Well, there is a little trick that is worth knowing that will not only instantly tell you if the battery has life left in it but will also impress your chums (it will impress the chaps in the office, but don’t go trying to woo those of the fairer sex with this little bit of knowledge, gals generally don’t rate tips about batteries highly in the entertainment rankings).
To do the test simply drop the battery on it’s end (either end, it doesn’t matter) onto a solid surface. A battery with charge left will drop without bouncing, where as a spent battery will bounce. Simple!
This video is slowed down so that you can see the difference in the bounce easier. You will notice that the good battery only bounces maybe 10mm where the dead battery bounces a good 80mm or so. A marked and noticeable difference.
This little knowledge might satisfy those who want to be able to detect a spent battery, but the knowledge hungry of you will be asking the question “why?”.
Well, inside a battery an electric current (the flow of electrons) is created as a chemical reaction takes place. Inside is both an anode (powdered zinc metal) in the inner core of the battery, and a cathode (a mix of manganese dioxide & carbon) which lines the case of the battery. They are separated by a type of fabric.
These electrodes are in powder form but are saturated with an electrolyte (Potassium Hydroxide solution in water) which makes a kind of very thick paste. It is this paste that acts to dampen the bounce as it moves slightly in the battery case lagging behind as the battery drops, then when the battery impacts the surface the paste catches up and dampens the bounce. That is why new batteries will not bounce.
As the battery is used, a chemical reaction takes place inside the battery and the Zinc powder and Manganese Dioxide react to create Zinc Oxide and Manganese (III) Oxide as in this below overall reaction:
Zn(s) + 2MnO2(s) ZnO(s) + Mn2O3(s)
This is a physical change inside the battery as it becomes spent. When totally spent the consistency of the products of the reaction are less like a paste and more solid in structure than the materials at the start of the reaction. This more solid structure cannot move about in the case of the battery and therefore cannot dampen the bounce. So spent batteries bounce!
So, now you know how to tell if a battery is dead or still has life in it and you also know why the test works. So next time you find some old batteries in your drawer give it a go.