Try this:

1. Using 7-zip or WinRAR, extract the contents of VAR00A11.exe into a new folder.
2. From the new folder, find and run InsydeFlash.exe.
   It will probably complain about your battery. Close it when it does.
3. A file named platforms.ini should now exist in the folder. Rename it to platform.ini.
4. Edit platform.ini and replace "BatteryCheck=1" with "BatteryCheck=0" and save it.

Now when you run InsydeFlash.exe and it should allow you to update despite the battery not working.