Changing the 12 volt without disconnecting the 400 volt

[KuramaKitsune]

Before work I pull into my local charger next to a Taco Bell
I watched a mobile technician from some random company change this other guys 12 volt

but he never disconnected the 400 volt and I stopped and asked him about it and he says it's "recommended" not "required"

What could happen ?

 

[jrweiss98020]

One hell of a spark if you happen to short the terminals!

 

[rrolsbe]

Before work I pull into my local charger next to a Taco Bell
I watched a mobile technician from some random company change this other guys 12 volt

but he never disconnected the 400 volt and I stopped and asked him about it and he says it's "recommended" not "required"

What could happen ?

They do not directly disconnect the 400V (all the high voltage cables are orange), they disconnect the black connector under the rear passenger side seat. This is the connector that provides power to the 12V bus. Pulling this connector also opens the contactors (disconnects the high voltage) assuming you have already removed the cable from the negative 12V battery post. If the contactors were closed (car is awake/high voltage enabled) and you did not pull the black connector under the seat, there would be 12-15V on the positive red battery cable. If you do not allow the positive cable to touch anything during the battery swap, he is right it's optional.

 

[Need]

The mobile tech who changed my 12v definitely disconnected the connector under the rear passenger seat. I watched the whole thing on my driveway security camera. He was done in under 5 min.