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 and ensure the battery is charged correctly, or more importantly, not over-charged. Solar charge controllers have been around for decades and used in almost all small scale off-grid solar power systems.
Basic Off-grid solar power system with MPPT and an inverter
Modern solar charge controllers have advanced features to ensure the battery system is charged precisely and efficiently They are categorised in two:
MPPT Vs PWM Solar Charge Controllers
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, improved performance and other advantages.
PWM Solar Charge Controllers
Simple PWM, or ‘pulse width modulation’ 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 modulate the current and maintain a constant battery voltage. This works ok, but the problem is the solar panel voltage is pulled down to match the battery voltage. This in turn pulls the panel voltage away from its optimum operating voltage (Vmp) and reduces the panel power output and operating efficiency.
PWM solar charge controllers are a great low-cost option for small 12V systems when one or two solar panels are used, such as simple applications like solar lighting, camping and basic things like USB/phone chargers. Note, if more than one panel is used, they should be connected in parallel, not series.
MPPT Solar Charge Controllers
The functioning principle of an MPPT solar charge controller is rather simple – due to the varying degree of sunlight (irradiance) landing on a solar panel throughout the day, the panel voltage and current continuously changes. In order to generate the most power, the maximum power point tracker sweeps through the panel voltage to find the ‘sweet spot’ or the best combination of voltage and current to produce the maximum power. The MPPT is designed to continually track and adjust the voltage to generate the most power no matter what time of day or weather conditions.
MPPT or ‘maximum power point trackers’ 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 operating voltage (Vmp) of the solar panel. The reasons for the increased efficiency and how to correctly size an MPPT charge controller is explained in detail below.
As a general guide, MPPT charge controllers should be used on all higher power systems using two or more solar panels, or whenever the panel voltage (Vmp) is 8V or higher than the battery voltage – see full explanation below.
PWM Vs MPPT
In this example, 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 to a 100W. With an MPPT charge controller, the panel can operate at its maximum power point and in turn can generate much more power: 250W
MPPT solar charge controller with battery voltage. 12V vs 24V
Besides the current A rating, the maximum solar array size that can be connected to a solar charge controller is generally limited by the battery voltage. As highlighted in the diagram, using a 24V battery enables much more solar power to be connected to a 20A solar charge controller compared to a 12V battery.
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.
Over sizing the solar array is allowed by some manufacturers to ensure an MPPT solar charge controller operates at the maximum output charge current, provided the maximum input voltage and current is not exceeded the set limit
Solar Panel Voltage Vs Battery Voltage
For an MPPT charge controller to work correctly, the solar panel operating voltage must be at least 4V to 5V higher than the battery charging (absorption) voltage, not the nominal battery voltage. On average, the real-world panel operating voltage is around 3V lower than the optimum panel voltage (Vmp).
All solar panels have two voltage ratings which are determined under standard test conditions (STC) based on a cell temperature of 25°C. The first is the maximum power voltage (Vmp) which drops slightly under cloudy conditions or more so when the solar panel temperature increases. The second is the open-circuit voltage (Voc) which also decreases at higher temperatures. In order for the MPPT to function correctly, the panel operating voltage must always be several volts higher than the battery voltage under all conditions.
12V Batteries
In the case of 12V batteries, the panel voltage drop is not a big problem as most (12V) solar panels operate in the 18V to 22V range, which is much higher than the typical 12V battery charge voltage of 14.4V. Also, common 60-cell (24V) solar panels are not a problem as they operate in the 30V to 40V range
24V Batteries
In the case of 24V batteries, there is no issue when 2 or more solar panels are connected in series, but there is a problem when only 1 solar panel is connected. Most common (24V) 60-cell solar panels have a Vmp of 32V to 37V – While this is higher than the battery charging voltage of around 28V, the problem is when the panel temperature increases on a hot day, the panel voltage can drop by up to 6V, and end up below the 28V battery charge voltage, thus preventing it from fully charging. Another way to get around this, when using only one panel, is to use a larger, higher voltage 72-cell or 96-cell panel.
48V Batteries
When charging 48V batteries, the system will typically need at least 2 panels in series but will perform much better with 3 or more panels in series, depending on the maximum voltage of the charge controller. Since most 48V solar charge controllers have a max voltage (Voc) of 150V, this allows up to 3 panels to be connected in series. The higher voltage 250V charge controllers can have strings of 5 or more panels which is much more efficient on larger solar arrays as it reduces the number of strings in parallel and in turn lowers the current.
As previously mentioned, all solar charge controllers are limited by the maximum input voltage (V – Volts) and maximum charge current (A – Amps). The maximum voltage determines how many panels can be attached (in series), and the current rating will determine the maximum charge current and in turn what size battery can be charged.
As described in the guide above, the solar array should be able to generate close to the charge current of the controller, which should be sized correctly to match the battery. Another example: a 200Ah 12V battery would require a 20A solar charge controller, and a 250W solar panel to generate close to 20A. (Using the formula P/V = I, then we have 250W / 12V = 20A).