My Nokia 100 phone was not able to charge its empty BL-5CB battery any more. The phone shop employee I consulted blamed the battery and advised to replace it. This type of battery can go through hundreds of charge-discharge cycles so I suspected something different was going on. My idea was that the phone was not able to charge the battery because of the empty battery state, probably the voltage being too low. I chose to increase it a bit by heating it in a hot air stream. This solved the problem: my phone is up and running again.
Of course one should be cautious with heating batteries. A safer but somewhat more complicated method is to charge the battery a little in an external battery charger: http://klkl.co.uk/2008/08/10/help-my-phone-wont-charge-heres-the-solution/.