This test works equally well for AA, AAA, C, D and 9 Volt alkaline batteries.
Bounce a Battery to Learn If It Is Still Good
Scientists prove that depleted AA batteries will bounce higher when you drop them, as inner material loses density
By Charlie Quigg and ChemistryWorld | April 1, 2015
Inspired by a YouTube video, scientists in the US have confirmed that a simple bounce test can be used as a technique to indicate charge in a battery.
Alkaline AA batteries are one of the most common battery types and there are a range of approaches to assess the charge in these batteries, often entailing electronic indicators. Now, a team led by Daniel Steingart of Princeton University have correlated the coefficient of restitution (COR), a measure of bounce, with batteries at various charges and determined their charge to a similar degree of accuracy as in situ energy-dispersive x-ray diffraction (EDXRD).
This level of accuracy is possible as the change in bounce is directly related to the physical changes that occur as the charge depletes.
An alkaline battery consists of a gel zinc anode and a MnO2 cathode. When the battery discharges, the zinc anode begins to oxidise, forming Zn(OH)42- ions in solution until the electrolyte is supersaturated. The ions then precipitate into ZnO around the zinc particles and create a percolation network throughout the anode, densifying the gel into a solid ceramic at approximately 50% charge. This change significantly increases bounce height at a rate that directly relates to the formation of ZnO. After the ZnO ceramic is fully formed, when the battery charge decreases to half, the coefficient of restitution will stabilise and battery bounce stays the same.
Batteries are regularly frozen when left outdoors in the winter. If a good alkalin battery is frozen, the electrolyte will stiffen sufficiently to cause a bounce. This is a room temperature test only.
It is our understanding that the following chemical reactions occur and helps to explain our observations.
A non-rechargeable alkaline battery begins life using zinc powder mixed into a gel containing a potassium hydroxide electrolyte separated from a paste of manganese dioxide powder mixed with carbon powder using a porous membrane. To minimize hydrogen out-gassing an extra measure of manganese dioxide is added. As the battery discharges manganese dioxide powder changes to manganese oxide causing the powdered granules to bond both chemically and physically. This packed-sand consistency reduces the anti-bounce effect exhibited by the gel mixture when the battery was fully charged.