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.
MPPT and PWM solar charge controller
There are two main types of solar charge controllers, PWM and MPPT, with the latter being the primary focus of this article due to the increased charging efficiency and several other advantages explained in detail below.
PWM Solar Charge Controllers
PWM solar charge controllers have a direct connection from the solar array to the battery and use a basic ‘rapid switch’ to modulate or control the battery charging. The switch (transistor) is open until the battery reaches the absorption charge voltage. Then the switch starts to open and close rapidly (hundreds of time per second) to reduce the current and maintain a constant battery voltage.This in turn pulls the panel voltage away from its optimum operating voltage (Vmp) and reduces power and efficiency.
PWM solar charge controllers are a great low-cost option for small 12V systems where one or two solar panels are used, such as simple applications like solar lighting, camping and basic things like USB/phone chargers.
MPPT Solar Charge Controllers
MPPT or ‘maximum power point tracking’ controllers are far more advanced than PWM controllers and enable the solar panel to operate at its maximum power point, or to be more precise, the optimum voltage for maximum power output. Using this clever technology, MPPT solar charge controllers can be up to 30% more efficient, depending on the battery voltage and voltage of the solar panel connected.
As a general guide, MPPT charge controllers should be used on all higher power systems using 2 or more solar panels, or whenever the panel voltage (Vmp) is 8V or higher than the battery voltage – see full explanation below.
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
RELATED SOLAR CHARGE CONTROLLER PRODUCTS
See Also
- How solar battery backup systems work
- What is a half-cut cell mono PERC solar panel?
- What is the best solar generator?
- How does solar inverter work
- Charge controller – Wikipedia
