The 12V battery runs the car’s systems when the car isn’t in a ready-to-drive state. As soon as you put the car into that ready-to-drive state, a high voltage contactor opens to feed the engines. This also feeds a DC-to-DC converter that produces the 12v needed for the car to run, taking over from the 12V battery (and also charging the 12v battery).

From what you’ve said, it sounds like:

• You hook the jump starter (pocket booster) onto the 12v battery.
• You hop into the car and “turn it on”.
• The car stays in this state for a few seconds then turns off.

If your jump starter is the Xiaomi 70mai, the manual says that it will turn off the jump start/boost function when it detects that the current draw has dropped. This could explain why the car doesn’t stay running - it doesn’t draw a lot of current, so the jump starter thinks it’s not needed and turns off. If the DC-to-DC converter hasn’t fully taken over by that stage then there won’t be enough power to keep the car running.

A better jump starter would help here. You want to get something that can keep supplying power rather than cutting out. Even if it’s something that turns off after 1 minute, that should be better.

You’ll also want to make sure that you turn the car on with your foot on the brake, so that it’s in a ready-to-drive mode. If you don’t have your foot on the brake, the car goes into accessory-only mode and doesn’t use the HV battery. You need that HV battery to be in use so that it can power the DC-to-DC converter.

The good news is that charging the 12v battery should make a difference. When the car is able to stay on, plug in the HV charger and also charge up the main battery. If the main battery gets too low, it can go into a failsafe mode where it refuses to provide charge (including to the DC-to-DC converter).

If you still have issues after this, you’ll need to get it into a dealership to get it looked at.