The power supply system in the HP Palmtop PC is designed to accept three input power sources. The palmtop PC also contains a built-in charging circuit to charge NiCd batteries installed in the unit. The charging control algorithm is controlled by software.
The input sources are 2xAA size alkaline or NiCd batteries installed in the unit, an external DC input supply through the DC jack and 1xLi CR2032 size battery installed in the unit to be used as a backup power source. With an external DC power source, the maximum power available to the system is 3W. For new alkaline batteries, power up to 3W is possible. For nearly depleted batteries, power available to the system can be as low as 1W. Therefore for batteries that are near the depleted state, power hungry devices such as PCMCIA card, CF card and backlight have to be disabled to conserve the battery life.
At any one time, only one input source is used to power the system. They work in the following order:
External power source
Main power source
Backup power source
This means that when the external power source is supplied to the system, the supply sources from both the main and backup batteries will be cut off. In the absence of the external power source, the main batteries will provide power to the system. In the case when the main batteries are either removed from the system or are weak, the backup battery will come into operation. However, the backup battery can only function to keep the system in suspend mode and it does not enable the users to operate the system. These controls will all be hardware controlled.