Low FPS VFC Aeg rifle

Hey guys.
Recently purchased a VFC KAC SR16 Carbine from a friend. It came with a Turnigy 7.4v 2S 2000mah li po battery and the gun is only doing 280 fps. Also the li po battery is old and a little puffy. Could this be the reason for its low fps? And what battery should I purchase for this? Thank you
Recently purchased a VFC KAC SR16 Carbine from a friend. It came with a Turnigy 7.4v 2S 2000mah li po battery and the gun is only doing 280 fps. Also the li po battery is old and a little puffy. Could this be the reason for its low fps? And what battery should I purchase for this? Thank you