did you send the phone back to Nokia in Singapore or to the reseller?
The exact same problem happened to me with an 808 from Singapore. I sent it back to the reseller(Tropical Computer Systems) who held it for 3 months and returned it still dead.
They said I should contact Nokia in Australia!
The battery life depends on the number of repeatedly charging and discharging, so should try to avoid more charge when the battery power, this will shorten the service life of the battery. Mobile phone off time more than 7 days, should first will phone the battery to completely discharge, enough electricity before use.