Design PWM based solar charge controller using Arduino program where battery charging will follow above stages. Firstly, when the Arduino controller is connected to the battery $(3\mathrm{.}7$V $9900$mAH$),$ the program will start the operation. Initially, it checks if the panel voltage is sufficient for charging the battery. If so$,$ then it will enter into the charge cycle. The charging cycle consists of $3$ stages.

Stage $1$: Arduino will connect the solar panel to the battery directly $(100\%$ duty cycle$).$ The battery voltage will increase gradually. In this case, please use voltage source instead of solar panel. When the battery voltage reaches $3\mathrm{.}7$V$,$ stage $2$ will begin. In this stage, the current is almost constant.

Stage $2$: Arduino will regulate the charging current by maintaining the voltage level at $3\mathrm{.}7$V for one hour. The voltage is kept constant by adjusting the duty cycle.

Stage $3$: The controller maintains the voltage level at $3\mathrm{.}5$V$.$ This stage keeps the battery to be fully charged if the battery voltage is less than $3\mathrm{.}5$V for $10$mins.

The charge cycle will be repeated.