I'm having problems with my battery so I'm trying to drain it completely inorder to re-calibrate it. Its been acting very strange. At time shows that I have only 29 mins left and the next moment it would close down. After I bring it back from sleep, I'm able to use the laptop for another hour comfortably. Anyhoo, inorder to recal the battery, I was trying to find the option that could disable the sleep mode during low battery or no battery. I was able to disable BIOS option for automatic shutdown for low battery but I can't find anything so far on Windows 7.
In Windows 7, you can change if/when the system goes to Standby in the Power Options menu.
Control Panel > Hardware and Sound > Power Options
Click "Change plan settings" for the currently in-use plan, then select "Never" for "Put the computer to sleep" (only on battery I assume).
If you let your battery drain past when it normally would go to standby, Windows 7 will hibernate the computer if the battery level gets critically low. You can change this option again under "Change plan settings" then "Change advanced power settings". Expand "Sleep" to "Hibernate after" and change this to an exceptionally large number, my Windows install has it set to 360 minutes, effectively disabling automatic hibernate (unless you battery can last more than six hours).
Try insomnia from Microsoft.It can prevent system going to sleep mode.