Had the Dreaded 12V Battery Drain Overnight

[Seajays]

So having never had an issue in the last year, and regularly checking the 12V battery every few months with a top up charger, I was happy that the 12V battery was good.

Unfortunately on Saturday I went out on the dual-carriageway and it was raining, but not dark enough for the auto lights to come on. So I switched them on manually. Then when I got home I forgot to switch them off.

When I came back to the car on Sunday it was totally dead. Had to use the mechanical key and the 12V battery had drained. I noticed I had left the lights switched to "on", but thought that the car would have automatically switched them off after a time out? Whether that was the problem or not I don't know. I'm kind of hoping it was as at least that would be an explanation, rather than a random problem that might strike at any time!

I recharged the 12V battery and all seems good again (after the dozens of errors that first came up of course!).

 

[Martinonline]

Yes the lights would have caused the drain. The car does not switch off the lights automatically unless you are in auto.

 

[EvTek23]

The 12V battery should get charged from the high voltage battery pack (Via the CCU) if the main battery is above 50%.

 

[T1 Terry]

So having never had an issue in the last year, and regularly checking the 12v every few months with a top up charger I was happy that the 12v was good.

Unfortunately on Saturday I went out on the dual-carriageway and it was raining, but not dark enough for the auto lights to come on. So I switched them on manually. Then when I got home I forgot to switch them off.

When I came back to the car on Sunday it was totally dead. Had to use the mechanical key and the 12v had drained. I noticed I had left the lights switched to "on", but thought that the car would have automatically switched them off after a time out? Whether that was the problem or not I don't know. I'm type of hoping it was as at least that would be an explanation, rather than a random problem that might strike at any time!

Recharged the 12v and all seems good again (after the dozens of errors that first came up of course!)

Had to do a You Tube tutorial on what the manual key door unlock was all about .... learn something new every day I tell ya

 

[MG Clive]

I guess it's like any car, old or new, if you leave your lights on overnight, you'll have a flat battery in the morning.

 

[Seajays]

Had to do a You Tube tutorial on what the manual key door unlock was all about .... learn something new every day I tell ya

Yes - it's probably worth doing before you have to as I almost broke the key twisting it, thinking it would go like a quarter of a turn and you'd hear some "clunk" or something, but it wouldn't move more than a short way... but no, you only turn it a small distance and then pull the handle while turning... I'm glad I didn't force it further!!!

 

[Martinonline]

I think the operative word is 'should'. There are a number of reports in the forum about the failure of the car to charge the 12V battery, including my post last year.

 

[T1 Terry]

I think the DC to DC only operates when the car is in ready mode ..... I could be wrong about that .... but I'm male, so I'm well versed in being wrong .....

T1 Terry

 

[siteguru]

I think the DC to DC only operates when the car is in ready mode ..... I could be wrong about that .... but I'm males, so I'm well versed in being wrong .....

T1 Terry

Page 23 of the Owner's Manual suggests that the latter may apply.

Low SOC in the MG4's case seems to be <50%

 

[decrep]

so if leaving the car for a while, make sure the HV battery is above 50%, maybe 60% ?

 

[T1 Terry]

so if leaving the car for a while, make sure the HV battery is above 50%, maybe 60% ?

Or disconnect the 12v battery ......

T1 Terry

 

[siteguru]

Or disconnect the 12v battery ......

T1 Terry

It won't ever charge that way. Of course you'd also have no parasitic drain - and no car data via the iSmart app, and have to reset everything when you return.

 

[Kithmo]

I'm surprised that it flattened the battery so quickly. The lights that stay on are the parking lights and are all LED, the headlights go off when the car is switched off. The 12v should be able to keep the parking lights on for a couple of days unless it's low to start with.
FWIW, if you don't switch the car off the headlights will stay on, but only for 30 minutes, then the car automatically switches off and the headlights go off.

 

[T1 Terry]

It won't ever charge that way. Of course you'd also have no parasitic drain - and no car data via the iSmart app, and have to reset everything when you return.

Hmmmm...... so I should put the repairable MG on the charger over night before we leave tomorrow for a few mths on the road ..... it's down to 28% now ..... What would need to be reset? The radio has stations for another state on it, so I don't mind if those are lost, it has a whole 480kms on the clock I seem to remember, so it hasn't actually learnt much for it to forget

T1 Terry

 

[siteguru]

The main things are resetting the window controls, and recentring the steering - both easily done. (I think the method is linked in the Wiki thread).

 

[JonnyG]

I guess it's like any car, old or new, if you leave your lights on overnight, you'll have a flat battery in the morning.

That's true, but most other cars I've driven bong quite loudly and consistently as soon as you open the door if you leave your headlights on. The MG4 just comes up with a message on the screen which you hardly ever see as you are already exiting the car by then. IMHO any facelift model should include an audible warning.