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  • Solar Charge Controller

  • Created: Jul 06, 2016

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Charge controllers are often in conjunction with solar or wind power generators. A charge controller prevents overcharging and may protect against overvoltage, which can reduce battery lifespan or performance. It also limits the rate at which electric current is added to drawn from electric batteries. In the solar-powered lighting systems, the solar charge controller plays an important role as the system's overall success depends mainly on it. It is an indispensable link between the solar panel, battery and load.

There are 2 methods of controlling the charging current: series and parallel regulation. In this circuit, parallel regulation is used because the control circuitry allows the charging current to flow into the battery and stop charging once the battery is fully charged. Also, instead of wasting the charging current as heat, the circuit allows it to be pulsed and applied to the battery to keep it topped-up. After power-on, the microcontroller reads the battery voltage with the help of the ADC0831 and displays the values on the LCD. It also monitors the input signal from dusk-to-dawn sensor and activates the load or charging the relay RY1 accordingly. In the ADC, as the Vref is connected to the Vcc(5V), the input voltage of the ADC cannot exceed +5V. A potential divider is used at pin2 to scale down the voltage from 0-20V to 0V-05V. The ADC output is multiplied four times and displayed on the LCD as battery voltage.

Initially, when the solar panel voltage is present, the dusk-to-dawn sensor provides signal to the microcontroller, which then displays 'charging' message on the LCD. During charging, the battery voltage is continuously monitored. The battery used here is a rechargeable battery. After the voltage reaches 14V, the microcontroller interrupts the charging current by energising the relay, connected to the Q2, and starts a 5-minute timer. During this stage, the LCD shows 'battery full'. After 5 minutes, the relay reconnects the panel to the battery, the charging current is pulsed at the intervals of five minutes and cycle repeats until the panel voltage is present. When the panel voltage falls below the zener diode (D1) voltage, the microcontroller senses this and activates the load by switching on Q3 via opto-isolator (U3) and displays 'load on' on to the LCD. When the battery voltage drops below 10V, the microcontroller then turns off the load by switching off the Q3 and displays 'battery low-load off' to the LCD. When the load is switched off, there are tendencies that the battery voltage tends to rise back and load oscillates between 'on' and 'off' states. In this case, the microcontroller employs a hysteresis control by entering into a 'lock' mode and comes out of the lock when the dusk-to-dawn sensor receives the panel voltage. During lock mode, the microcontroller keeps converting the ADC value and displays the battery voltage on the LCD.

Solar-powered lighting systems are already available in rural as well as urban areas. These include solar lanterns, solar home lighting systems, solar street lights and many more. All of them consists of four components: one of these is the solar charge controller and load. The circuit is suitable for a 10-40W solar panels and can operate when 10A load is connected.