This is approximately how it works. Imagine ten buckets, each 42cm high with some water in them. The levels are all slightly different. A tap (the charger) pours water into all buckets at exactly the same rate for each bucket until water spills over a rim and falls on the floor. As soon as water is on the floor, the tap is blocked to stop further spillage. When the tap is blocked, it turns itself down to a trickle, but the trickle can't go anywhere because it's still blocked.
Each bucket has a tiny hole drilled in the side at a height of 41.8cm, so all the time the water is above the hole, some will leak out and go down a drain (not on the floor). If you fill the bucket to the top, it will end up with water 41.8cm deep because of the hole. As soon as the level drops below 42cm, the tap is unblocked, but now it has adjusted itself to just a trickle, so a trickle of water goes into all buckets, which fills them very slowly, while all those ove 41.8cm deep are leaking down the drain.
There is enough pressure in the pipe to fill all buckets to 42cm, but as the buckets get close to that, there is no longer enough pressure to pump the water through the taps, so the flow slows right down when the buckets become full until it's just a trickle.
42cm is equivalent t0 4.2v and 41.8cm is equivalent to 4.18v.
