Eli5: Why do small batteries sometimes need their seemingly clean contacts wiped clean in order to work in a device?
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The small batteries might not have the spring contacts forceful and sharp enough. Bigger batteries you might insert with a snap and rotate them a bit, which would scrape the contact points clean. I’ve noticed that coin cells in calculators and similar are usually not held with much force.
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In addition to oils from your hands, there can be a thin layer of corrosion or oxidation on the batteries. In addition, some new coin batteries have a bitter coating to prevent children from eating them that can interfere with some devices.
The small batteries might not have the spring contacts forceful and sharp enough. Bigger batteries you might insert with a snap and rotate them a bit, which would scrape the contact points clean. I’ve noticed that coin cells in calculators and similar are usually not held with much force.
In addition to oils from your hands, there can be a thin layer of corrosion or oxidation on the batteries. In addition, some new coin batteries have a bitter coating to prevent children from eating them that can interfere with some devices.
So as you’ve already concluded: corrosion is the main factor. Contaminants are another. Smaller batteries with bad contacts are often held together by friction. One or two springs keep the battery in place. Spring wants to extend creating a force lateral to the battery this in turn creates a higher friction perpendicular to the battery. This creates more corrosion.
Now this is all well and good but this is just one aspect because just minor corrosion doesn’t reduce conductivity. Small batteries also tend to be used in small appliances that tend to be jostled around. This jostling allows air to occasionally interrupt the contacts but do to the voltage it might create tiny sparks. These sparks also create corrosion, at a much larger scale as it also promotes oxidation of the contact points. A very small layer of “rust” can significantly reduce conductivity.
When a battery becomes old, some might also leak acid (that white crap on your electrical christmas decorations when you forgot to take the battery out). Acid is highly corrosive and if not in an aqueous solution a pretty bad conductor.
So as you’ve already concluded: corrosion is the main factor. Contaminants are another. Smaller batteries with bad contacts are often held together by friction. One or two springs keep the battery in place. Spring wants to extend creating a force lateral to the battery this in turn creates a higher friction perpendicular to the battery. This creates more corrosion.
Now this is all well and good but this is just one aspect because just minor corrosion doesn’t reduce conductivity. Small batteries also tend to be used in small appliances that tend to be jostled around. This jostling allows air to occasionally interrupt the contacts but do to the voltage it might create tiny sparks. These sparks also create corrosion, at a much larger scale as it also promotes oxidation of the contact points. A very small layer of “rust” can significantly reduce conductivity.
When a battery becomes old, some might also leak acid (that white crap on your electrical christmas decorations when you forgot to take the battery out). Acid is highly corrosive and if not in an aqueous solution a pretty bad conductor.
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