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System Fault 12v battery low and car doesn't start

GalacticSkies

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Hi there,
New to the forums (just joined)!
My dad's 72 reg MG5 pre facelift no longer turns on and I suspect that he left the heating on for too long whilst on the 12v battery. He had put the car to charge at the supermarket and all I know is that the car was at roughly 40 percent (don't know whether it was ac or DC). He was waiting in the car and said he was using the heating - it was -3C outside so fair enough - but he must have forgotten about the 12v battery voltage becoming too low.
Now the car doesn't start and we got it towed home but it will not take any home ac charge. On the display the battery was at 11.6v and so we put it on a battery charger which slowly charged it to 12.9v (on my multimeter) and when we turned the car back on we had a lot of warning lights like hill ascent and lane keeping unavailable and finally system fault. The battery then dropped to 12.4v - all in 1 minute. I don't think the below freezing temp helps either. We will be leaving the 12v battery on charge overnight and I will leave a reply if it doesn't work but I just want to see if anyone else had this problem and how they resolved it - did it just start working or did you need to clear codes? If the problem was constant did you try and swap it out for a lithium battery and did that help?
Thanks for your time!
 
Sounds like the 12v is to low.
If you are sat in the car while charging then you must put it in ready mode if you want to use the heater then the HV battery keeps the 12v one on charge.
 
I had a similar issue with my previous Hyundai Ioniq. You probably just need to give the 12v battery a good charge to get it back up to full voltage.
 
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It is fairly normal for the car to start throwing up multiple warning, when the 12 volt battery is running super low.
It like a chain reaction, one is initially triggered, which then sets of an avalanche of warnings messages.
Full charge the 12 volt battery and expect the pending 12 volt warnings, to self clear themselves from the dash.
If you sitting in the car with the heater running, you have to be in READY mode and not STANDBY mode as this will not maintain the SOC of the 12 volt battery !.
 
Last edited:
Back again,
The car started working again after letting the 12v battery charge for another overnight and some more time and clearing the codes on the obd 2 port. Unfortunately, I wasn't there to see what happened but my brother said he clicked clear all and the car started working.
I tried at 7am in the morning to get the car started but the 12v voltage dropped to 12.4v fairly quickly so I left it on charge for the rest of the day. Not sure why the codes didn't clear automatically but I guess the issue is resolved for now. I'll get my dad a jump start kit and an obd 2 scanner for himself. Does anyone recommend an obd 2 scanner that is completely compatible with the MG system - maybe one that can integrate with ABRP? Also would a lithium battery be a suitable replacement for the lead acid batter of the MG to prevent similar events from occuring again?
 
Back again,
The car started working again after letting the 12v battery charge for another overnight and some more time and clearing the codes on the obd 2 port. Unfortunately, I wasn't there to see what happened but my brother said he clicked clear all and the car started working.
I tried at 7am in the morning to get the car started but the 12v voltage dropped to 12.4v fairly quickly so I left it on charge for the rest of the day. Not sure why the codes didn't clear automatically but I guess the issue is resolved for now. I'll get my dad a jump start kit and an obd 2 scanner for himself. Does anyone recommend an obd 2 scanner that is completely compatible with the MG system - maybe one that can integrate with ABRP? Also would a lithium battery be a suitable replacement for the lead acid batter of the MG to prevent similar events from occuring again?
If you feel you must replace the battery then replace with a new lead acid. No need to go to the expense of a lithium 12v.
 
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