A quick look at battery leaks and longevity
“Daddy, why do batteries rot?”
While my third-grader is planning to use potatoes and lemons in an upcoming science fair project, I don’t think my five-year-old was thinking of produce when he asked this question. Most batteries are made, or at least sealed, in inorganic materials, and so they’re unlikely to become lunch for bacteria or fungus like an old apple might. A leaking alkaline battery does look a little like it’s growing mold though, so that seemed like a decent place to start with his question.
The white crust that can build up on a battery is the byproduct of a leak in the battery’s shell. As a battery is used, ages, or just reacts to changes in temperature, the internal chemical reactions produce extra hydrogen gas. In good conditions, that gas can be vented through a gasket at one end of the battery (along the negative end of a AA battery, for instance) so that pressure doesn’t build up in the shell and cause it to burst. If the hydrogen is produced too quickly for the gasket to handle, pressure will build up and rupture the shell, allowing some of the potassium hydroxide, which is the battery’s electrolyte, will start to leak out.
Potassium hydroxide is a caustic base, and can cause eye, skin and respiratory irritation. You don’t want to touch or inhale it, which is tricky since you won’t usually see that it’s there. The white crust you do see on a leaky battery is actually potassium carbonate, which is the product of leaking potassium hydroxide reacting with carbon dioxide in the air. The resulting power is a bit like rock salt, and is basically inert. However, since it’s likely to have bits of the liquid electrolyte on it, it’s still best to avoid handling it.
Batteries that have burst shouldn’t be used, but they might not be “dead” in the way my five-year-old was thinking. Another form of battery decomposition is the way a battery can lose its charge over time, even when it’s not being used. You may have run into this with the rechargeable lithium ion batteries in your cell phone or laptop, assuming you’ve put them down long enough for them to lose their charge.
There are two main reasons for a rechargeable battery to unexpectedly turn up dead. The first is that irregular charging cycles and changes in temperature wear the battery’s cathode out faster. Since the cathode helps control the flow of electrons through the battery’s electrolyte, its degradation reduces how much the battery can be recharged. Eventually, its capacity is reduced, and your device seems to run out of juice in what feels like the blink of an eye. Alternatively, a battery that is left with no charge at all will likely trigger a protection circuit, preventing the battery from ever charging again to avoid putting power through what may be damaged cells.
From the look on my five-year-old’s face, this wasn’t the “rot” he was thinking of. He just wanted to know why batteries in his toys get used up, which is actually a similar scenario to the depleted rechargeable batteries described above. To simplify things a bit, an alkaline battery creates power by moving electrons between the manganese dioxide cathode and the zinc anode. Those materials are consumed in this reaction, eventually leaving the battery with no way to get electricity moving again. So to return to the biological analogies my son seemed to favor, it’s a bit more like the battery runs out of food, although in the case most AAs, we don’t have a way to feed them again later.
Source: Alkaline battery, Wikipedia