The accessory socket is neither an output or an input, it's a connection to the battery... Electricity, like water, flow from the higher pressure (voltage) to the lower pressure.
If you connect something with a lower pressure (anything that uses electricity) current will flow out of the socket to operate what's plugged in it.
If you connect a charger (to charge a 12v battery, you have to 'push' more than 12v in it) with a higher voltage, current will flow to the battery.
That is somehow oversimplified, a regular charger will not work because the CanBus system 'cuts' the connection between the battery and the socket when the motor is switched off, but, keeps on 'an eye' the socket.
CanBus compatible chargers send a signal for the CanBus to connect the socket to the battery, allowing the charge to reach it's destination.
If you connect something with a lower pressure (anything that uses electricity) current will flow out of the socket to operate what's plugged in it.
If you connect a charger (to charge a 12v battery, you have to 'push' more than 12v in it) with a higher voltage, current will flow to the battery.
That is somehow oversimplified, a regular charger will not work because the CanBus system 'cuts' the connection between the battery and the socket when the motor is switched off, but, keeps on 'an eye' the socket.
CanBus compatible chargers send a signal for the CanBus to connect the socket to the battery, allowing the charge to reach it's destination.