NorthGuy , thank you for the fast reply
you wrote " Battery gets more and more charged. As it approaches the full charge, it can take less and less current. So, the controller has to decrease the current to maintain the same voltage."
1: Why it can take less and less current? I understand that's the battery is more charged ,does it mean its resistance is rising? if the voltage of the battery is constant , the voltage of the Controller is constant, why the current between them changes? I guess I need a battery model , does it behave like a resistor ,capacitor or coil , some superposition of them?