What is the function of solar charge controller?
3.1 Preventing overcharge
When a battery is completely charged, it cannot store more solar energy as chemical energy. But if power is continuously applied to the fully charged battery at a high rate, the power will be turned into heat and gassing, which would present as a flooded battery with a lot of bubbles from the electrolytes. That is the hydrogen gas, which is generated from a chemical reaction. These gases are dangerous since they are explosive. Overcharging also accelerates battery aging. And then we need a solar charge controller.
The main function of the solar charge controller is to regulate the voltage and current that is generated by solar panels going to the batteries to prevent batteries from overcharging and guarantee the batteries a safe working condition and a longer lifespan.
There are 3 types of regulators:
A current regulator acts like a switch. It simply switches the circuit on or off to control the energy flow to the battery bank, just like stage 1 bulk charging. They are usually called shunt controllers, which are no longer used due to their obsolete technology.
2. Pulse width modulation (PWM)
Shunt controllers shut down the current completely, while the PWM controller reduces the current gradually. PWM is more similar with stage 3 float charging.
We will have an in-depth discussion about PWM and MPPT when we start the topic: PWM VS MPPT which one is better.
3. Voltage regulator
Voltage regulation is common. The solar charge controller regulates the charging in response to the battery voltage. It is quite simple. When the voltage of a battery reaches a certain value, the controller protects the battery from overcharging by reducing the power. When the voltage of a battery drops because of a large sum of power consumption, the controller will allow bulk charging again.
3.2 Blocking reverse current
The second main function is to prevent reverse current flow.
At night, or whenever there is no sunlight, the solar panel does not have power to convert into electricity, and, in a solar power system, the voltage of the battery bank will be higher than the voltage of the solar panel, since we all know electricity flows from high voltage to low voltage. So, without a charge controller, the electricity will flow from the battery bank to the solar panel, which is a waste of power, as the solar power system takes efforts to collect energy during the day but wasting a little of them at night. Although the loss is only a little in proportion to the total energy collected, it is not hard to solve.
A solar charge controller can deal with this problem.
Most controllers allow the flow to go only from solar panel into a battery bank by designing into the circuit a semiconductor, which only passes currents in one direction.
Some controllers have a mechanical switch, which is also called a relay. When the relay clicks on and off, you will hear a clatter sound. When the voltage of the solar panels is lower than that of the battery bank, it detects and then switches off the circuit, disconnecting the solar panels from the battery bank.
3.3 Load control
Some solar charge controllers are designed with load control, allowing you to connect a DC load, such as an LED lamp (a concrete example is on our website all-in-one solar LED street lights), direct to the solar charge controller, and the load control will turn the lamp on and off according to its pre-settings (the voltage of battery, photocell sensor, or a timer).
For example, there commonly are timers in LED solar street lights, and the load control will read the time from the timer and then execute the command: turn the LED on at 7:00 pm at dusk and turn it off at 6:00 am the next morning. Or the load control will read information from the photocell sensor and then control the LED on and off according to the brightness of the ambient environment.
3.4 Low voltage disconnect (LVD)
Imagine that you are boiling water in a pot and you forget to turn off the fire until the boiling water is totally evaporated; no longer any water in the dry pot and the pot overheats. The pot is destroyed permanently. In the same way, discharging a solar battery completely will result in permanent damage to a battery.
Deep cycle batteries are widely used in solar power systems. The Depth of Discharge (DOD) could be as large as 80%; however, they are susceptible to permanent damage if discharged up to 90% or, even worse, 100%.
If you wait to switch off the DC load from your batteries until you find your lights dimming, the battery damage could have already happened. Both battery capacity and life expectancy will be decreased every time when over-discharge happens. If the battery were set to work in this kind of over-discharge state for a period of time, it would be ruined quickly.
The only practical solution to protect batteries from over-discharge is to switch loads (such as appliances, LED lights and so on) off and on, provided that the voltage has recovered from bulk charging.
Typically, if a 12V battery drops to 10.9 volts, the battery would be on the verge of over-discharging. In the same way, 21.9 volts for a 24V battery.
If your home solar system has some DC loads, the LVD feature is necessary. Some LVDs are integrated into charge controllers while others aren’t.
3.5 Overload protection
When the input current flow is much higher than what the circuit can safely deal with, your system overloads. This can lead your system to overheat or even cause a fire. Overload can be caused by different reasons, such as a wrong wiring design (short circuit), or a problematic appliance (a stuck fan). Commonly, a push-button reset is designed for the overload protection circuit.
However, there is a built-in overload protection in each solar charge controller; large solar power systems usually require double safety protection: fuses or circuit breakers. If the wire carrying capacity is smaller than the overload limit of the controller, then setting up a fuse or breaker in your circuit is a must.
The displays of solar charge controllers vary from LED indicators to LCD screen displays, with information of voltage and current. Displays to solar power systems are what console dashboards are to cars. They provide you with detailed data so that you can monitor the state of your battery bank: how much energy you are using or generating.
If your system already has a self-contained monitor, then the display feature would not be important. Even the cheapest monitor would include basic meters, just as controllers have.