How does MPPT charge controller work

Hi, I have been trying to learn more about MPPT charge controllers and how they work because I would like to build one. I have read several papers but non of them talk about the part I am interested in.

For what I understand, they have a buck converter to lower the voltage the wind turbine is venerating to the battey voltage. So since the battey voltage is considered constant in most of the cases, if the duty cycle of buck converter is 100% it would be like connecting turbine directly to battery and if duty cycle decreases then turbine voltage and velocity increase. This way the MPP is reached.

What I do not manage to understand is how could you achive stage charging of battery. So for the bulk charging there shouldn't be any problem because the battery will just take current and rise its voltage. But how do you make it stop at absorb voltage and later reduce to float voltage?

Thanks in advanced