Ben;

No matter how hot the car gets while parked in the sun that alone will not damage the battery and the adjacent systems. The problem begins when you start the car and the IPU (Power control unit behind the rear seat) is fed the hot air inside the car as its main cooling stream. Under normal operation the IPU gets much hotter than the heat you are likely to find in the passenger cabin after leaving the car parked in the sun for hours.

However, when the car is operating the IPU needs to be cooled effectively for it to operate normally. The IPU will (by design) deny assist or regen functions if it exceeds the threshold for safe operation.

What I (and many others) will recommend is that you open the doors (or windows) for a while before starting the car in a very hot day. This will allow the IPU to benefit from the coolest air from the onset and hence avoid any similar situations.

One last thought: Do operate the air conditioning in a measured and conservative way. In other words, operate it on Auto and start with a higher temperature while periodically adjusting it to a lower temperature where you find it confortable. Many people find that stabilizing the cabin temps at 27C (78F-82F) is very confortable. By keeping the cabin cool you'll also keep the IPU happy. On our 06 HCH's it is possible to operate the A/C with minimal (or no) impact on your fuel economy. Manage your battery state of charge well and stay cool.

Cheers;

MSantos