What is the round button for on the top of the battery pack on the Pride GoGo folding scooter?

There is what appears to be a round button on the top of the sealed lead acid battery pack on the Pride GoGo Folding scooter:

This is actually a plug that covers the hole and serves no other purpose on the sealed lead acid battery pack. For GoGo Folding scooters equipped with a Lithium ion battery pack, the plug is replaced with a key lock that locks the battery pack to the scooter.