The primary function of a solar charge controller is to protect your battery bank from being over-charged by your solar array. Charge controllers continually monitor battery voltage, solar array voltage and output current and ensure your batteries get correctly charged. Moreover, most charge controllers will manage 3, 4 or 5 different stages of battery charging cycles to properly charge your batteries at different times and battery voltages. The most common battery charging cycles are:
Bulk Charge: This first stage of battery charging is used whenever your batteries are low on energy. Most charge controllers automatically begin bulk charging as soon as power is drawn from the battery bank. In this stage, current is sent to the batteries at the maximum safe rate they will accept until voltage rises to near (80-90%) full charge level.
Absorption Charge: This 2nd stage of battery charging is used to safely bring your battery bank up to 100%. Voltage remains constant and current gradually tapers off as internal resistance increases during charging.
Float Charge: The 3rd stage of battery charging is a “maintenance” charge cycle. After your batteries have reached full charge, charging voltage is reduced to a lower level (typically 12.8 to 13.2) to reduce gassing and prolong battery life. It’s main purpose is to keep an already charged battery from discharging. PWM, or “pulse width modulation” accomplishes the same thing. In PWM, the controller or charger senses tiny voltage drops in the battery and sends very short charging cycles (pulses) to the battery. This may occur several hundred times per minute. It is called “pulse width” because the width of the pulses may vary from a few microseconds to several seconds.
The following illustration shows how a typical three stage charge controller works (illustration courtesy of Xantrex Technologies):
Equalization Charge: Some charge controllers are capable of a manually engaged equalization charge on wet lead acid batteries. Equalizing refers to an overcharge performed on flooded lead-acid batteries after they have been fully charged. This maintenance step helps eliminate stratification and sulfation. This charge cycle should always be done while you are present to monitor your batteries, as they can release excessive amounts of gas, or become overheated during this process. Make sure to carefully follow all battery, and charge controller manufacturer instructions.
Types of Charge Controllers:
PWM: Pulse Width Modulated, or PWM charge controllers offer a relatively inexpensive and effective way to regulate battery voltage. They are commonly found in mobile, marine, telecommunications and smaller off-grid battery charging environments. The use of PWM controllers in medium to large off-grid battery charging systems is waning due to the fact that these controllers need the solar array nameplate voltage to match the battery voltage being charged. Many people prefer wiring their solar array at a voltage higher than battery voltage to reduce voltage drop on the home wire run and also to take advantage of cost per watt savings available on larger “grid-tie” solar modules which do not have standard battery charging voltages. These newer solar arrays must use an MPPT controller. Please see the next section for more on MPPT controllers.
MPPT: Maximum Power Point Tracking, or MPPT charge controllers (in general) offer the user the ability to operate a solar array voltage at higher than battery voltage. These controllers (in general) are able to convert the higher input voltage into a lower voltage for battery charging, and convert that loss in voltage to an increase in charging current to the batteries (Ohms Law). These newer charge controllers are generally used on medium to large scale off-grid solar systems. They are much more expensive than traditional PWM controllers, and care must be taken to properly wire your array to prevent mismatch or overly high solar voltages.
Charge Controller Selection
Please see the chart below. Clicking on the chart will open it up in PDF Format for easier viewing.
The charge controller you select for your project will be based on:
- The size of your solar array, and
- The voltage of your solar array, and
- The battery bank voltage, and
- Site specific variables, and
- Desired monitoring or programming options
We’ll go through each of the above selection criteria below.
Sizing your Controller:
Charge controllers are, first and foremost listed by their charging capacity in amps. This is the output capacity of the controller. The proper way to determine the maximum output current of your array is:
Array Isc x 1.56
Why do we multiply by 1.56? Well, 1.56 represents two things. It represents 1.25 x the Isc (for temperature compensation) and then 1.25 x that value because the solar circuit is continuous).
Note that this is the “Array Isc”, not the solar module Isc, and that that the array Isc is assumed to be at (nominal) battery voltage. Therefore, the calculation is can get complicated when the array voltage is dissimilar to the battery bank voltage, as is almost always the case when using MPPT controllers.
