You have probably used some Lithium Polymer batteries in some of your projects or toys. They pack a bunch of power in a small package (they can also be a bit dangerous if not handled correctly). Our friend Csaba has built a Lithium Polymer Internal Resistance Meter that will let you measure the resistance of the battery which is a good way to determine how good the battery is.
“This device can be used to measure the internal resistance of Lithium Polymer batteries used in electric RC models. The power consumption of these models is quite high, and the batteries wear out in 100-200 recharge cycle. The internal resistance is a good measurement of the age of the batteries. New cells are below 5mohm, and the ones at the end of their life are above 20mohms. It also depends on the original quality. “