I use double-pack batteries so when my polarcells went from around a combined 3000ma/h to about 1000 in three months and used up completely lasing around 20 hrs in a N900 mostly on standby with a few short calls or checks of email I had no recourse or warranty as I had seriously modded the battery. BTW I didn't apply any heat to the battery when soldering the tabs.
I am actually soldering a double cell battery today from fake Nokia BL-5Js so we will see how they last.