Analog Solar Charge Controller System With Battery Health Monitor.

This project is a completely analog solar charge controller that can take care of a lithium battery bank when 5V solar panels are used. This project includes a real-time battery status check with two power outputs 5V as well as 12V. In contrast to purely digital solar charge controllers, this project makes use of highly reliable operational amplifiers (LM324) to execute the logic for switching.

Semiconductors & ICs:

1. U1, U4 (LM324): Quad Operational Amplifier used for comparator logic.
2. U2 (L7805): 5V Linear Voltage Regulator for stable logic power.
3. Q1 (BC558): PNP Transistor to drive the solar relay.
4. Q2 (BC547): NPN Transistor to drive the load relay.

Switching & Protection:

1. K1, K2: 5V SPDT Relays for solar and load switching.
2. D2, D4 (1N5408): High-current Power Diodes.
3. D1, D5 (1N4007): Flyback protection diodes for relay coils.

Passive Components:

1. RV1, RV2 (10kΩ Potentiometers): Used for calibrating charge and discharge limits.
2. R9, R15 (100kΩ Resistors): Provide hysteresis to prevent relay chatter.
3. C3, C4 (10µF Electrolytic Capacitors): For regulator input/output stability.
4. C1, C2, C5, C6 (100nF Ceramic Capacitors): Noise decoupling for Op-Amp inputs.

Hardware:

1. Aluminum Enclosure: Conductive housing for the final build.
2. Voltage Reader: Digital display for battery monitoring.
3. 3x 1800mAh Batteries & 3x 5V Solar Panels.

Understanding the Analog Logic Description:

The brain of the system is the LM324 Quad Op-Amp. I used two of its four stages as comparators:

1. Solar Cut-off (U1A): Compares battery voltage via RV1. When the battery is full, Q1 (BC558) activates Relay K1 to disconnect the solar ground.
2. Load Cut-off (U4A): Monitors deep discharge. If the battery falls below the limit set by RV2, Q2 (BC547) switches Relay K2 to turn off the load. I included R9 and R15 (100kΩ) for hysteresis to ensure the relays don't switch rapidly (chatter) when the voltage is right at the threshold.

Breadboarding and Threshold Tuning Description:

Before moving to the final build, I assembled the circuit on a breadboard. I used a variable power supply to simulate the battery voltage.

1. Adjusting RV1: I set the power supply to 12.6V (full) and turned RV1 until the solar relay clicked off.
2. Adjusting RV2: I dropped the supply to 10.5V (low) and adjusted RV2 until the load relay disconnected.
3. Hysteresis check: I ensured the 100kΩ resistors provided a stable 'swing' so the system remained steady.

Preparing the Aluminum Chassis Description:

I chose an aluminum enclosure for its durability and heat dissipation. I drilled cutouts for the voltage reader, the main toggle switch, and the USB/12V outlets. Important Note: Because aluminum is conductive, I had to ensure the L7805 regulator and the PCB were properly insulated from the chassis to avoid ground loops or short circuits. And also i add another L7805 regulator for take 5v out for the 5v phone charger outlet(USB). Then I stick a insulating black color sticker on enclosure to get a good look.

Real-world Calibration and Lessons Learned Description:

The final assembly in the metal box showed some slight variations compared to the prototype. I had to recalibrate RV1 and RV2 slightly to account for the voltage drop in the internal wiring. The system now successfully charges via the 5V solar panels and provides a safe, regulated 5V output for mobile devices and a 12V output for heavier loads.