Say I have my phone on 5% and a large battery pack on 35% and I charge the phone. By the end the phone is on 100% and the pack is on 12%.
How can the battery pack charge the phone up to a higher percentage of its current charge?
I would expect that the phone would stop charging when the percentage on the power pack was equal to the percentage on the phone, but clearly it doesn't, so what's happening?
Best Answer
Sometimes it is easier to understand circuitry in the context of water. What you're imagining is two tanks of water of equal size linked together by a pipe that has been sealed off. If one tank holds 5% water and the other holds 35% water, when you remove the seal, the tanks equalize and you end up with 20% in both tanks.
What you're forgetting is that like batteries, water tanks come in varying capacities. A large battery pack "water tank" is more equivalently like a tank 4 times the size of the phone "water tank". When you release the seal, the phone water tank is filled easily and the source, the battery pack water tank still has plenty of water. Charge works similarly, except you cannot "see" charge like you can with water in a water tank.
So what happens when you have a large tank with water at the same level as the smaller tank? Despite the larger tank holding more water, since they are at the same water level, they are at equilibrium and so one doesn't "charge" the other. Likewise a sufficiently uncharged battery, even with plenty of capacity, can have low charge and not charge the phone.
Hope that explains it.
Edit: In light of several comments, perhaps it would be imprecise to imagine two tanks one next to the other, but rather it would be more accurate to imagine the large battery tank on top to fill the phone battery tank below, since potentially all of the charge could transfer from the large battery pack to the phone battery.