No, it is not that easy...
I try to explain in short words.
Current always flow from the higher potential (higher voltage) to the lower. So some current (call it capacity) will move form the source to the weaker battery. Fine so far. But you will reduce the capacity of the source and thereby pull down the potential of it while the receiving battery raises its potential. The bulk loading will end up soon, when both potential are equal. Tests have shown, that connected batteries with a different potential will have a high current flowing in the beginning, only a few seconds up to some minutes, which then drops to a low current and nothing relevant happens anymore in a short time. If you keep them connected over a longer period, of course, things will continue (to get worse). You always have losses of power while loading, going into the sky as heat. So in the end you loose power and there is no new power coming into the system as there is no power source, only batteries. If you leave them connected you always have a certain rate of self discharging, both batteries will empty themselves over time (with healthy batteries that takes some time). They give power to each other, no battery will stay on its potential level and thereby pull current from the other and vice versa. But due to the losses while loading, the overall capacity decreases.
To load a battery you need a source which gets new power all the time and which is able to keep its potential above the potential of the battery to be loaded. That also compensates the losses of loading. Loading of a battery is running through different phases, for a reason:
1. Bulk loading phase - The potentiol of the source is raised such high, that the maximum loading current is achieved. This currents depends on the internal resistance of the loaded battery and the voltage of the loader. While loading the internal resistance of the receiving battery increases slowly. To keep up the current the voltage must also be increased (Law of Ohm: I = U/R. If R increases, U must increase as well to keep I the same). You can monitor that wth a battery monitor very well.
2. Absorbtion phase - When a certain percentage is reached, the potential is not increased anymore, but the internal resistence still increase. Thereby the current decreases. That is the final loading phase of the battery. When the current fells below a certain level, the battery is considered full. Thereoretically it should become zero, but in the real world, that could take a loooong time, so all battery charging devices and montiroing devices define a certain current, which when reached the battery is considered full.
3. Floating phase - The floating phase keeps the battery fully charged and puts power in which was sucked out or was lost due to self discharging mechanisms.
Why just use the source as the power to crank the engine. That what it is made for and the source battery doesn't know which engine it starts.
AWo
