You can also see the updated schematic here.

The input power connector to the solar panels is the screw terminal JP1 and JP2 is the output screw terminal connector to the battery.The third connector JP3 is connection for the load.

F1 and F2 are the 5A safety fuses.

The buck converter is made up of the synchronous MOSFET switches Q2 and Q3 and the energy storage devices inductor L1 and capacitors C1 and C2 The inductor smooths the switching current and along with C2 it smooths the output voltage.Capacitor C8 and R6 are a snubber network,used to cut down on the ringing of the inductor voltage generated by the switching current in the inductor.

The third MOSFET Q1 is added to allow the system to block the battery power from flowing back into the solar panels at night.In my earlier charge controller,this is done by a diode in the power path. As all diodes have a voltage drop a MOSFET is much more efficient.Q1 turns on when Q2 is on from voltage through D1. R1 drains the voltage off the gate of Q1 so it turns off when Q2 turns off.

The diode D3 (UF4007) is an ultra fast diode that will start conducting current before Q3 turns on. It is supposed to make the converter more efficient.

The IC IR2104 is a half bridge MOSFET gate driver. It drives the high and the low side MOSFETs using the PWM signal from the arduino (Pin -D9) .The IR2104 can also be shut down with the control signal (low on pin -D8) from the Arduino on pin 3. D2 and C7 are part of the bootstrap circuit that generates the high side gate drive voltage for Q1 and Q2. The software keeps track of the PWM duty cycle and never allows 100% or always on. It caps the PWM duty cycle at 99.9% to keep the charge pump working.

There are two voltage divider circuits( R1,R2 and R3,R4) to measure the solar panel and battery voltages.The out put from the dividers are feeds the voltage signal to Analog pin-0 and Analog pin-2 .The ceramic capacitors C3 and C4 are used to remove high frequency spikes.

The mosfet Q4 is used to control the load.The driver for this mosfet is consists of a transistor and resistors R9 ,R10.

The diode D4 and D5 are TVS diodes used for over voltage protection from solar panel and load side.

The current sensor ACS712 sense the current from the solar panel and feeds to the Arduino analog pin-1.

The 3 LEDs are connected to the digital pins of the microcontroller and serve as an output interface to display the charging state.

Reset switch is helpful if the code gets stuck.

The back light switch is to control the back light of LCD dislay.