Solar charge controller is the heart of the off grid solar system, so we need to pay a special attention in selecting and sizing. Thus, we can fully utilize our solar power system.
What is the solar charge controller and why do we need it?
The solar charge controller is an electronic device that works as a voltage and current regulator in off grid solar system. It is used to charge batteries from solar panels during daytime hours and discharge batteries when there is no enough power coming from the sun or during night hours.
So, its main function is to protect batteries from overcharging or deep discharging under operation conditions. Without this regulation, the batteries may subject to damage as the charging voltage coming from solar panels varies and may go above the max charging voltage level of batteries.
Other benefits we get from the solar charge controller is to prevent power reverse from batteries to solar panels at night where no power coming from solar system, provide solar panel short circuit protection, and also regulate voltage/current required for feeding small DC loads.
Moreover, for modern charge controller, it has a display showing us some important measurements like solar input DC current/voltage, batteries charging voltage/current, DC load current …etc. From these information, we can alway be informed of the state of charging.
Main types of solar charge controller
In the market, you will see two types with two different technologies, the first is PWM charge controller (Pulse Width Modulation) and the second is MMPT charge controller (Maximum Power Point Tracking).
There exists big difference in between. And let us clarify the main difference as follows.
When MMPT charge charge controller receives higher voltage from the solar panels (Vmp) than batteries charging voltage level, it reduces the voltage to match charging batteries level, but while reducing the voltage, it increases the current, so charging power will increase as well. The result is that charge controller has succeeded to increase its output batteries charging power, consequently achieving higher efficiency and reduce losses.
But for PWM solar charge controller, it just reduces the voltage to match batteries voltage charging level while the current is still the same, so batteries charging power still the same. As a result, the PWM charge controller has failed to make use of all solar panel power, consequently lower efficiency and higher losses.
Conclusion: Although MPPT charge controller is more expensive, but it is worth to pay extra money for, as it is more efficient and more reliable than PWM charge controller.
How to size the solar charge controller?
Sizing a solar charging controller is based on your solar array current and voltage. Simply select the charge controller to be larger enough so can handle the power, voltage and current which come from solar energy system.
The solar charge controller has max acceptable DC input voltage and current.
For voltage, the solar array open circuit voltage (max at no load) should be less than charge controller max input voltage.
For current, the solar array short circuit current (max at no load) should be less than converter max input current level.
Generally, when sizing the solar charge controller, it will be better if its input DC current is higher than solar array short circuit current by good reasonable factor like 150% or so, this will not affect the system performance, and would be useful if you decided one day in the future to increase the solar array capacity.