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

Sounds like the 12V is too 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.

It doesn't sound as if there is an actual fault. The warnings should clear with a good battery when the car is driven.

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 !.

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?

GalacticSkies said:

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?

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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.

GalacticSkies said:

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?

Click to expand...

We had a similar issue at a rapid, family in the car waiting while we charged, had the aircon going, radio on and the car just turned off, disconnected the charger and wouldn’t restart to readiness. My wife looked up on ChatGPT and it suggested turning the car off, then back on but with the drivers door open during cycle - when the door was closed the system fault had reset and we were able to drive away, with the aux battery quickly regaining its 14V reading once again.

Lessons learnt but car door trick might be worth a try in a similar situations where you have to get goin - we were due to board a ferry imminently - you can imagine it was a v nervous 10mins while I tried to figure out how-the-fudge to get the car to wake up!

ChrisTheTaurean said:

We had a similar issue at a rapid, family in the car waiting while we charged, had the aircon going, radio on and the car just turned off, disconnected the charger and wouldn’t restart to readiness. My wife looked up on ChatGPT and it suggested turning the car off, then back on but with the drivers door open during cycle - when the door was closed the system fault had reset and we were able to drive away, with the aux battery quickly regaining its 14v reading once again.

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That’s odd, the CCU should also charge the 12V aux during a rapid session, or was the car not ‘READY’?

Car was initially’ready’ after plugging in for the rapid and it tethering successfully but I think the issue was the drain on the aux was more than the trickle charge via rapid charging & it’s winter so the aux has been under pressure for the last three months - it like an ice would behave I guess, after a cold dark winter - it gets low and takes more power to feed whilst more power is constantly demanded of it.
That’s my take at least.
Once it reset via the door trick tho it seemed to be fine. Got us safely to our destination anyway.

ChrisTheTaurean said:

Car was initially’ready’ after plugging in for the rapid and it tethering successfully but I think the issue was the drain on the aux was more than the trickle charge via rapid charging & it’s winter so the aux has been under pressure for the last three months - it like an ice would behave I guess, after a cold dark winter - it gets low and takes more power to feed whilst more power is constantly demanded of it.
That’s my take at least.
Once it reset via the door trick tho it seemed to be fine. Got us safely to our destination anyway.

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The 12V aux is actually doing very little, it's only required to provide the initial power for the HV contactors to 'kick in' to get READY; all subsequent low power demand is supplied from the main HV battery via the CCU.

Problems will arise (especially in winter) when the car isn't READY and folks run lights/radio and blower straight off the 12V aux battery.

Does that mean the heater on a zs ev when activated on ismart uses the 12V not the HV. I thought it used HV.

JohnV6 said:

Does that mean the heater on a zs ev when activated on ismart uses the 12v not the hv. I thought it used hv

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I can tell you that it uses HV when connected to my home charger when I remotely activate heating/cooling.

JohnV6 said:

Does that mean the heater on a zs ev when activated on ismart uses the 12V not the HV. I thought it used HV.

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Blower (fans) and heater are two different things, the heater and A/C are always powered by the HV battery..

As long as you are in ready mode
If you are in accessory only in MG5 they will use the 12V

Pebblefeline said:

As long as you are in ready mode
If you are in accessory only in MG5 they will use the 12V

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The PTC heater and A/C compressor need HV to run, a 12V aux cannot provide that, therefore in accessory mode the air is just blown around (no actual heating or cooling takes place)

Sitting in the car without it being READY for a prolonged time is a sure way (especially in Winter) to drain your 12V aux sufficiently to be left stranded, waiting for the AA to jump start it.

Ok just like an ice car then.thanks for clarifying

MickeySw said:

The 12V aux is actually doing very little, it's only required to provide the initial power for the HV contactors to 'kick in' to get READY; all subsequent low power demand is supplied from the main HV battery via the CCU.

Problems will arise (especially in winter) when the car isn't READY and folks run lights/radio and blower straight off the 12V aux battery.

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Ah so in the mg5 (pre facelift) the ac and heater only work when the blowers on to move the air through it so if you use the ac/heater while charging on a rapid presumably the blower, required to move the air around is what drains the 12V system, because the HV system whilst in charge mode presumably can’t keep the 12V topped up? That about right?

ChrisTheTaurean said:

because the HV system whilst in charge mode presumably can’t keep the 12V topped up? That about right?

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I'm highly sceptical about the claim that the 12 V battery doesn't get charged while rapid charging.

During any charging, AC or DC, the main contactors must be on. Usually, the contactor is in the middle of the HV battery, so that when the battery is idle (contactors off), there is a maximum of around 200 V from any one point to any other. Usually, when the main contactors are on, the DC-DC converter would be running, which charges the 12 V battery and supports the 12 V loads. That means you don't need another contactor to isolate the DC-DC from the converter. I suppose that since one of the car's computers has control over the DC-DC charge current, it could choose to set the current to zero, effectively turning it off, but why do that?

When rapid charging, there are substantial fans and pumps working to reject waste heat from all that charge power. I would think that if the 12 V battery was not charged during rapid charging, then it could and would run flat just running the fans, pumps, and computers needed for the rapid charging, much faster than while AC charging, since the losses will be so much higher during a rapid charge.

So if you turn on the HVAC during charging, I can't see how it would drain the 12 V battery, unless the DC-DC is so weak that it can't support these loads plus the HVAC blower. The DC-DC is typically capable of well over 100 A, or some 1200 W, so I don't buy that argument either.

Using HVAC during a rapid charge will use a tiny fraction of the charge power to run the HVAC blower and the HVAC itself (the heat pump, PTC heater, or air conditioner). So if you're in the car, why not be comfortable?

It does get charged as long as you are In Ready mode

If you are not in ready mode then the 12 V will not be charged and any accessory use will drain the 12V battery.