What you want doesn't really exist. Handheld devices need 5VDC into the device to charge, laptops need whatever DC voltage comes through the transformer (the brick thing on the cord) to charge their batteries.

Where will you be that a computer is of benefit, but no electricity to charge the computer will exist? If you have a second battery, it can always act as a spare, only used if you drain the first and have no way to charge it.