Rechargeable (secondary) batteries are electrochemical storage cells that function on the basis of a reversible chemical reaction, allowing depleted cells to be replenished and reused numerous times before requiring disposal. Typically, secondary cells store less energy in a given space than comparable primary cells and exhibit higher levels of self-discharge, making them less stable in long-term storage. While generally more costly on a unit basis than primary cells, their potential for reuse makes them a more economical choice for many applications. While the term "battery" is most properly reserved for an assembly of multiple individual cells, its use in reference to a single cell is common.