MG 4 & MG ZS EVs both dead - 12V flat while charging.

[Coulomb]

I was honestly surprised they have a lead acid battery. I am sure there are better options available.

Yes, but they are more expensive, and there is already a huge industry servicing starter batteries. Teslas are moving to higher voltages (16 V and 48 V) and more suitable chemistries, but a lot of Teslas still have boring lead acid 12 V batteries. Again, if your 12 V map light blows, you can replace it with one from a servo. Not so easy for a 16 V or 48 V bulb. Then again, I'll bet most bulbs in the cybertruck (48 V) are LED with in-built DC-DC. So they're way less likely to blow than the typical Edison bulb anyway.

 

[decrep]

Looking at the reliability list of EVs, the most common problems seemed to be with the 12V battery.
This surprises me, because we've been doing lead/acid batteries for over a century now, isn't it time we had it together? Or is there something radically different with the EV 12V system?

 

[Lovemyev]

Just to clarify, the HVAC system uses a combination of both battery sources in its use.
The high voltage traction battery is used to provide the heat etc, while the blower motor fan is using power supplied from the standard 12 volt battery normal found under the bonnet.
The different intensity levels of air delivered to the vents is controlled by using a resistor pack, this gives the motor the ability to be reduced to a desired speed.
Without the resistor the motor would spin at max / full power instantly when it was powered on with 12 volts.
This normal behaviour a good indication if your resistor pack has failed.
Lower speeds are lost and only max air flow is possible.
Some ZS EV owners have experienced this situation.

 

[Coulomb]

Or is there something radically different with the EV 12V system?

EVs tend to have a few more computers, to deal with the main battery and charging. They also have more need to be monitoring things when idle, like has the charge port just been plugged in. That tends to keep computers on for longer, flattening the 12V battery.

Finally, in an ICE, you get warning immediately when the 12V battery is getting weak, because you do a severe load test every time you start it. You can also hear when it's starting to get weak, as the starter motor sounds slower. So in an ICE, the 12V battery is replaced sooner. In an EV, it can limp along for years when weak, no longer smoothing the 12V power, sometimes causing a raft of issues.