The definitive guide to Solar Charge Controllers, what are they? The different types of controller technologies explained, fundamental knowledge, the different types of charging technologies, PWM & MPPT, and sizing your charge controller according to your system requirements. The following topics will be covered in this article.

1. What is a Solar Charge Controller?

  • 1.1 What is PWM Charging?
  • 1.2 What is MPPT Charging?

4. Charge Controller Sizing:

  • 4.1 PWM Charge Controller Sizing
  • 4.2 MPPT Charge Controller Sizing

5. What is a hybrid solar charge controller? 

What Is a Solar Charge Controller?

A solar charge controller controls the power flow and charging rates from the solar array to the battery bank. It ensures that the deep cycle batteries are not overcharged throughout the day and that the current, for whatever reason, don’t run in reverse due to short circuit or incorrect connection to the solar panels to protect and prolong your batteries lifespan. Some charge controllers come natively with extra capabilities, for instance, day/night sensors for automatic lighting and load control, but managing sustained & effective battery charging is its primary function.

Though there are two main types of charge-controller, they both perform these main functions:

  • Multi-stage charging modes for efficient charging of the battery bank
  • Built-in reverse polarity and short circuit protections
  • Overload voltage protection (Automatic shut-off when an overload is detected)
  • Undervoltage protection (Automatic shut-off when the batteries are low)
  • A temperature sensor to automatically shut-off when abnormally high temperatures are detected (optional)
  • Monitor and track the state of charge/discharge on the battery system, by measuring amps and battery voltage

Solar Charge Controller Wiring Diagram

A solar charge controller has two main technologies for its charging function, PWM (Pulse Width modulated) and MPPT (Maximum Point Power Tracking). How they operate in a system are very different from each other.

PWM Solar Charge Controllers

PWM Solar Charge Controllers have been around for a number years and increase or decrease the voltage of your solar panel array to function at the same voltage as your battery bank during charging. They tend to be less expensive than MPPT charge controllers which we are planning to describe after this topic, but they're also less efficient than the MPPT type.

PWM Controller functioning by making a connection directly from the solar array to the battery bank. When the period of 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 increases as well, using more of the solar power as it charges. As an effect, you need to check that you match the nominal voltage of the solar array with the voltage of the battery bank.

*Normally when refer to a 12V solar panel that means a panel that is designed to work with a 12V battery. The voltage of a 12V solar panel, when connected to a load, is approximately 18 Vmp (Volts at maximum power). This happens because of a higher voltage source is required to charge a battery. Also when battery and solar panel both functioning at the same voltage, the battery would not charge.

Actually, by using a 12V solar panel, we can charge a 12V battery. A 24V solar panel or solar array (two 12V panels wired in series) is required for a 24V battery bank, and 48V array is required for 48V bank. If you get an effort to charge a 24V battery bank with a 12V solar panel, you will be miss 100% of the panel’s potential, not charging at all, and, may actually drain (damage) the batteries as well. Same goes for attempting to charge a 12V battery bank with a 24V array, a lot of the power provided by the PV modules will be lost during the charging process.

MPPT Solar Charge Controllers

Maximum Power Point Tracking Solar Charge Controllers are different from traditional PWM charge controllers. This controller has higher efficiency and also most of the cases this provide more features than PWM.MPPT solar charge controllers allow your solar panels to operate at their optimum power output voltage, increasing their performance by as much as 30%. This controller will measure the Vmp voltage of the panel and down-converts the PV voltage to the battery voltage in-turn increasing the charging amperage.

Since the power into the charge controller equals power out from the charge controller if the voltage is dropped to match the battery bank, the current is increased accordingly, so we
can use more of the available power from the panel. If need, we can use a higher voltage solar array than a battery, as there are more readily available panels, as an example we can use 60 cell nominal 20V grid-tie solar panels. By using single 20V solar panel, we can charge a 12V battery bank, or two panels in series can charge up to a 24V battery bank, and three panels in series can charge up to a 48V battery bank.

