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

Coulomb said:

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?

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ChrisTheTaurean said:

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.

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MickeySw said:

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....

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Pebblefeline said:

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

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Gomev said:

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

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Pebblefeline said:

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

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Coulomb said:

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.

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Gomev said:

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.

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