Solar Charge Controllers: Solar Power Components

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Solar Charge Controllers

This blog post is about Solar Charge Controllers.

This is the third in a series as we take a look at some of the main components in a solar electric system: the solar panels, batteries, solar charge controllers (SCC) and inverters. Technology is moving forward at a very fast rate. Most of the generalities are true today, but there are likely to be exceptions.

If you missed the earlier blog posts, I recommend that you read them to get a good foundation:

All other solar power articles may be found HERE.

Solar Charge Controllers (SCC)

Solar Charge Controllers are important components in a battery based system. There are two (2) main functions of solar charge controllers:

Prevents battery overcharge;
Prevents battery discharge at night through the solar modules.

Solar charge controllers charge the batteries by sending different voltages and currents to the battery bank based on how full the battery is. Much like pouring a glass of water. When the glass is empty you can have the faucet on full blast. But when it starts to get full you want to turn down the faucet to prevent overflowing. Likewise solar charge controllers send a lot of power to the battery when it is low but as it approaches full, it slows it down. Once it is full, it will send a smaller amount of power, a trickle charge, to keep it topped off.

Stages of Charging:

Bulk Charging
Absorption
Equalization
Float

BULK CHARGING – When the battery is low it will accept all the current provided by the solar system and send it to the battery.

ABSORPTION – The battery has reached the regulation voltage. The controller begins to hold the voltage constant. This is to avoid overheating and over gassing the battery. The current will taper down to safe levels as the battery becomes more charged.

EQUALIZATION – Done only with flooded batteries not with sealed batteries. Many batteries benefit from a periodic high-voltage boost charge. This is to stir the electrolyte, level the cell voltages and complete the chemical reactions. Your battery specs will tell you how often and at what rate it wants to be equalized.

FLOAT CHARGING – This is when the battery is fully recharged. The charging voltage is reduced to prevent further heating or gassing of the battery.

There are three main types of solar charge controllers:

Shunt (rarely used nowadays)
PWM (Pulse Width Modulated)
MPPT (Maximum Power Point Tracking)

Shunt chargers just turn the flow to the batteries on or off. They are rarely used anymore so we won’t discuss them.

The two main types you’ll find these days are PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking).

PWM

PWM solar charge controllers are the less expensive option. A PWM charge controller pulses the power sent to the battery bank. This allows it to do the different charging stages we discussed. When using a PWM charge controller, the voltage of the solar panel must be the same nominal voltage as the battery bank. If you are using a 12-volt battery you must use a 12-volt solar panel. If you have a 48 volt battery bank you must wire four (4) 12 volt panels or two (2) 24 volt panels in series to make 48 volts. Make sure the charge control you select is designed for that battery bank voltage. Some can support multiple voltage ranges. Others are designed for only one voltage. If a PWM charge controller says it can support 12V or 24V, both the panel and battery bank must be one or the other. It is not saying it can take a 24-volt panel to charge a 12-volt battery. It is saying it can work in either a 12 volt or 24 volt system.

Summary of PWM

Less $$$ than MPPT

Same volts in as out

Uses variable (duration and spacing) pulse charging

MPPT

MPPT solar charge controllers are the more sophisticated and more expensive type of solar charge controller. It tracks the output of the solar array and adjusts itself so the output is always maximized. In doing so, it can increase the production of the array by up to 30%. Another great advantage is that most MPPT charge controllers can take a higher voltage array. For example, a 60V array to charge a lower voltage battery bank like a 48V. This is required if you have a 60 cell 20V grid-tied solar panel and then want to use it to charge a 12-volt battery. It is also very useful if you have to go a distance from your array to your battery bank. The higher the voltage of the solar array, the lower the current going across the wire. You can use smaller gauge wire which will cost less and have a lower voltage drop which gets more power to the batteries. There are also a few MPPT charge controllers that can take a lower voltage panel and charge a higher voltage battery bank. But most MPTs require either higher or equal to voltage panel. Be sure to read the specs carefully.

Summary of MPPT

Tracks the optimum voltage/current ratio from array

Can provide up to 30% increase in performance over other controller types

Supports different voltage in than out

SCC Optional Features

There is a wide range of features that are optional on some but not all solar charge controllers.

In most cases, a display does not automatically come with a controller but can be added separately for remote display.

A few even have ethernet connections, allowing you to monitor your system across the web.

Temperature compensation will improve the battery bank charging by adjusting its output based on the temperature.

Low voltage disconnect (LVD) is a great feature that allows you to connect your DC load straight to the charge controller. If the battery voltage gets low, it will turn off the load preventing the batteries from becoming too low and getting damaged.

Some solar charge controllers can be used as a diversion or dump load controller turning power onto a heater to burn off excess power. There are others that have light control functions turning lights on and off automatically.

SCC Sizing

Solar charge controllers are rated by both voltage and amps. As we said, PWM charge controllers support the same voltage in as out. Check the specs to make sure what voltage that controller supports. MPPT specs will list the maximum Voc voltage it can support. This is higher than the nominal voltage. A typical 150V charge controller can support up to three (3) 38V panels in series. We must remember that cold weather increases the voltage output of a solar panel. If we say that the Voc of a panel is 38 volts, three (3) in series equals 114V. If we also figure in the cold temperature in winter, we increase the voltage. You can see why at least in cold climates three (3) 38V nominal panels would max out the 150V charge controller.

Voc x in series = Max Voltage

38V x 3 = 114V

Cold weather compensation (winter)

114 x 1.2 (temperature compensation) = 136Voc (close to max of 150V SCC)

There are now higher voltage charge controllers available with some accepting as much as 600V. This is very useful if the array is a long distance away from the battery bank.

Charge controllers are also rated by the current range they are able to support.

PWM charge controllers just pass the power through from the panels to the battery bank. There is no adjustment, current in equals current out. You would select the charge controller based on the Isc (short circuit current) of the solar array.

MPPT charge controllers are rated by their current output not their input. If you are inputting 60V and outputting 24V, the voltage is going to drop. This is because power (watts or W) equals volts times amps (W = V x A). To keep the power constant, anything done to the Volts, the opposite needs to be done on the Amps. When the volts drop on the output of the charge controllers, the amps will increase. To figure out what the output will be from the charge controller, take the total watts of the solar array and divide it by the voltage of the battery bank. For example, a 1000W solar panel array into a MPPT SCC that is connected to a 24V battery bank will output 41A. You need to find a MPPT SCC that can output at least 41A.