This problem can happen mainly due to a duplicate battery, try replacing the battery with an original blackberry battery. Other reason maybe of the battery not getting proper contact with the connector. the battery connector maybe corroded. just scribe the battery connector with a metal blade or screw driver to remove the corrosion. now try and put back the battery and see if it connects. an other issue maybe of the battery connector not getting proper contact with the motherboard, in this case replace the battery connector or clean the battery connection points on the motherboard and also the battery connector with IP or PCB cleaner. This problem can also rarely happen due to an software issue and some times due to a problem in the charging circuit