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

[ChrisTheTaurean]

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?

Ok, why then would my mg5 shut down the charging and disconnect with the 12v showing only 12v charge on the display and system warnings flagging up paralysing the car - the only thing we had going was the ac and a radio. Genuinely interested in understanding why that happened.

Also to note we managed, after several minutes and ons and offs of the car to reset the system by opening the driver’s door while doing an ignition off and on to ‘Ready’

Any explanations would be very welcome. Trying to understand how the car functions and why we had this panic when trying to get a simple rapid charge whilst keep car occupants warm.

 

[MickeySw]

Ok, why then would my mg5 shut down the charging and disconnect with the 12v showing only 12v charge on the display and system warnings flagging up paralysing the car - the only thing we had going was the ac and a radio. Genuinely interested in understanding why that happened.

Only thing I can think of is a temporary CCU fault, for whatever reason the DC/DC converter stopped working and all low voltage power came off the 12V battery which drained it quite quickly.

I'd say disconnecting the 12V battery briefly (hard reset) should also have sorted the issue, best to have a 10mm spanner with you, better look at it than for it....

 

[ChrisTheTaurean]

Only thing I can think of is a temporary CCU fault, for whatever reason the DC/DC converter stopped working and all low voltage power came off the 12V battery which drained it quite quickly.

I'd say disconnecting the 12V battery briefly (hard reset) should also have sorted the issue, best to have a 10mm spanner with you, better look at it than for it....

Will check that out when we get home. Cheers and yeah, agree with logic that

 

[Gomev]

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

Quite agree, but it has to be full ready mode, i.e. foot on the footbrake and press start, not just the green light from pressing the start twice.

 

[Coulomb]

Quite agree, but it has to be full ready mode,

Ready mode is when you start with a foot on the brake pedal.

But I disagree that you need to be in ready mode. Ready means ready to drive away. You can't be ready to drive when charging. I claim that the car should have been charging the 12V battery, and thus also carrying the 12V loads with a small part of the rapid charge power, when rapid charging.

I can only agree that it was some sort of glitch, and should not have happened.

Was it perhaps near the end of the rapid chsrge? For example 85% SoC or higher? If that was the case, the car would have been asking for a small amount of power, and if ut neglected to add on enough for the auxiliary loads (HVAC, pumps, etc), then there might not have been enough power for the auxiliary loads. That would have choked off the main battery charge, which could well have triggered the end of the rapid charge. Now that I type this, it seems quite unlikely.

 

[Pebblefeline]

Even if you are charging if you are not in ready mode the car will not charge the 12V battery sufficiently to run the car's systems.

No power from DC charging is diverted to systems, the charge goes to the battery and the systems run from the battery.

Think of ready mode as HV battery is connected to the car's systems.

 

[JohnV6]

Just like an ICE car with ignition off and radio on. Or if auto stop start is in stop but the ICE car either shuts down or starts the engine. Surely MG should at least alarm.

 

[MickeySw]

Even if you are charging if you are not in ready mode the car will not charge the 12v battery sufficiently to run the cars systems

No power from dc charging is diverted to systems, the charge goes to the battery and the systems run from the battery

Think of ready mode as hv battery is connected to the cars systems

Does this mean that the MG5’s 12V aux powers the complete car during an AC charging session overnight?

Or does above only apply to rapid charging?

 

[Gomev]

Ready mode is when you start with a foot on the brake pedal.

But I disagree that you need to be in ready mode. Ready means ready to drive away. You can't be ready to drive when charging. I claim that the car should have been charging the 12V battery, and thus also carrying the 12V loads with a small part of the rapid charge power, when rapid charging.

I can only agree that it was some sort of glitch, and should not have happened.

Was it perhaps near the end of the rapid chsrge? For example 85% SoC or higher? If that was the case, the car would have been asking for a small amount of power, and if ut neglected to add on enough for the auxiliary loads (HVAC, pumps, etc), then there might not have been enough power for the auxiliary loads. That would have choked off the main battery charge, which could well have triggered the end of the rapid charge. Now that I type this, it seems quite unlikely.

As I stated, 'i.e. foot on the footbrake and press start, not just the green light from pressing the start twice.' - as if your were starting the car normally when not charging. The former action will charge the 12V, whilst charging on a rapid and allowing use of the lights, radio etc. etc., the latter won't.
Of course you can't drive away, I never implied that even though you may have inferred it.

 

[MickeySw]

The former action will charge the 12v, whilst charging on a rapid and allowing use of the lights, radio etc. etc., the latter won't.

Most (all?) EVs will automatically activate the DC/DC circuit when a charging session (AC or DC) takes place.

The MG5 is no exception and will charge the 12V aux regardless what state the power button is (off, green or amber)

 

[MartinJ]

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!

Hi , I am very sorry to hear what ur dad experienced. We had a major issue like this 2 weeks ago. Car charging station charger was not working for us and we tried to unplug the EV charger cable but it got jammed. We somehow managed to pull it out after many attempts however as a result it stopped our car working, as whilst the car turned on, it came up with errors and would not go into Drive (forwards) gear or Reverse gear. Our car just wouldn't start and go into gear. The battery warning light kept flashing saying "HV battery shutdown". We next tried to disconnect the 12 v battery (the negative side) under the bonnet for a while and left it for 2 hours before starting it again. After few hours gone, we realised that didn't help. So, we found an car engineer to come to us to tow the car back to his garage in order to fix it. However, MG electric car cant be towed by normal tow truck as it needs to towed by a hydraulics tow truck instead. So, our engineer had to fix it on the spot. He started to jump start the car but it won't work. Then , he released the positive terminal side of the 12v battery by using a screwdriver metal part touching the positive terminal for 3 minutes. He said that will release all the memories of the car memory registering system. Still no joy. So, after that he using a OBD computer to clear all the faults in the car system. As a result of that the car can start again and go into gear again. Hooray! So, it is a common occurrence/ (I would say shocking & anxiety experience for us) in some electric car even in Tesla"s. I hope this would help others MG5 electric car users if such incident happens to them.

 

[MickeySw]

So, after that he using a OBD computer to clear all the faults in the car system. As a result of that the car can start again and go into gear again. Hooray! So, it is a common occurrence

It used to be a lot more common after failed DC charging sessions. FW upgrades over the years seem to have helped but many drivers got their own OBD dongle/phone app for just such occasions..

Apart from clearing error codes this will allow you to monitor SoH and cell balancing.