This is the same battery that you would have received with your HTC Phone .  Use as an additional/replacement for the original.

The battery life of a PDA is limited like any other device containing rechargeable batteries. Battery capacity drops little by little as you use it more.
When the available battery time is shortened considerably, a probable cause of this is that the battery has reached the end of its life.
The battery life will vary according to how it is stored and through general operating conditions.