why completely used up batteries seem to have a small bit of energy again after some time.
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Batteries have electrodes in them that are in contact with a solution containing the reactive compounds. I’m saying “solution,” but that doesn’t always mean liquid. Sometimes it’ gel or even a nanopourous solid. In any case, It’s the compounds in solution that provide the electrons on one side and take up the electrons on the other. They need to be at the surface of the electrode to react. That means that the area right at the surface of the electrode can be run out of the compounds it needs. It takes time for fresh compounds to diffuse to the electrode. This effect is going to be strongest when the battery is almost dead, because the concentration of the compounds is very low.
Apart from the chemical explanation, phone batteries seem to gain charge because of a software solution: they measure battery voltage when you turn them on, but after that they just measure how much power they use and estimate your battery level. It’s generally a good idea to state a lower level than what’s actually there compared to showing let’s say 15% and then turning off because it was overestimated and it ran out. This means that the phone undershoots the percentage and says for example 10% for a 15% battery, but when you restart it, it measures the real voltage again and returns to 15
ELI5: they act like you after doing some sports.
When you are working out, you provide your energy.
At some point you may be out of juice and stop working out.
Your body will rebalance out.
5 minutes later, you will have energy left to continue.
Batteries are generally a solid state system. The electrons flow out from one side, so you get a zone of depletion near the exit. With a well-charged battery, electrons flow relatively quickly from further away and replace a good portion of the lost electrons because the battery still has a huge amount of charge. The concentration of free-electrons is pretty high throughout.
As the charge depletes, the average concentration decreases, so it takes longer for charge to move from far away and get to the exit zone. A “dead” battery is generally not truly “dead”, but it has basically no mobile electrons near the exit and can’t get replacements fast enough, so it stops providing energy.
However, the battery generally does have residual charge further away, and the charge will slowly spread (average itself) throughout the battery, rejuvenating the battery a little bit. Eventually, there will not even be enough residual charge far away to get a useful charge anywhere.
Liquids tend to “mix” from areas of high concentration to areas of low concentration much faster than solids. The molecules are more mobile, but even liquid batteries like a car battery can see a zone of depletion in the region of contact between solid (lead) and solution, so the battery can rebound a bit if left to sit, if given a bit of time.
But you can never get back to full charge, only the average charge of the entire battery at the time it “died”.
In this post I think the OP is mainly referring to the chemical aspect of the phenomena that can be observed in non-smart devices, or purely mechanical devices (that have no software-power-management).
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For example:
If you’re old enough to remember using a Walkman, you’ll know that if your double AA batteries run out… but you then wait a while, then try pressing play on the tape again, it will spin up and run again for a little bit, before… running out of juice again.
You could do that a few times, before the 2 AA batteries would no longer spin the tape-motors.
After that, you would then switch to just running the radio (which used a lot less energy than spinning motors). And then again the radio stops working. So you wait a bit again, and then the radio’s working again! But then you wait again, etc… etc…
This process repeats and repeats over and over again, until finally the battery can’t even power the radio anymore.
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Anyways, so ya, this cycle works with other purely mechanical devices too (not just the 80’s/90’s Walkman example I gave).
So that’s what I think the OP is asking about.
But, it’s worth noting that in today’s age, this question does sometimes have a new twist to it, involving software-based energy management of a device, like the kind done by the latest smart-phones.
For that additional new software based element, u/ProPolice55 gives a good description of that above.
You should check out the designs for zombie flashlights that use the residual to power an LED….I learned a lot from that project
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.
Say you have a bottle of ketchup, and as you’re putting it on your burger it sputters and seems to run out, but you shake it to make it settle near the nozzle, and you can get a little bit more. I don’t recommend shaking your dead batteries, but the charge inside will settle on its own after a bit of time.
Batteries are like balloons.
A balloon that your Mom blew up for you is a fully charged battery and you can use the air that’s inside it to run your toy.
When the balloon runs out of air your toy stops, but if you leave the balloon alone for a bit it will inflate itself just a bit (this is depending on what the balloon is made of). You can then use that tiny bit of blown up balloon for your toy, but it probably won’t go far.
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Batteries are like balloons in a lot of ways. Leaving a battery on the charger is bad just like inflating a balloon to its max is bad as it will break. Same with leaving a rechargeable battery dead for too long, like a flat balloon eventually the sides of the balloon stick together and it won’t inflate, a battery that’s been dead for too long won’t recharge.
So don’t overcharge or leave your rechargeable toys or electronics dead for too long or they’ll break.
Could be they are just cooling down which would let them last a little longer because of reduced resistance. That being said normal batteries like AA, AAA, D, whatever never get close to being completely dead, they just fall below the useful voltage. Lots of time this means they are full at 1.5 volts, and dead at 1.0. If you wanted to jerry rig 2 dead batteries together you’d end up with a good one
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