12V battery charge level

R

Required

Standard Member
Joined
May 31, 2023
Messages
20
Reaction score
6
Points
8
Location
Cumbria
Driving
Not an MG
So, a week before Xmas my mg5 mk1 died showing HV errors etc and the 12V battery was showing at 11.7V. No amount of jumpstarting etc would bring the car back to life.

The AA dragged it off to the dealer and after they've replaced the 12V battery everything appears to be working again. When collecting the car the 12V system was showing 15V but this dropped to 13.7 / 13.8 over the next few days but has stabilised at that.

Is this drop likely to be the system "recalibrating" itself to a new battery or is it likely that the drain will continue again and leave me stranded?

Similarly - what is the acceptable range for a new 12V battery?

Cheers
 
Last edited by a moderator:
Mine typically shows 13.8V. I expect the higher voltage was a result of being charged by an "normal" battery charger, which may operate at a higher voltage than the charging circuit from the traction battery.
 
13.8 - 14.0V is a good charging voltage but depends on what demands are being put on it. A healthy battery "at rest" (measured with no load on battery, fully charged, disconnect charger, leave for 24 hours) is c.12.7V.
Many articles on the web do not mention depletion rates. It's no good testing the voltage of a battery that's come stright off charge as it may continue to deplete (lose charge over period of time, eg. several hours). Hence the reason for leaving it 24 hours to get a true resting voltage. Ideally this should be done with the battery disconnected in isolation.
Alternatively, get soemone to do a drop test of the battery. But remember, any battery should be fully charged BEFORE doing a drop test.
As for working range of a 12V battery, see the chart in this link: Test Your Car Battery Voltage With Or Without Using A Multimeter
 
Last edited by a moderator:
This is commonly what the standing state of the 12 volt battery is displaying on our ZS EV LR.
When it drops lower than 12 volts I would be concerned myself.
I always carry a jumper pack in my car.
You could install a 12 volt battery monitor that will ping a notification to your phone if the battery state drops too low.

Battery Monitor BM2 device for Cars & Campervans 12V Battery/leisure battery tester Compatible with Android iOS and Ipad with bluetooth 4.0 and above. https://amzn.eu/d/a7miqtl


IMG_2291.png
 
Required said:
So, a week before Xmas my mg5 mk1 died showing hv errors etc and the 12v battery was showing at 11.7v. No amount of jumpstarting etc would bring the car back to life.

The AA dragged it off to the dealer and after they've replaced the 12v battery everything appears to be working again. When collecting the car the 12v system was showing 15v but this dropped to 13.7 / 13.8 over the next few days but has stabilised at that.

Is this drop likely to be the system "recalibrating" itself to a new battery or is it likely that the drain will continue again and leave me stranded?

Similarly - what is the acceptable range for a new 12v battery?

Cheers
Click to expand...
My battery is normally around 12.8V, rises to just over 13V when the car has been charged overnight but drops down to 12.8V. From what I have read this is a healthy level.
Having read all the posts about MG batteries dropping below 12V and the errors caused, I keep an eye on mine.
 
Last edited by a moderator:
The main problem is caused by the fact that the battery provides the only source of power when the start/stop is pressed and is showing the yellow led. If the car is lights etc are then used, it can drain the battery quite quickly especially if auto lights and so on are on. The dealer had this problem when I test drove the car and it was because the car was shown to prospects without fully switching the car on. Typically they didn't want to get in and press the brake pedal to go green. As a result the battery can get out of sync. Jumping it with a pack to raise the voltage will let the car switch on. The critical voltage is around the 11.6 V level.
 
Last edited by a moderator:
Lovemyev said:
This is commonly what the standing state of the 12 volt battery is displaying on our ZS EV LR.
When it drops lower than 12 volts I would be concerned myself.
I always carry a jumper pack in my car.
You could install a 12 volt battery monitor that will ping a notification to your phone if the battery state drops too low.

Battery Monitor BM2 device for Cars & Campervans 12V Battery/leisure battery tester Compatible with Android iOS and Ipad with bluetooth 4.0 and above. https://amzn.eu/d/a7miqtl


View attachment 22544
Click to expand...
Good advice as usual 🙂
 
Required said:
So, a week before Xmas my mg5 mk1 died showing HV errors etc and the 12V battery was showing at 11.7V. No amount of jumpstarting etc would bring the car back to life.

The AA dragged it off to the dealer and after they've replaced the 12v battery everything appears to be working again. When collecting the car the 12V system was showing 15V but this dropped to 13.7 / 13.8 over the next few days but has stabilised at that.

Is this drop likely to be the system "recalibrating" itself to a new battery or is it likely that the drain will continue again and leave me stranded?

Similarly - what is the acceptable range for a new 12V battery?

