So count me in for the 12 volt discharged battery problem

[erikatl]

I have been an almost perfectly happy MG4 owner since December 2022. A wonderful car. Returning from a two week flight to the Mediterranean, the 12 volt battery was dead and the car was towed away to our service centre. No cost for the towing (because of having a 3 year service agreement), but I was charged for fault search and a replacement 12 volt battery, a total of 2861 SEK (£222). So much for the promised 7 year warranty I guess...
/Still a happy MG4 owner

 

[Tuareg]

The 12V battery doesn't have a 7 year warranty, normaly these are covered for 1-2 years.

 

[erikatl]

Well, I guessed so as well. What I would like to have known is the fact that you can't leave the car for two weeks unattended. No ICE car would need that kind of attention... Still learning...

 

[Kithmo]

Well, I guessed so as well. What I would like to have known is the fact that you can't leave the car for two weeks unattended. No ICE car would need that kind of attention... Still learning...

Any modern ICE car with the same tech would probably be the same with a 3 year old 12V on it.

 

[webhiker]

Well, I guessed so as well. What I would like to have known is the fact that you can't leave the car for two weeks unattended. No ICE car would need that kind of attention... Still learning...

I left my MG4 for 7 weeks, no problem

 

[erikatl]

Any modern ICE car with the same tech would probably be the same with a 3 year old 12v on it.

My previous car was a Volvo V50 D bought new 15 years ago, changed the 12V battery once. I kind of expected similar experience with the MG. No big thing though.

 

[m4rmite]

4 years with my MG5, just recently came back after 15 days away, it started up fine.

I did buy a battery jump booster just in case, it's never been used so far.

Meanwhile when I watch youtube and there are exotic cars in it, they can't go a week without a trickle charger plugged in.

 

[erikatl]

I left my MG4 for 7 weeks, no problem

Kind of what I expected. But I have now been recommended by the service guys to connect a trickle charger to the 12V battery if leaving the car unattended for more than a week. Does this make sense? Not to me.

 

[pjmmk]

EVs and hybrids have very low capacity 12V batteries, they don't hold a charge for long if there's any sort of parasitic drain. All they're intended for is suppling current to monitoring/unlocking systems and starting up the electrical systems then the cars use the traction battery to run things. ICEs have much larger batteries to support the starter motor. I had this same problem with my Prius many moons ago. If I didn't use it for a couple of weeks I'd have a totally flat 12V battery.
Having said that, the MG should be able to connect the traction battery in order to maintain a charge on the 12V battery. There's obviously a limit to how long that can continue - I don't know what SoC the MG stops trying to charge the 12V battery.
Personally I'd aim to store the car with at least 50-80% SoC to give it as much time as possible. Too little and it won't keep charging the 12V battery for long.

 

[erikatl]

EV's and hybrids have very low capacity 12v batteries, they don't hold a charge for long if there's any sort of parasitic drain. All they're intended for is suppling current to monitoring/unlocking systems and starting up the electrical systems then the cars use the traction battery to run things. ICE's have much larger batteries to support the starter motor. I had this same problem with my Prius many moons ago. If I didn't use it for a couple of weeks I'd have a totally flat 12v battery.
Having said that, the MG should be able to connect the traction battery in order to maintain a charge on the 12v battery. There's obviously a limit to how long that can continue - I don't know what SoC the MG stops trying to charge the 12v battery.
Personally I'd aim to store the car with at least 50-80% SoC to give it as much time as possible. Too little and it won't keep charging the 12v battery for long.

I forgot to say that I left the car with 54% SoC in the main battery. I assumed that the "system" would see to keeping the 12 V battery at a reasonable charge. I was obviously wrong there...

 

[Ian Key]

It seems some cars keep the 12 V battery charged up when left and others don't. I wonder if there's a software update that has been applied to some and not others.

Some on here have speculated that the car needs to be above a certain SoC but I've just had another thought. Could it be related to the discharge % set in the car? I've looked at mine and it's set at 50%.

 

[securespark]

Could you explain that please, Ian?

 

[Ian Key]

Could you explain that please, Ian?

If you tap the charging tile on the infotainment screen then select the Discharge Settings tab you can move the slider to limit the % that the car will allow the traction battery to go down to. It's usually used for when you are using V2L to make sure you have enough juice to get home.

I'm speculating it will also stop charging the 12 V battery if the traction battery reaches that percentage.

 

[securespark]

Gotcha, thanks.