Batteries typically lose half their capacity after 18 months. After 30-36 months they are either completely dead or will only hold a minimal charge.
This is typical of the technology and not a quality control issue. The reason they wear down over time is because like all batteries they require that ions physically move around inside the half cells. This is the same reason that batteries can't be charged up almost instantly like capacitors can, and for the same reason the number of charge/discharge cycles are limited.