If, like many you are confused by the jargon and technology available to help RV'ers live off-grid and enjoy the more remote parts of our beautiful country, then hopefully this article will help clarify this topic for you. We will be publishing a series of articles to help RV'ers who would like more information to make informed decisions on their RV needs.
There are two main types of solar charge controller, they differ significantly based on their mode of operation. The two main types are PWM and MPPT charge controllers.
PWM
The Pulse-Width Modulated (PWM) charge controller was the first model that appeared in the market and is the most basic (and cheapest) type of controller that you can find. PWM charge controllers act like a switch that connects the output of the photovoltaic modules with the battery bank. Once the switch is closed, the voltage at the terminals of the charge controller will be the nominal voltage of the battery. The charging process of the PWM controller consists of closing the switch during the first stage of the process to the maximum possible current value available from the PV array, as voltage gradually increases. When the voltage reaches the absorption voltage value, the current decreases slowly by disconnecting and reconnecting the switch multiple times. This creates a pattern that has the shape of small pulses until the current drops to zero.
The following image shows the charging process of a PWM charge controller.
The PWM charge controller always sets the output voltage close to the nominal voltage of the battery bank (generally a little higher to be able to charge it). Then, the regulator will provide the corresponding current in the I/V curve of the solar array. When using a PWM charge controller, you must closely match the solar panel's voltage to the battery bank. There will be high losses if you do not closely match the voltage to the battery voltage. This means that when operating a PWM controller, you will likely have to wire the panels in Parallel, which increases wiring cost. Here is an example of what happens when you connect a PWM charge controller without matching it to the battery voltage. The voltage into the charge controller will be 20 Volts. The PWM charge controller cuts the voltage to 13 Volts to charge the battery, so you lose about 7 Volts because the PWM doesn’t track the MPP (maximum power point). The power loss over the PWM charge controller will be 35%, resulting in a 65% efficiency. Therefore, it’s essential to match your solar panels to the voltage of your battery bank. PWM charge controllers are available in different voltages to match the battery at 12V, 24V or 48V.
MPPT
Maximum Power Point Tracking - This controller is essentially a DC-DC converter (a device that takes a DC signal and transforms it into another DC signal with other parameters). The operation mode of this type of controller consists of adjusting the voltage in the output terminals according to the voltage required to charge the battery bank. At the same time, the controller tracks the changes in the solar array across the day in the I/V curve and locates the maximum power point of the curve every time.
After locating this point, the controller determines the amount of electric current needed to provide the same amount of power as the MPP would provide, but at the battery's nominal voltage. Instead of simply assigning the corresponding electric current in the I/V curve (like the PWM controller does), the MPPT model increases the electric current to reach the maximum power point. In comparison to a PWM, which can have a 35% loss, an MPPT charge controller has a loss of 2 to 6%. In the following image, we can see that the MPPT controller increases the current at the output.
When deciding on a charge controller you would need to consider shading. Under shading conditions, the MPPT charge controllers will have a much better performance than the PWM models because the MPPT models can track the array's maximum power point (which will become affected by the presence of shade). Advances in solar panel technology have reduced the gap between the two different types of controller however MPPT still leads. Therefore, if your solar panel installation is expected to have shading conditions, it’s better to choose an MPPT model. Despite all of the advantages of MPPT charge controllers, they can be considerably more expensive than PWM models. Therefore, the decision comes down to a balance between costs and performance. A PWM charge controller can be very handy for small loads or a trickle charger to reduce costs.
Conclusion
Because off-grid applications like a motorhomes, caravans or boats have limited space to put solar panels, we need to extract as much energy as possible from the panels. The best way to do this is with an MPPT charge controller.
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