Because batteries are not a 'static' load, there is a multitude of variable activity that simply can't be summed up in a single phrase or even concept.
At cut-in, the battery is said to be 'clamping' the generator, meaning that is the point at which the battery begins to present itself as a load to the charging source. Below the cut-in voltage, and behind a diode, there is no current flow because the generator cannot overcome the potential at the battery terminals.
Once cut-in is reached, a new set of rules begins to apply, and are dependent on things like terminal voltage, state of charge, internal impedance, and so on. Those are the three that have the most notable effect (in terms of understanding the concepts) in what happens when a battery is connected to a generator.
If you leave the battery out of it for a moment, you'll recall that if you short the genny output, you see instant loading on the shaft, regardless of speed. The counter-torque is always there. The torque increases with speed.
Without overcomplicating things, the current that flows in the wiring at that point is limited by little more than the internal impedance of the windings in the generator, the wiring leading to the short, the short itself, and any diodes that might be in place. Any input on the shaft is converted to heat in these components, and the more energy going into the shaft (in torque and RPM), the more heat will come out. They are directly proportional.
The same thing happens with a battery involved, except, you must reach the cut-in voltage first before current will begin to flow. Until cut-in, the genny only effectively sees an open circuit at it's output.
If you could spin the genny fast enough by hand, this would become more than apparent, but normally, it requires more RPM than one can muster by hand. A 'pedgen' comes to mind as a better example, but not everyone has ever experienced such a thing.
Long story short, the shaft will spin freely up until cut-in, and then significant resistance to further increases in speed will 'suddenly' show up. Only then is current actually beginning to flow.
The battery's terminal voltage will affect what speed the genny will cut in at, and the internal impedance of the battery will affect how much current it will draw. Both are affected by the state of charge of the battery, and with internal impedance in particular, age and use of the battery is also a factor.
So, first you have to overcome cut-in, then you have to get the shaft spinning faster than that, while also providing torque (the force that will make the RPM of the shaft actually mean something in terms of power), and then your battery has to accept what it is being given.
Hence "complex mechanisms".
Indeed. I agree.
Hope this helps a little.
Steve
