Hi,
Here's the [http://h10032.www1.hp.com/ctg/Manual/c04775894.pdf|maintenance and service guide for the laptop.
Go to p.56 to view the ''necessary pre-requisite steps'' and then the procedure to remove/ replace the RTC battery (Real Time Clock aka CMOS).
When the battery is disconnected measure its voltage. If it is <2.5V DC replace it.
Search online for ''702853-001'' to find suppliers that suit you best.