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What is a solar charge controller?
A solar charge controller, also known as a solar regulator, is essentially a solar battery charger connected between the solar panels and battery. Its job is to regulate the battery charging process to ensure the battery is charged correctly, or more importantly, not over-charged. DC coupled solar charge controllers, have been around for decades and used in almost all small scale off-grid solar power systems.
Modern solar charge controllers have advanced features to ensure the battery system is charged precisely and efficiently, plus functions like DC load terminals for lighting. Generally, most smaller 12V-24V charge controllers up to 40A in size are used for caravans, RVs and camping, and have DC load output terminals built-in. While most larger, more advanced 60A+ MPPT solar charge controllers do not have load output terminals and are specifically designed for larger off-grid system with solar arrays and powerful off-grid inverters.
Solar charge controllers are rated according to the maximum input voltage (V) and maximum charge current (A). These two ratings determine how many solar panels can be attached as explained in detail below ‘Sizing a solar charge controller’.
- Current Amp (A) rating = Maximum charging current.
- Voltage (V) rating = Maximum voltage (Voc) of the solar panels.
Which is better PWM or MPPT?
In the example above, a common 60 cell (24V) solar panel with an operating voltage of 32V (Vmp) is connected to a 12V battery bank using both a PWM and a MPPT charge controller. Using the PWM controller, the panel voltage must drop to match the battery voltage and so the power output is reduced dramatically. With an MPPT charge controller, the panel can operate at its maximum power point and in turn can generate much more power.
Battery Voltage VS Solar Charge Controller Voltage
Unlike battery inverters, most solar charge controllers can be used with a range of different battery voltages. For example, most smaller 10A to 60A charge controllers can be used to charge either a 12V or 24V battery, while most larger capacity or higher input voltage charge controllers are designed to be used on 24V or 48V battery systems.
The maximum solar array size which can be connected to the solar charge controller is generally limited by the battery voltage. As highlighted in the following diagram, using a higher 24V battery enables more solar power to be connected to a solar charge controller with a maximum charge rating of 20A.
Based on Ohm’s law and the power equation, higher battery voltages enable more solar panels to be connected. This is due to the simple formula – Power = Voltage x Current (P=V*I). For example 20A x 12.5V = 250W, while 20A x 25V = 500W. So using a 20A controller on a higher 24V volt battery, as opposed to a 12V battery, will allow double the size solar array to be connected.
- 20A MPPT with a 12V battery = 260W max Solar recommended
- MPPT 20A with a 24V battery = 520W max Solar recommended
- 20A MPPT with a 48V battery = 1040W max Solar recommended