How old is the charger Asterix
I used to have this with my Ericsson Bluetooth headset (old style small adaptor), If I left it plugged in it went flat. I changed the charger I had in the car to one that came with a newer phone and it was fine.
Since I have left my new Ericsson (larger adaptor) plugged in and it's been fine.
I had a 12 volt battery plugged into my boot accessory point on my RX4 on a camping trip, when I arrived and switched off the ignition the stereo kept playing