Calling all nikon experts! I've been having problems with my camera and i need your

Okay so basically my camera, nikon d40, has been running on en-el9 battery for three years and recently i got it changed to the en-el9a which was recommended by the shopkeeper. At first the battery was fine but then i charged the battery and when i proceded to use it it kept saying 'this battery is not compatible for your camera'. I've bought a new battery twice and it works alright at first but after charging it once or twice, the message 'this battery is not compatible for this camera' shows up and no one seems to be able to find a solution to this problem. Is it the charger? Is it the camera? I don't know, just thought maybe some of you may have encountered the same problem and may be of assistance.

According to Nikon, the en-el9a is not compatible with the Nikon D40.
http://www.nikonusa.com/...atibleWith

The en-el9a is 7.2V @ 1080mAh, while the en-el9 is 7.4V @ 1000mAh.

While 0.2V might not seem much of a difference, the camera is probably detecting the voltage difference. The reason it probably worked the first or second time is as the battery was probably not fully charged. The voltage increases as the battery charges, and a half-charged battery will have a slightly lower voltage.

Buy the correct battery.

http://www.althephoto.com/tips/aa_batteries.php