Hi there Koropip,
Thank you for posting to Apple Support Communities.
Great question about the expected behavior of the MagSafe Battery Pack. The green light may flash when it is charging.
It's a great article that you've referenced with How to use your MagSafe Battery Pack
The status light on your MagSafe Battery Pack will stay lit for about 8 seconds.
While your MagSafe Battery Pack is charging, the status light might flash. If it flashes green, your battery pack is fully charged. If it flashes amber, your battery pack might need more time to charge.
Using an adapter that isn't 20W or higher will result in a slower charge.
If you don't expect this to be expected behavior, we recommend contacting Apple Support for assistance: Contact Apple Support
Let us know if you have any questions. Take care.