Sadly, the simply battery trick hasn't worked in a long time for modern BIOS systems, which have mostly transitioned to long-lived flash memory. You'd have to wait much longer than 20 seconds---more on the order of 20 lifetimes.

However, many manufacturers have programmed secret "recovery password" prompts that can be accessed on only certain date/times that are the default after a BIOS battery reset. For example, many ASUS laptops (possibly all post-2002?) on Jan 2. 2002 can be bypassed by pressing Alt+R and entering ALAA4ABA. This will not only remove the BIOS passwords, but also any encryption passwords on the harddrive. You can then reboot and set them however you like (and you should---the BIOS is typically buggy and stores nonsense in the admin region after this procedure.) Presumably other manufacturers have thought of similiar secrets that aren't supposed to be known for use when laptops are mailed in under warranty.