Sometimes your laptop computer, your lamp or even your car need to be powered and what you have are batteries only.
I have had the experience on using 20 AA batteries (packaged in 15V) to jump start my car.
In case of emergency, maybe you can power EVERY DEVICE which you MUST.
Step 1: what you need
what you need are very simple:
2.aluminum foil for cooking
4.bulb(for this example - desk lamp)
5.some chipboard cut from box