Whilst battlegrounds can be entered more than twice a day, shards can only be collected the first two times.
Collecting them in the middle of the battleground slows down the movement by 50 %. They need to be turned in at the Resource Recovery Officer located nearby the spawn area. After they have been turned in movement speed is restored back to normal.
Every turn in has a chance to result in one, two or three treasure shards as well as some battleground points and honor. 60 shards can be synthesized into one Battleground Treasure Chest.