You can't use standard rechargeable batteries with the K912. If you still have the original batteries, and compare them to the generic batteries you are trying to use, you will see the difference on the - ( minus ) side of the battery. On the original battery that comes with the headphones, there is a bare metal band on that side, which connects to a sensor in the headphones ( you can also see this sensor inside the battery compartment, on the - side ). A generic rechargeable battery does not have this metal band for the sensor, so the charger won't charge them. This is for protection, in case you have non-rechargeable batteries in the headphones and put them on the charger. This could cause damage to the headphones.