Eli5: How does a power bank charge a device until it dies, wouldn’t equilibrium be reached?
In: 551
Power banks for charging portable devices contain circuitry to regulate their output voltage to 5v (or higher for certain fast charging standard), and as such can continue providing power until a determined low-cutoff value is reached.
It’s not like connecting two water containers together and letting them equalise. The power bank is essentially pushing electrons into the device, like a delivery driver delivering a package to your door. The delivery driver doesn’t care what you do with the package once it gets into the house – you could open the package now, or you could put it in a room to store it for later. Similarly, the power bank doesn’t care what happens to the electricity when it gets inside the device.
As long as there is a voltage differential and a clear circuit, current will flow. Your phone doesn’t feed electrons back through the direction they came from, so there’s always a difference in voltage, until the difference is too low to maintain a flow of electrons, aka, the power bank goes flat.
The power bank always output at negotiated USB voltage and the phone always receive at that voltage, it doesn’t change, there is no equilibrium.
It’s a little more like water draining from a container that’s intentionally slightly higher up. All or almost all of the water can drain into the lower container as long as the water levels don’t catch up with each other.
We just use trickery and clever design to always make sure the thing that charges is higher on the shelf than the thing being charged
Edit for clarity: height, in this case, is analogous to voltage
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Power for most devices are one way for power. Power bank has an output and the phone has an input. If you plug two phones together they don’t charge each other. It’s like filling a toilet tank there is a valve (Transistors) that stop the flow when the receiving device is full.
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There’s no charging that goes back from the device to the bank, so the power only goes one way.
An analogy using water is like transferring water from bucket A (initially full) to bucket B (initially empty). Yes, on level ground the water levels would just equalize, but what the circuitry inside a power bank essentially does is to raise bucket A as it empties (Eg someone lifts it as it empties). This way, the water level in bucket A stays constant relative to the ground, even if the water level relative to the bottom of bucket A changes. Accordingly, bucket B continuously fills.
Eventually, you just run out of water in bucket A. Lifting it any higher won’t give you more water. That’s when the power bank runs out of battery.
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