The Compaq laptop battery pictured here is not especially dangerous by the standards of its genre. But have you ever done the numbers on the energy stored in one of these innocent-looking lumps of hardware?
At the batterys bottom left appear the values 14.8 volts and 3.2 ampere-hours. Thats less than 50 watts for a period of 1 hour, about what you get from the light bulb in your refrigerator. But modern batteries get their efficiency from low internal resistance (enabling high current with little reduction in voltage). Any of several forms of abuse could release something closer to 500 watts over a period of a few minutes. That spare battery could become a truly hot accessory.
If youve never read the fine print on a battery, do it now. Believe it, and treat these things as carefully as youd treat a can of gasoline.
The principle is the same: The difference between utility and danger is just a matter of keeping that energy under control.