I had troubles with this battery too. My hx 4700 is over 4 years old, so I didn't care for garantie and opened it. It is easy to replace the battery!
The battery is Varta V 6 HR and costs only some Euros.
Have a look at:
HP-iPAQ-HX4700-PDA-Backup-Battery-Replacement-005
http://www.varta-microbattery.com/en...ts/DS55996.PDF
Data by Technology - VARTA Microbattery Corporate Website