why completely used up batteries seem to have a small bit of energy again after some time.
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A battery stores energy by putting a bunch of electrons somewhere they’d rather not be normally. Hooking it up to a circuit lets the electrons go “downhill” to a place they’d rather be, and any devices in the way are able to take some of the energy from the flowing electrons kinda like a water wheel in a river. If not enough electrons are flowing, you don’t have enough power to “turn the wheel” and run whatever you’re trying to run.
But the electrons aren’t just floating around in the battery. Most of them are tied up in molecules that have to undergo chemical reactions to let go of them. These reactions go both ways all the time, but they’ll balance out to a set ratio of tied up electrons to available electrons. So if you take out available electrons, more will free up to restore that balance. For the sake of ELI5, we’re going to say the tied up electrons are like ice and the ones that can move are like water.
So essentially you have a partially frozen reservoir uphill from a water wheel. After the liquid water empties out, the trickle coming from the ice melting isn’t flowing strongly enough to turn the wheel. But if you block off the reservoir and let the ice melt, you can build up enough liquid water to move the wheel a little bit when you unblock it.
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