The below is a copy and paste from Gazelle Australia, I take it from the reaction that it isn't a common practice thankfully:
Is the battery protected against theft?
Yes, in 2 ways. The battery is in the frame with a separate key lock. The key is the same as the lock on the wheel. There is also a battery with a unique serial number. This serial number of the battery is stored digitally in the computer. If the number on the battery does not match the number on the computer, the bike will not operate electrically.
The frost outside affects the batteries range. Why is that?
At temperatures below -5 degrees Celsius, batteries lose some capacity. Unless the battery is fully discharged, this won't cause any problems.
Are spare batteries available?
Yes, through your Gazelle dealer. Sometimes it is handy to have more batteries for longer trips. Batteries must be registered to your bike through the Gazelle dealer.
Can I also use the battery from my partner's bike?
Yes you can if the dealer has reprogrammed the battery on the computer. If this has not happened, you will get error message "1.1" on your monitor. Contact your dealer for more information.