Solar Charge controllers: all you need to know

A solar charge controller is an electronic component that controls the amount of charge entering and exiting the battery, and regulates the optimum and most efficient performance of the battery. Batteries are almost always installed with a charge controller. The controller helps to protect the batteries from all kinds of issues, including overcharging, current leaking back to the solar panel during the night, the prevention of Undervoltage and it helps to monitor the status of the batteries.

How do Charge Controllers work?

Power generation solar power plants, wind power plants, tidal power plants, fuel cells, etc., are intermittent in nature, which means these power sources are not producing electricity constantly and at any moment of the day. For that reason they require a battery for storage of electricity. The battery does not perform any modification or transformation of the electrical current; it simply stores it. Whenever electricity is not available, the stored charge inside the battery is used to provide power to the loads. Batteries are almost always installed with a charge controller. As the name implies, a charge controller is an electronic module, which controls the amount of charge entering and exiting the battery. Charge controllers are installed for optimum and most efficient performance of the battery, and to protect the battery from over-and undercharging.

Voltage and battery (dis)charging

There’s an interesting relationship between the charging / discharging of batteries and its voltage. This relationship is graphically shown in below Figure 1. As the battery begins to discharge, it experiences a slight reduction in its output voltage. This relationship is used in the working of the charge controller. Charge controllers have built-in voltage sensing instruments (potentiometers), which sense the output voltage. Depending upon the output voltage, the charge controller determines the charge percentage of the battery, using a similar curve below. Figure 1: Typical discharge curve (voltage versus % charge) for a 24 volt lead acid battery Then depending upon the programmed parameters, the charge controller determines its course of action. For example: Discharge curve (voltage versus % charge) 24v lead acid battery

The charge controller can be programmed to disconnect the battery whenever it reaches 80% discharge. When the potentiometer senses an output voltage of around 22.8V (see curve above), it will open the breaker of the battery to disconnect it from the system, thereby protecting the battery. A large number of parameters can be programmed, each with a different action to be taken, depending on the user’s requirements.

Major functions performed by solar charge controllers

While the primary function of any charge controller is to control the amount of charge entering and exiting the battery, it is not its only function. Modern solar charge controller perform several other useful functions:

1. Block reverse current

   This function facilitates a unidirectional flow of current from the solar panel to the battery, and blocks the reverse flow during the night. This helps to prevent batteries from unnecessary discharging and it increases the battery uptime.

2. Under voltage protection

   Under voltage occurs when the batteries have lost 80% of their charge. It is recommended to take the battery out from the circuit and connect it back only during charging. As the charge level is proportional to voltage level, the charge controller cuts out the battery at a certain voltage level, preventing Under voltage situation.

3. Prevent Battery Overcharge

   Overcharging of batteries could significantly reduce the lifetime of the batteries and is therefore not recommended. The charge controller stops the charging of the batteries once these are sufficiently charged.

4. Configure Control Set Points

   Various set points could be edited and re-programmed using the charge controllers. This helps in fine tuning of your battery charging and discharging cycles to ensure the most efficient performance and longer life.

5. Displays and Metering

   Various parameters could be monitored using the display screen of charge controllers. Some commonly monitored parameters include: Voltage level, Charged percentage, Current Discharge time at fill load, etc.

6. Troubleshooting and Events History

   Some charge controllers have an inbuilt memory to save events and alarms with a date and time stamp. This events and alarms history helps for quick troubleshooting.

Pre-settings: Four Key Parameters

Below you find four key parameters that can be programmed in any charge controllers. They are:

1. Regulation Set-Point

   This is the maximum set-point voltage. Any charge controller will protect the battery to reach a voltage in excess of this Voltage. At this point, it will discontinue any further battery charging.

2. Regulation Hysteresis Set-Point

   This is the difference between the Regulation Set-Point Voltage and Voltage when full current is reapplied, also called Regulation Hysteresis Voltage Span. This set point should be as high as possible to prevent switching disruptions and harmonics.

3. Low Voltage Disconnect Set-Point

   This is the minimum set-point voltage. Any controller will not allow the battery to reach a voltage lower than this Voltage. At this point, it will disconnect the load to prevent battery under discharge.

4. Low Voltage Disconnect Hysteresis Set-Point

   This is the difference between the Low Voltage Disconnect Set-Point and Voltage at which the load will be reconnected, also called Low Voltage Disconnect Hysteresis Voltage Span. This set point should be as high as possible to prevent frequent disruptions to the connected load. Please note that these set-points shall be recommended by the manufacturer. If you use these parameters well, altering these set-points can result in a more efficient operation of your solar energy systems.

Spending money on a quality charge controller is a good investment

Without any doubt, a quality charge controller will protect and increase the battery life of your solar system and also helps in monitoring and quick troubleshooting. When using the right charge controller the lifetime of your battery bank can easily be extended with several months. As a charge controller only accounts for a small portion of the overall solar system cost, it’s highly recommended to purchase a quality charge controller.