I have three Nokia phones, each one has a BL-5C battery. Suddendly one of them stopped working, it was a battery fault. I could'nt get it to a full charge again. I also tried charging that battery using the other phones I have at hand but nothing works. The phone that had the faulty battery in the first place still works with any other BL-5C.
I measured the voltages of the faulty battery:
A-B: 0 Volt
A-C: 2.70 Volt
B-C: 2.50 Volt
Reference image: http://www.laubfrosch.it/tmp/bl-5c/batteria_nokia.jpg
It looks like the battery still works (it's not 0 Volt on A-C!) but couldn't get to be full recharged, hence the voltages stay at such a low level that the mobile does not turn on.
I wonder if anyone else had this very same problem, I'd like to share the details.