I believe there is a tool that can be used to unlock the brakes if the battery goes flat but i am not sure how easy it is to use or who has access to it. (i.e. it isn't in the tool kit that comes with your car).
Below is the process they use for the CC in the US for a fully drained battery.
If the battery is not 100% drained i believe you can dissengage the brakes by doing the following:There are only two ways to manually disengage the parking brake, both involve crawling under the back of the car with some tools:
1) Unbolt the parking brake motor, mounted to the backside of each caliper,, and use a power-driver tool to retract the pads.
2) Pull the electrical connector off each parking brake motor, and apply 12VDC from a portable jump-starter (has a 12V battery inside) to the motor contacts, to retract the pads. If the pads don't retract, reverse the polarity of the applied 12V.
(a) remove key from ignition;
(b) remove both battery cables from their respective terminals (10mm hex);
(c) touch the ends of the battery cables together for about 30 sec. (I held them there for about 60).
(d) reconnect the battery cables.
All our local skidpans got shut down due to noise restrictions so i have never had to use my handbrake for anything other than the stop the car.
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