Charge Controller Sizing

As we all know, the most important job of all solar charge controllers is properly charge the batteries and to give them as long a life as possible. When are sizing the appropriate charge controller, there will be some steps that you must follow to make sure that you are selecting a suitable controller for the job.

The following information will be required to do a manual calculation to figure out the amperage of the controller needed:

  • The wattage of the solar array
  • The battery-bank voltage (12V, 24V, 36V or 48V).

Now Ohm’s Law comes into play: Amps x Volts = Watts

We must ensure that we do not exceed the input voltage of the controller. Also, Temperature and open-circuit voltages have to be considered for calculations. If brief, Since PV open-circuit voltage (Voc), goes higher as temperature drops, we will need to make sure the input voltage ratings of the controller can bear this in the cold of winter.

PWM Charge Controller Sizing

Since the nominal voltage of the charge controller is close enough to your battery bank, this will allow proper charging without overcharging the cells. Using OHM’S law, we can use the below method to get an idea about the charge controller sizing.

12V Example:

140W of solar / 12V battery = 11.66A
Add 25% for safety margin to account for the short circuit current (multiply by 1.25)
11.66A x 1.25 = 14.57A
Round up to the nearest charge controller size that is the same voltage as your system. Any charge controller that is rated for at least 20 amps at 12V will work.

24V Example:

300W of solar / 24V battery = 12.5A
Add 25% for safety margin to account for the short circuit current
12.5A x 1.25 = 15.6A
Round up to the nearest charge controller size in the same voltage as your system. For this situation, any charge controller that is rated for at least 20 amps at 24V will work.

MPPT Charge Controller Sizing

MPPT charge controllers are capable to take a higher input voltage from your solar array to the lower voltage of your battery bank without overcharging your batteries. As MPPT controller sweeps the voltage curve throughout the day to maximize your energy generation, this will affect to increase the generation by 10 – 30% (on average).

Note that you will get the most out of your system if your solar panel input voltage is over twice that of your battery bank with an MPPT charge controller.
For charge controller sizing, we should always use the open-circuit voltage (VOC) and the short circuit current (ISC) to confirm that your system falls within the electrical guidelines of the charge controller manufacturer.

Using a 60 cell 300 watt Canadian Solar panel: CS6K-300MS with the below electrical specifications:

Watts (STC): 300W
Max Power Voltage (VMPP): 32.5V
Max Power Current (IMPP): 9.24A
Open Circuit Voltage (VOC): 39.7V
Short Circuit Current (ISC): 9.83 A
Max System Voltage (UL): DC1000V

12V battery system – One (1) solar panels (32.5 VMP/39.7VOC)
24V battery system – Two (2) panels wire in series (65 VMP/79.4VOC)
48V battery system – Three (3) panels wired in series (105.75 VMP/119.10VOC)

** For a 3600 watt PV array consisting of Twelve (12) 300 watt solar panels
Four (4) parallel strings of Three (3) panels wired in series:

Each string: ISC (9.83) x four (4) parallel strings = 39.32 x 1.20 = 47.18A
VOC (39.7V) x 3 panels wired in series = 119.1VOC

Any charge controller that is rated for the above ratings will work.
Please note that for colder environments you also want to account for the temperature coefficient.

What is a hybrid solar charge controller?

A solar hybrid charge controller is a solar charge controller, DC to AC power inverter and AC battery charging combination unit. It reduces the use of mains and provides solar supplemented power throughout the daytime it mechanically re-connects it after sundown or if the battery voltage goes down than pre-programmed limit, switching back to grid-supplied power.

Hybrid Solar Inverter System

A hybrid system works to drastically reduce your utility bill by providing constant power supply by a combination of using grid-supplied and backup power generated using solar panels or AC battery charging. The hybrid inverter will primarily use Solar Power until a point where the demand exceeds supply where it will supplement Solar generated power with grid provided mains electricity supplied by your utility.