Cheers
Click to expand...
Is that 11.7 V when the DC-DC convertor is energized and it's charging or 11.7 V when the HV battery is disconnected?
 
Last edited by a moderator:
dragon2611 said:
Is that 11.7v when the DC-DC convertor is energized and it's charging or 11.7v when the HV battery is disconnected?
Click to expand...
That will be the battery voltage on its own which is below the threshold and is why it won't start (connect the HV pack). When the DCDC is energised, the voltage should be around 13-14v and then the HV pack is connected. If it can't be jumpstarted then I suspect the battery was faulty which is why it was replaced, as it would have pulled the voltage below the threshold.
 
Required said:
So, a week before Xmas my mg5 mk1 died showing HV errors etc and the 12V battery was showing at 11.7V. No amount of jumpstarting etc would bring the car back to life.

The AA dragged it off to the dealer and after they've replaced the 12V battery everything appears to be working again. When collecting the car the 12V system was showing 15V but this dropped to 13.7 / 13.8 over the next few days but has stabilised at that.

Is this drop likely to be the system "recalibrating" itself to a new battery or is it likely that the drain will continue again and leave me stranded?

Similarly - what is the acceptable range for a new 12V battery?

Cheers
Click to expand...
Hi - there are 2 key figures to consider. First is the one you are raising which is the alternator current sent to the 12v battery. Usually between 12.5v and 14v, - lower figure when running headlights, etc. This juice keeps the 12v battery charged. Second figure is the SOC - State of Charge of the 12 v battery. This should always be 100% or very close.
My 2nd generation MG5 fudgeed up as the control unit failed and the alternator stopped charging the 12v battery. As the 12v controls all the main functions, its failure is serious. I got a warning message that the 12v SOC was low, but this message is easy to miss. There is no "Alternator not charging" warning. My MG5 cut out on a fast dual carriageway - no drive power, no lights or dash display. Scary. RAC charged the totally flat 12v battery in 22 minutes (!) and followed me to the MG dealership where they took 2 weeks to get the replacement control unit and fix it. Its been ok since, but I now check these 2 figures every time I drive.
 
Required said:
So, a week before Xmas my mg5 mk1 died showing HV errors etc and the 12V battery was showing at 11.7V. No amount of jumpstarting etc would bring the car back to life.

The AA dragged it off to the dealer and after they've replaced the 12V battery everything appears to be working again. When collecting the car the 12V system was showing 15V but this dropped to 13.7 / 13.8 over the next few days but has stabilised at that.

Is this drop likely to be the system "recalibrating" itself to a new battery or is it likely that the drain will continue again and leave me stranded?

Similarly - what is the acceptable range for a new 12V battery?

Cheers
Click to expand...
Could I ask how you use your car for.
 
Just checked mine, less than 3k miles, received last November and the battery shows 12.1V while HW battery is at 80%. I am a bit concerned tbh, I might speak with the leasing company for a check. Anyone else has similar experience? Editing to add that during normal operation (D mode) the voltage shows 14+ volts, so the system is definitely trying to charge the battery. As soon as I lock the car and check the battery though, within 1-2 minutes it is down to 12-12.2Volts.
 
Last edited:
NikosP said:
Just checked mine, less than 3k miles, received last November and the battery shows 12.1V while HW battery is at 80%. I am a bit concerned tbh, I might speak with the leasing company for a check. Anyone else has similar experience? Editing to add that during normal operation (D mode) the voltage shows 14+ volts, so the system is definitely trying to charge the battery. As soon as I lock the car and check the battery though, within 1-2 minutes it is down to 12-12.2Volts.
Click to expand...
I wouldn't expect the battery voltage to be above 12.4 ish. It reports as 14V while the HV is powered on because that is the charging voltage that is sent to the 12V battery. In theory the car is supposed to notice if the 12V battery drops to a worrying level and fire up the HV to charge the 12V up a bit. I think that in earlier versions of some MGs this process didn't work reliably.
My own MG5 FL (just over a year old now) sits at 12.1 - 12.2 if it isn't charging and I think that is normal.
 
kevin ellard said:
I wouldn't expect the battery voltage to be above 12.4 ish. It reports as 14V while the HV is powered on because that is the charging voltage that is sent to the 12V battery. In theory the car is supposed to notice if the 12V battery drops to a worrying level and fire up the HV to charge the 12V up a bit. I think that in earlier versions of some MGs this process didn't work reliably.
My own MG5 FL (just over a year old now) sits at 12.1 - 12.2 if it isn't charging and I think that is normal.
Click to expand...
Same here - MG5 FL about 10 months old
 
Support us by becoming a Premium Member

Latest MG EVs video

MG3 Hybrid+ & Cyberster Configurator News + hot topics from the MG EVs forums
Published
Subscribe to our YouTube channel
Back
Top Bottom