if your 5800 is an official unit (distributed by zitron/avaxx/sapura), the battery got 06 months warranty from the date of purchase..
normally service center will replace a new battery if the battery faulty.

for claim battery, like the one totally can not charger, cannot power on the phone or the battery "bengkak" within six month, is easy there just change (may take time for the approval from Nokia) a new one, but for those who claim the battery not up to the expected performance like short of stand by time, cannot last long, or bla bla bla, then there will test on the battery only there will decide want to change for you or not..in this case, normally u can make a tiny mark on the battery so u can know whether there change the battery for u or not..every battery have a different serial no. on the battery.
remark : only yourself who know the battery got problem or not, they just use a equipment to test only, not really testing on using the battery..so if they didn't change the battery for you and u found the same problem to your battery then u have to lock the case again..the second time normally there will change a new one..(this is for the battery that really have the problem, not for those used until the 5th or 6th months before the warranty expired and simply want to change a new battery.)
what ever they change or claim for u, they won't give u back the parts or component..

just an entry level of stereo bluetooth headset..don't expect too much on the sound quality..frequent disconnect problem, not sure, not body feedback on this yet..

normally won't change on the spot..have to leave the battery..