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.
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
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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