how do alkaline batteries have a charge but can’t be recharged?
In: Chemistry
The charge of an alkaline battery is the result of electrons moving from one chemical to another inside the battery; most alkaline batteries send electrons from zinc metal to manganese dioxide. This process chemically transforms both! The zinc is oxidized (similar to iron rusting) into zinc oxide, and the manganese dioxide is reduced to Mn2O3.
This process can, in fact, be reversed – you *can* recharge alkaline batteries by feeding electricity into them. But this reaction, and the battery the materials are contained in, aren’t designed for reversal. The Mn2O3 is much harder to push back into MnO2 and completely changing it all back into its original form isn’t feasible. Additionally, the reversed reaction is very exothermic (heat-generating) and is going to build up pressure, heat, and potentially small amounts of hydrogen gas. Hydrogen gas tends to explode under high heat and pressure (as any fan of German zeppelins will tell you), so this is not a very good idea.
Technically, they can be, but it’s not safe because the process can cause a buildup of gas in the cell and it can explode caustic electrolytes all over the place. They also don’t charge very well using a standard charger so even if they don’t leak or explode you won’t get very good reuse from them
Batteries work by doing a chemical reaction, usually akin to something like a piece of metal getting rusty when you leave it exposed to water/salt. This process creates electricity.
Different types of batteries use different metals and chemicals – nickel metal hydride, nickel cadmium, lithium ion, lithium iron phosphate, etc.
Some of these chemical reactions can only happen in one direction – you leave a shiny piece of metal out and it rusts and that’s that. Others are reversible, like if you could put electricity into a rusty piece of metal and make it shiny again. These are rechargeable batteries. Some reactions are reversible but not easily and only under certain conditions.
Regular alkaline batteries are one direction, and they are “charged” when you get them from the store but not in the sense that somebody built a battery and then added electricity to it before boxing it up. Someone put a piece of shiny metal and salt water together in a factory, and using it will rust the metal and make electricity.
As an aside, these different chemical reactions determine what the voltage of the battery is. Regular alkaline batteries are 1.5v because that is how much voltage the reaction generates. NiMH is 1.2v. Lithium ion is 3.6-4.2v. The only way to get higher voltage batteries is to take multiple cells of the same reaction and stack them together. You can’t buy a 6v lithium battery, but a 20v lithium battery is just 5 cells in a row (5 x 4v).
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