Battery backup device not charging

I have Panasonic KX-TG3615BX cordless landline phone.

When i turn the power supply on, status indicator of battery backup device starts flashing and then becomes constant for few seconds and then again starts blinking. This goes on.

I think batteries of backup device does not charge because when i turned power supply off status indicator shows it is not providing backup to the phone.

When i tried switching the batteries with handset batteries the status indicator becomes constant but handset shows do not use this battery. (i am using batteries provided with the product).

Please help me with this issue…