product ASIN B0BNPMVMZH

I've got a big Duracell car power bank thing. It's ace. Also doubles as a laptop charger ryc.

Sara4711 said:

This time I am on vacation and left my car at my brothers place. I did check from the app after 1 week and it seemed ok but today I was doing another check after a little over 2 weeks of vacation and I could not connect to the car.

Click to expand...

Actually, going by my own experiences of the iSmart (or as I prefer to call it, the iDumb) app, not connecting to the car is far more likely to be a server/network/whatever problem with the app, rather than the 12v battery in the car. What was the displayed voltage of the 12v battery last time you were able to connect?
As a secondary thought - do you have anything like a dashcam connected?

Sara4711 said:

Hi,
I have a question about your experience with the 12v battery.

The thing is that about a month ago I parked my Mg4 mat home and the next day I could not unlock the car and had to call Mg assistance to help me with this. I suspect that due to some unknown circumstances something like the headlights was on even if I locked the car and everything looked normal.

Now I hav had another instance of a similar problem where I suspect that it is caused by the 12v battery. This time I am on vacation and left my car at my brothers place. I did check from the app after 1 week and it seemed ok but today I was doing another check after a little over 2 weeks of vacation and I could not connect to the car. I think that the 12v battery should not die after only 2 weeks of inactivity. This is bad and I will contact MG service as soon as I come back home.

Click to expand...

I've just had the same issue. The AA came out and started it up with a portable charger. He then cleared all the error codes using a laptop plugged in.
Here's what I learned from all of this.
I could have done that myself except clearing the codes. The codes remained on the screen even when cleared. I was advised they usually reset themselves as they need a calibration drive. Sure enough, they did after a 10 min drive and a restart.
The AA man said this issue is routine among EVs. Most of his calls now are for this.
I'll be buying my own portable charger as suggested above.
While it's a solution, it does suck. During lockdown my Petrol Audi TT sat for over a month without moving. Started with no issues at all. Let's put that in perspective. The Audi was an ICE car. They can only charge when the engine is running. An EV has a battery it could (presumably) use at any time to keep the 12V sufficiently charged or at the very least send a warning that it's running low. Of course you might loose some range but a dead car has zero miles range ;-)

I have to admit I do find it puzzling what the problem is with using the HV to charge the 12v battery. I presume the Tesla must do something like this as it must have a fairly high idle current draw with the fact that all the cameras are working all the time etc, much more than on the MG4 where people seem to be hitting problems.

Thinks like accessing the iSmart app should not result in a flat battery

The MG4 manual, page 23, says ‘The 12V battery SOC (State Of Charge) is constantly monitored, when the vehicle is power OFF it is possible, under certain conditions, that the HV battery will automatically charge the 12V battery to ensure the vehicle starts. This function will activate and switch off automatically.’ ‘The intelligent charging function is suspended when the high voltage battery is in a low SOC.’

So I don’t understand why the 12V battery should ever go flat if the HV battery has a charge above about 20%.

Never had an issue and mine's been parked for three weeks and two weeks while I was on holiday

That's good to know.

timeades said:

Have to admit that in preparation for the arrival of my vehicle I took delivery of one of these today, not that I think the battery is going to be an issue....honest


Amazon product ASIN B0BNPMVMZH

Click to expand...

I bought something similar for my mg5 2 years ago.
I've never had to use it on my car, however I've used it on other people's cars and vans to start their ICE vehicles