It is not appropriate to discuss this problem in the physics part, because the charger is a complex engineering problem. I am engaged in power supply design, including charger. In fact, the charger is a regulated power supply. You will never think that a seemingly simple internal circuit of the charger is actually a complex circuit containing hundreds of components.

Because our daily electricity consumption is industrial AC power grid, not only 220V is AC, but also 10% fluctuation, so an ordinary power conversion circuit will basically include: bridge shaping circuit-converting sine wave signal into sine positive half-wave signal, capacitor filtering circuit-preliminarily filtering AC component, series feedback voltage stabilizing circuit (some with integrated voltage stabilizing tube) and so on. Many very complicated power supplies can be derived from this basic linear regulated power supply. Now the charger is more and more complicated, which is beyond your understanding.

When I was in high school, I often thought about charging the battery like you. Now it seems so funny. Not yet. Let's focus on other issues.

It is even more unrealistic to charge another battery with a regulated battery. . This thing is not as simple as you think, because it is not irrigation. As long as both water bottles are opened, they can be poured into each other. .