Touchcard doors - being locked out

One factor that has concerned me about the potential switch away from keys to touchcards for residential properties is the possibility of being locked out if the power goes down. The obvious solution to this is a backup battery, which should definitely be a feature. And of course if you have a backup battery, you need to check it on a routine basis to ensure it's holding charge.

But what if your backup battery does fail? I think it would be useful to have an external "power in" socket to allow you to get your touchcard door working again without having to break down the door to sort out the power problems.