We often meet this question “ I have discharged the battery to zero volts, and fully discharge and fully charge as your instruction, but the battery can not charge to 100% capacity.” It is involved with overdischarge and overcharge .

The battery pack is made up of several cells. If you discharge the battery to zero, what will most likely happen is that one or more of the cells will "reverse" because they are weaker than the other cells. The reversed cell begins to accept a "backwards" charge from the other better charged cells around it. This will kill the battery. So we should stop the discharge at one point, rather than letting the pack slump down to zero volts.
In the meantime, when the battery is at its full capacity, we should not plug the AC adapter to trickle charge the battery. It will overcharge the battery.
Important infomation: Overdischarge and overcharge the battery will damage the battery.