Rechargeable batteries or secondary batteries contain active materials that can be regenerated by charging. When the energy produced by rechargeable batteries drops below optimum efficiency, secondary batteries may be recharged in a couple of ways, depending upon their construction. In the United States, secondary batteries carry designations such as AAAA, AAA, N, 1/3 AA, 2/3 AA, AA, 1/2 A, 2/3 A, A, 4/5 Cs, Cs, C, 1/2 D, and D. Nonstandard rechargeable batteries or secondary batteries include prismatic cells, coin or button cells, sachet cells, lantern batteries, and battery packs.