My canon EOS 450D battery loses power within 5mins

1.1.0 and this site from Canon, you can download the firmware.
Canon EOS REBEL XSi 450D Firmware Download
Only when you have a new lens, not yet supported by your came, I would get the firmware.

Canon Support Drivers EOS 5D Mark II You can download any camera manual directly from Canon's website.

bad main circuit is most probably cause

Canon's error 99 is a catch all for connection problems. You can clean the gold contacts with a pencil eraser by lightly polishing the surface and whip with a lens cloth not tissue. However after reading your description about the battery and charger I would tend to believe you have a battery problem. Before changing the battery see if the camera recognizes it and give a state of charge.

I would proceed with this problem with
Cleaning the lens contact with a new pencil eraser
Clean battery contacts in the camera the pencil eraser can be used her as well
Check state of charge with the battery and make sure it isn't leaking

If all of the above fails I would enter the Canon web site and update the firmware of the camera.
Then recheck the battery.

This should bring you back to a working camera

Maybe. Charge the battery and take off the lens. Fire off a few shots and check to see that the mirror flips up all the way.

I think the charger of the camera might be your problem maybe you'll have to buy a new charger goodluck:) and please rate this solution...

I think so :)

Mine is over a year old. I typically need to charge it once a month if not in use and otherwise after about 800 shots without flash or 300 shots with flash.

Err 99 usually means your lens contacts are dirty either on the lens itself or on the camera.

To isolate the cause of the issue, do the following: -
1. Turn off the camera.
2. Remove the lens, battery, and CF card.
3. Allow the camera to sit without power for approximately 20 minutes.
4. Insert only a fully charged battery, and turn on the camera.
5. Depress the shutter button as you would to take a picture.

Does the “ERR 99" message appear? If so, the camera may be faulty and requires a service. If it does not, then: -
1. Turn off the camera.
2. Insert the CF card.
3. Turn on the camera.
4. Format the CF card in the camera.
5. Depress the shutter button as you would to take a picture.

Does the “ERR 99" message appear? If so, then the CF card is faulty. Try a different card. If the message does not appear: -
1. Turn off the camera.
2. Clean the gold contacts on the lens by GENTLY rubbing them with a pencil eraser or soft cloth. Keeping the camera face pointing down, also clean the gold contacts on the camera, being careful that you do not let any debris fall into the camera body.
3. Reattach your lens.
4. Turn on the camera.
5. Depress the shutter button as you would to take a picture.

If the “ERR 99" message only appears when one particular lens is attached, then that lens should be examined by a service technician.
If you see the “ERR 99" with a different Canon lens attached, then the camera is faulty and should be serviced.

Error 99 is a "catch all" error that has several causes. It could be that your battery is not fully charged, especially if it is a generic, non-Canon battery. Charge for at least over night even if the charger says it's full. If that's not the cause, switches inside the camera will have to be cleaned by a repair shop.