why do rechargeable batteries go bad over time and lose their ability to hold a charge?

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why do rechargeable batteries go bad over time and lose their ability to hold a charge?

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I’m not 100% sure but I think the battery acid can only be recharged x amount of times and each time it does looses y amount of total capacity. Which eventually it means the charge doesn’t last long at all.

Batteries have chemistry going on inside them. Every battery is a little self-contained chemistry set. The chemical reactions that we call “charging” and “discharging” are harsh to the materials of the battery. Sometimes crystals form inside the battery. Sometimes contamination causes unintended chemical reactions. Sometimes environmental factors like hot and cold degrade the battery. Sometimes the electrolyte degrades into other molecules over time. However the battery dies, it’s really just a matter of the materials they are made of degrading over time. Once the materials are degraded enough, the chemistry just doesn’t happen.

Batteries are chemical things, they rely on specific reactions happening, and not only happening, but in specific spots. That is the reaction only happens where the electrolyte contacts it, but that spot needs to be the right spot. For example, an electrode might me a mesh of carbon coated in the battery material. When it reacts it contracts or expands. The layers must always be thin enough to let the electrolyte through the mesh, and if it gets too thick it might hit the other side and crack the electrode. Both of those would reduce capacity.

In the long term, it’s never perfect. Impurities mean the wrong reactions happen and they can’t be undone (basically removing theoe chemicals from the battery). Also, since ions are moving around, eventually then end up in the wrong spot and cause physical damage.