I have got a Toshiba qosmio F20 laptop. the battery pack is damaged. How can I assemble a battery pack with new cells in the old cover . can you provide me the circuit diagram and know how for this work

Consider a new battery. How old is it? A battery has a Cycle Life, when it's reached it's time to replace it. If so, always replace with a genuine replacement, some aftermarket batteries don't work well at best, and COULD be a Fire hazard at worst. You could try a different Cord/Charger with the EXACT same power specs as the original.

You can use a can of compressed air or an air compressor to clean it, or you can also use a vacuum with a hose attachment to clean it. Or you can use any combination of the three.

Instructions

Shut down your Toshiba laptop, and then disconnect all cables. Close the LCD lid, and then place the laptop face down on a flat surface. Position the laptop with front edge facing you.

Locate the battery pack near the top section of the laptop's bottom casing. Press on the battery release button. Remove the battery from the laptop.

Locate the cooling fan compartment cover. Its location varies from laptop to laptop, but it will be the only hardware cover with a vent in it. Remove the Phillips-head screws securing the fan's cover in place. Remove the cover from the laptop.

Disconnect the fan's cable from its connector on the motherboard. Remove the Phillips-head screws securing the fan in place. Lift the old fan out of the fan compartment and set it aside. Removing the old fan will reveal the processor chip. Apply a thin coat of thermal grease to the processor's surface using a clean paper towel. This will prevent your new processor from frying once you power on your laptop.

Place the new cooling fan assembly inside the fan compartment. Align the screw holes on the new fan with the screw holes inside the compartment. Replace the four retaining Phillips-head screws, and then reconnect the fan's power cable to the motherboard. Replace the fan compartment's plastic cover, and its retaining Phillips-head screws.

Replace the battery pack and reconnect all power cables and external devices. Turn on your laptop to test the new cooling fan

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Here's the procedure:

1. Shutdown the laptop and remove the AC adapter. Disconnect any devices, such as a printer or external DVD-ROM or drive, that are attached to the laptop
2. Remove the laptop battery. Turn the laptop upside down, slide the battery release latch and then pull the battery out of the bay.
3. Unscrew the hard drive cover. Grab the hard drive tab and shift the hard drive to the left to unplug it. Lift the hard drive out of the bay.
4. Unscrew and remove the PCI memory cover, the optical drive cover, and switch cover.
5. Unscrew the two screws that hold the keyboard to the base, these are located in the middle of the rear panel. Flip the laptop over so it is right side up, open and remove the keyboard by using the keyboard release switches located above the keyboard. Next, unplug the Toshiba Qosmio E15-AV101 Fan cable.
6. Close the laptop and turn it over so it is bottom up. Unscrew the display assembly screws located under the laptop screen hinges. Turn the laptop over so it is right side up. Open the laptop and unplug the display assembly cable and disconnect the wireless antenna from the display assembly. Unscrew the display assembly from the laptop base and remove.
7. Unplug the mouse pad cable. Turn the laptop over so the bottom side is up and unscrew the base screws which are located along all four sides of the laptop base. Gently remove the base of the laptop.
8. Turn the laptop over so it is right side up. Unplug the fan cable from the circuit board. Unscrew the screws holding the Toshiba Qosmio E15-AV101 CPU fan assembly to the laptop. Lift the fan assembly out of the laptop.
9. Insert the new Toshiba Qosmio E15-AV101 CPU fan assembly into the lap top. Screw the assembly unit to the lap top and plug the Toshiba Qosmio E15-AV101 CPU cooling fan cable into the circuit board.
10. Reassemble the laptop by repeating the previous steps in reverse order.

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To reset the BIOS read the information from this link.

In case of your laptop the easiest method would be using software from Hiren's Boot CD as it's pretty hard to open it up and reach for the CMOS battery or the jumper.

Minimum (everything off and/or on minimum): 31.8 Watts
Idle (max. brightness): 42.1 Watts
Maximally (full load inclusive. WLAN): 64,8 Watts

Dear Sir,
Try this, remove the power plug and battery, wait for five minutes and plug the computer to power with out the battery and try to start it.

The easy option is to Disassemble, i recommend you give it
  to a technician.  

These instructions should help.

Toshiba® Satellite 100, 105, 110, 115, 200, 205, 210, 400, 405, 410, 420, 430 Series

Hard Drive Installation Instructions

1. Turn notebook upside-down.
2. Remove the battery compartment cover.
3. Remove the battery pack.
4. Remove the 2 Phillips screws that mount the hard drive down.
5. Slide the hard drive away from its connector and gently lift it out of the notebook.
6. Remove the tray from the old hard drive and assemble it onto the new hard drive.
7. Slide the new hard drive into its connector inside the notebook.
8. Replace the 2 Phillips screws used to mount the hard drive into place.
9. Replace the battery pack.
10. Replace the battery compartment cover.

Try entering the system setup at startup, usually pressing the Delete or F2 key, it may also indicate whick key to press. Once in the setup look for a battery utility. Running the utility, if there is one, will calibrate the battery.