A solar charge controller is needed in virtually all solar power systems that utilize batteries. Solar charge controller regulates the power going from the solar panels to the batteries, in order to ensure full charge of batteries to its capacity and prevent overcharging or overvoltage.
The most basic charge controller simply monitors the battery voltage and once the battery voltage reaches to boost voltage level, it regulates charging current to keep the battery voltage at boost voltage level.
Another function of solar charge controllers is preventing reverse-current flow. At night, when solar panels are not generating electricity, electricity can flow backwards from the batteries through the solar panels, draining the batteries. The charge controller detect when no energy is coming from the solar panels and open the circuit, disconnecting the solar panels from the batteries and stopping reverse current flow.
Solar charge controller is available in two different technologies, PWM and MPPT. They perform in a system differently from each other. An MPPT Solar charge controller is more efficient than a PWM charge controller.
PWM SOLAR CHARGE CONTROLLER
PWM stands for “Pulse Width Modulation”. PWM is often used as one method of float charging. Instead of a steady output from the controller, it sends out a series of short charging pulses to the battery – a very rapid “on-off” switch. The controller constantly checks the state of the battery to determine how fast to send pulses, and how long (wide) the pulses will be. In a fully charged battery with no load, it may just “tick” every few seconds and send a short pulse to the battery. In a discharged battery, the pulses would be very long and almost continuous, or the controller may go into “full on” mode. The controller checks the state of charge on the battery between pulses and adjusts itself each time.
The PWM Solar Charge Controller operates by making a connection directly from the solar array to the battery bank. During bulk charging, when there is a continuous connection from the array to the battery bank, the array output voltage is ‘pulled down’ to the battery voltage. As the battery charges, the voltage of the battery rises, so the voltage output of the solar panel rises as well, using more of the solar power as it charges. As a result, you need to make sure you match the nominal voltage of the solar array with the voltage of the battery bank. *Note that when we refer to a 12V solar panel that means a panel that is designed to work with a 12V battery. The actual voltage of a 12V solar panel, when connected to a load, is close to 18 Vmp (Volts at maximum power). This is because a higher voltage source is required to charge a battery. If the battery and solar panel both started at the same voltage, the battery would not charge.
MPPT SOLAR CHARGE CONTROLLER
The most recent and best type of solar charge controller is called Maximum Power Point Tracking or MPPT. MPPT controllers are basically able to convert excess voltage into amperage.
Solar panels can deliver far more voltage than is required to charge the batteries. By, in essence, converting the excess voltage into amps, the charge voltage can be kept at an optimal level while the time required to fully charge the batteries is reduced. This allows the solar power system to operate optimally at all times.
The two main types of solar charge controllers are the PWM and the MPPT. While PWM is essentially a switch that connects a solar array to the battery, the MPPT is more sophisticated, since it will adjust its input voltage to harvest the maximum power from the solar array and then transform this power to supply the varying voltage requirement of the battery plus load.
To compare them on the technical level, below you find a comparison table, that takes into account five different aspects: the array voltage, the battery voltage, the system size, Module specifications and the array sizing method.
SUMMARY OF COMPARISON
|PWM Charge Controller||MPPT Charge Controller|
|Array Voltage||PV array & battery voltages should match||PV array voltage can be higher than battery voltage|
|Battery Voltage||Operates at battery voltage so it performs well in warm temperatures and when the battery is almost full||Operates above battery voltage so it is can provide “boost” in cold temperatures and when the battery is low.|
|System Size||Typically recommended for use in smaller systems where MPPT benefits are minimal||≈ 200W – 300W or higher to take advantage of MPPT benefits|
|Module specifications||Must use 36 cells PV modules typically with Vmp ≈ 17 to 18 Volts for every 12V nominal battery voltage||Enables the use of lower cost 60/72 cells PV Modules helping bring down the overall PV system cost|
|Array Sizing Method||PV array sized in Amps (based on current produced when PV array is operating at battery voltage)||PV array sized in Watts (based on the Controller Max. Charging Current x Battery Voltage)|