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.
cheaper lead acid battery's have a set charge life. most time users leave the battery in the laptop and the laptop does a trickle charge and drain..so that the battery is always in a drain/charge mode. this can called a memory effect. battery having full charge only last 30-50 min. your better off if you use your laptop as a desktop to pull the battery and change it up once every 30 days.