What is the importance of solar charge controller?

28/1/2015

The solar charge controller is a small component in the overall solar power system but it plays a crucial role in the proper functioning of the solar power system. A right configuration of the solar charge controller helps in producing optimum output and the smooth functioning of the system.

Definition

A solar charge controller is required in almost all solar systems which require the battery backup.
The solar charge controller is an electronic device when placed between the solar panels and the battery regulates the voltage coming from the solar panels to the battery.

Function

It protects the battery from being overcharged. The solar charge controller monitors the voltage of the battery and when the battery voltage reaches to the certain level it opens the circuit and the charging of the battery from the panels get stopped. It also blocks the reverse current from flowing back to the solar panels. During night when there is no sunlight hence no electricity is coming from the solar panels. The current from the battery can leak pass to the solar panels and can drain the batteries. The solar charge controller detects when no energy is coming from the solar panels, it opens the circuit and disconnects the batteries from the panels hence avoids the reverse flow of the current.

Example

A 140 watt panel at STC (Standard Test Conditions) has the following specifications:

voltage of 17 volts
current of 8.24 ampere

This amount will charge the 12 volts/100Ah battery. As the battery requires 12 volts to get charged the remaining (17-12) volts i.e. 5 volts will get wasted. The power which is going into the battery is 12 volts*8.24 amperes=98.9 watts while the panel is producing 140 watts. The remaining power of {(140-98.9) watts} 41.1 watts gets wasted. The efficiency of the system is reduced by almost 30%.
It will take almost (12 volts*100 ampere-hr)/ (98.9 watts) = (1200 watt-hr)/ (98.9 watts) = 12.13 hours to charge the battery (assuming the output power from the panel is constant). As the battery gets fully charged the charge controller circuit will get open and the charging of the battery will get stopped. Hence preventing it from the overcharging.
Here we saw that even though the normal charge controller is able to protect the batteries from overcharging but at the same time the efficiency of the system is reduced by 30%. So we need something which in addition to protecting the batteries from overcharging also keeps the system effective and efficient.

The MPPT solar charge controller

Here comes MPPT (Maximum Power Point Tracking) charge controller in picture. The basic function of this charge controller in addition to protecting the batteries is to provide the maximum power to the batteries and reduce their charging time.
Let us take the same case in which the voltage from the solar panel is 17 volts. As the requirement of the battery is 12 volts, the remaining 5 volts is converted to equivalent amount of amperage by the MPPT charge controller. Here the power generated by the panel and the power reaching to the battery for charging is the same i.e.140 watts. (Ignoring other losses like heat loss while passing through wires)

A 140 watt panel producing 17 volts and current 8.24 amperes under STC. The MPPT charge controller will convert the excess 5 volts into the equivalent amperage to keep the power same.
Power from the panel=Voltage*Current=17volts*8.24 amperes=140 watts
Power reaching to the battery=12 volts*11.67 amperes=140 watts (the excess voltage from the panel gets converted to equivalent amperage to keep the power same)
The charging time of the batteries in this case will be

= (12 volts*100 ampere-hr)/140 watts
= (1200 watts-hr)/140 watts
= 8.6 hours (Assuming the power from the solar panel is constant).
Hence the charging time is reduced.

The output of panel depends on temperature & the intensity of solar radiation

The voltage and the current from the panel is the function of the temperature and the amount of the solar radiation falling on the surface. As both of these parameters change continuously which cause the output power from the panel to vary throughout the day; the MPPT charge controller detects the maximum power produced by the solar panel and transmits the same to the battery for charging.
Even though the MPPT charge controller is expensive than the normal charge controllers but paying high price is worth in a view of the increased efficiency of the system.