Roger Alcindor

I connected a battery charger to my Audi A1( 2014), connecting the positive clip to the large Red cable connection and the negative clip to a hexagonal bolt. It was dark and upon reflection, I used the wrong connecting point for the negative clip.
The battery charger indicated that it was charging so I left it to charge. I took a look at the battery charger after about half an hour and it indicated that the battery was charged which was suspicious as the battery was rather flat to begin with and should have needed several hours to fully charge,
I disconnected the charger and tried to start the car but to no avail; absolutely dead. I suspect that the electronics have been damaged and would like to know what the hexagonal bolt was that I connected the negative charger clip to.
I attach a picture showing the silver hexagonal bolt. What is this item and could I have damaged the electronics by using it as a negative terminal with a battery charger. The battery charger was rated as charging at 8 amps maximum.