The so called BIOS battery is called RTC battery in the service guide. There should be no need to remove it unless it needs to be replaced. An example is when the laptop no longer keeps time when it is powered off. The BIOS settings are also maintained by the RTC battery. It is not trivial to replace it since it is soldered to the mainboard.
Removing the RTC battery would work but not necessary. Jumper J1 is used instead to clear the BIOS setting which include the BIOS password.
The directions are a little confusing. The ones on page 42 seem different than the ones on page 150. I would have to have the laptop in front on me to verify which one of them applies.
In any case the laptop should first be unplugged and the main battery removed.
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On page 42 it shows very simple directions:
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Removing BIOS Passwords:
To clear the User or Supervisor passwords, open the RAM door and use a metal instrument to short the J1 jumper.
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On page 150 it shows more involved directions.
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Steps for Clearing BIOS Password Check
If users set BIOS Password (Supervisor Password and/or User Password) for a security reason, BIOS will ask the password during systems POST or when systems enter to BIOS Setup menu. However, once it is necessary to bypass the password check, users need to short the HW Gap to clear the password by the following steps:
1. Power Off the system, and remove HDD, AC and Battery from the machine.
2. Open the back cover of the machine, and remove the Modem Board (see “Removing the DIMM Modules” on page 55.
3. Disconnect the RTC Battery cable and locate the J1 jumper.
4. Use an electric conductivity tool to short the two points of the HW Gap.
5. Plug in AC, keep the short condition on the HW Gap, and press Power Button to power on the system till BIOS POST finish. Then remove the tool from the HW Gap.
6. Restart system. Press F2 key to enter BIOS Setup menu.
7. If there is no Password request, BIOS Password is cleared. Otherwise, please follow the steps and try again.
NOTE: These steps are only for clearing BIOS Password (Supervisor Password and User Password).
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References:
Pages 8-9: Memory compartment is #5 in diagram
Page 42: Removing BIOS Passwords:
Page 55: Removing the DIMM Modules
Page 150: Clearing Password Check and BIOS Recovery