How Do AA Rechargeable Batteries Work?
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Basic Function
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The battery works by creating the movement of charged particles, which is usually just called electric current. This is created by the electrochemical reactions between two electrodes, the cathode (positive terminal) and the anode (negative terminal). These two electrodes are not in physical contact, as that would cause a short circuit. Instead, the circuit is formed by a medium called an electrolyte.
What makes battery designs different is the specific electrochemistry used: what the cathode, anode and the electrolyte medium are made of are different for every battery type. This is the heart of battery design, and much more important in categorizing them than details like what their casings look like.
Alkaline Batteries
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Some AA batteries are of the zinc-manganese type. These are usually not rechargeable, and trying to recharge a non-rechargeable battery of this kind will rupture the casing and spill out dangerous potassium hydroxide fluid. This is why it is very important to never try to charge a non-rechargeable battery.
Nickel Cadmium Batteries
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Other AA batteries are of the nickel cadmium design, using nickel oxide hydroxide and metallic cadmium as the cathode and anode. Just as with the zinc-manganese battery, the electrolyte is potassium hydroxide. These batteries usually are meant to be rechargeable.
Recharging
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Applying an electric current to the battery reverses its electrochemistry, causing it to "recharge." In the case of a nickel cadmium battery, overcharging is a risk with the danger of causing the electrodes to break down and release gases. The cadmium in particular is dangerous, since it can release highly flammable hydrogen gas. As a safety measure, it is normal for these batteries to be made so their cadmium anodes have a higher capacity than their nickel cathodes. Also, in their AA form, these batteries are sealed to contain any oxygen and hydrogen that is released by overcharging until they combine to form water.
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