I got a new battery for my HP COMPAQ 6710b yesterday for 4350 buks......the previous one was kaput and lasted less than 10 mins on full charge......the crappy "compatible" battery was for 2500 buks and the OEM for 3750 buks......the shopkeeper himself admitted the OEM's are half the time duplicates and carry 1 yr dealer warranty only......i ensured the battery box i picked was box pieced.....initially i was offered a "box'ed" piece which was "opened" for signature on the battery....which i refused and demanded a HP sealed battery....which was given joyfully on the condition that 1 yr warranty would be entertained directly by HP and i will have to go to HP service center for it , and i readily agreed to it.....
also refer to this --->
http://www.laptop-battery.org/batterytips.html
Answering your questions :
1. From where in NCR should i buy a new battery with warrenty ? - No clue as im from Mumbai
2. Are there different types of batteries for the same laptop ? - I doubt that.
3. How much would a 6 Cell Battery cost ? - Depends model to model , shop to shop
4. Can i fix a 9 Cell battery instead of a 6 Cell to get better backup ? - Yes, ofcourse.
5. If yes then what are the so called side effects of doing the same ? - None AFAIK. besides the fact the 9 cell would have a higher charge time.