Let’s say we have a 100W (36 Cell, 5.56 Isc, 18.5 Vmp) solar panel and a 12V (nominal) battery bank.
5.56 (Array Isc) x 1.56 = 8.67A Minimum Charge Controller Size
In reality you’d be selecting (at least) a 10A charge controller. You would probably even select a (minimum) 20A controller to give you room to add a similar solar panel in the future.
WARNING FOR SIZING CONTROLLERS WITH GRID TIE PANELS!
The above example works just fine when the solar array output voltage is nominally the same as the battery bank voltage, ie – 12V solar array charging a 12V battery bank, etc… The above example doesn’t work when using a solar voltage dissimilar to the battery bank voltage. This situation occurs all the time, particularly when using 60 cell (grid tie) solar panels and MPPT charge controllers to charge battery banks.
Let’s say we have a 260W (60 Cell, 8.95 Isc, 30.3 Vmp) solar panel and a 12V (nominal) battery bank.
8.95 (Array Isc) x 1.56 = 13.96A Minimum Controller Size
But, assuming that an MPPT controller can convert solar voltage into usable battery charging current, we could have the following:
260W (Array Power) / 14.4V (Battery Charging Voltage = 18.05A
What should you do? I do two things:
I look at the battery type I’m charging (wet lead acid, gel, AGM, etc…) and use the charge controller specification sheets to see what the charging voltage will be for the given battery type and pick the lowest charging voltage that I know will be applied to the battery bank. I then use this number to divide into the solar array power (in Watts).
I look to the solar controller manufacturer(s) and use their on-line string sizing tools. Here are some links to them:
I inputted the above data into the MidNite Solar KID Sizing Tool and discovered:
as you can see, the “Array Watts” divided by the “battery charging voltage” method yielded a much closer representation of what might be seen in the field in situations where the array voltage does not match the battery bank voltage and an MPPT controller is present to compensate.
Once you know how big your solar controller needs to be, the rest of the selection process gets easier, but there are still site specific variables and monitoring issues to be considered. For instance:
If your installation requires 3000 watts of solar to supply your home or cottage, you would look at:
Your battery bank voltage, where 3000 watts of solar charging could mean:
208A of charging current with a 12V battery bank, or
104A of charging current with a 24V battery bank, or
52A of charging current with a 48V battery bank.
Unless you plan on installing multiple charge controllers, you will likely look to running a 48V battery voltage system.
Also, the distance between your solar array and solar controller will be a factor, where you could wire your array at 48V to match your battery bank and use a less expensive PWM controller provided your solar array was very close to your battery bank. The wire size to carry 52A of current across any distance will quickly become an overwhelming cost issue. For instance:
52A of array current at a distance of 10′ from your controller = 6 AWG
52A of array current at a distance of 50′ from your controller = 2 AWG
52A of array current at a distance of 100′ from your controller = 2/0 AWG
As you can see from the above, if your array is any distance at all from your battery bank then any cost savings in the PWM controller will be more than lost in wiring costs. You could easily use an MPPT controller and double the array voltage (thereby lowering the array current) and significantly reduce your wire size. For instance:
26A of array current at a distance of 50′ from your controller = 8 AWG
26A of array current at a distance of 100′ from your controller = 6 AWG
The online MPPT calculators (above) allow you to play with your series and parallel numbers to see the impact to the array current and array voltage. If you have any questions, just call!
Solar Array Voltage Considerations
The implementation of MPPT solar controllers has provided a great deal of versatility in designing solar systems since the solar array voltage can be (and most often is) higher than the battery bank voltage. There are still design issues such as:
- The solar array nominal Maximum Power Voltage (Vmp) must be (at least) 17V for 12V nominal battery banks, 34V for 24V nominal battery banks and 68V for 48V nominal battery banks. MPPT controllers can not raise the array voltage to meet the needs of battery bank charging. This is the reason that parallel strings of individual 60 cell solar modules can not fully charge 24VDC nominal battery banks.
- The maximum input voltage of a solar controller is the array Voc (Open Circuit Voltage) x 1.25. The 1.25 adder compensates for the higher voltage of the array during low temperatures. This is especially critical in Canada. All solar controllers will be damaged by voltages higher than their maximum rating.