1) Is your power management settings on the Laptop set to turn the Hard Drive off after a few minutes? Assuming you're using a newer version of Windows, right-click the battery on your start bar and select power options. You should see a window with Power Plans. For the Power Plan that you have selected, click 'Change Plan Settings' and adjust the values to suit your needs.
2) If the above isn't the problem I'd look at your battery. I had a faulty battery in a laptop that would discharge from 100% to 0% within a few minutes. The only way I could fix that was to buy a new battery.