Piggy Bank is a feature that allows players to accumulate gems and cards for each opened chest (earned in battle or purchased in the Shop).
With each completed level, the Piggy Bank must be opened with gems. If the Piggy Bank is 100% filled, it must be opened within 24 hours. If it is not, the percentage will reset.
Also, it is not possible to "accumulate chests" - once the bar is full, any other chests opened afterwards will not be considered either to the current or to the next Piggy Bank.