Solar Charge Controller | What is it and How it works in a Solar Power System

A Solar Charge Controller is an important component in battery based systems. They are not used in straight grid-tied systems, as they do not have batteries to charge. Their primary role is to manage charging the battery bank. It prevents it from overcharging, and many of them control the rate of the current and voltage at which it charges.

Why do we need Charge Controller for Solar System?

First let’s take a look where a solar charge controller fits into an off grid solar system. The charge controller goes between the solar panel and the deep cycle battery.

Some solar charge controllers have load control, where you connect the DC load right to the charge controller instead of to the battery, and it will turn it on and off based on voltage of battery and/or time of day, for example turning the load off if the battery gets too low or turning on a light from dusk to dawn.

At night, the voltage of the battery bank is higher than that of the solar array that is connected to it. Since electricity flows from high voltage to low, without a charge controller, the tendency would be for the electricity to flow out of the battery bank.

A charge controller prevents that from happening, allowing the flow to only go one way, into the batteries.

Many charge controllers manage charging the batteries by varying the voltage and current to the battery bank based on how full the battery is.

Much like pouring a glass of water, when the glass is fairly empty, you can have the faucet on full blast, but when it starts to get full, you want to turn down the flow to prevent overflowing.

Likewise, a charge controller sends 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.

Do I always need a Charge Controller?

Not always, but usually. Generally, there is no need for a charge controller with the small maintenance, or trickle charge panels, such as the 1 to 5 watt panels. A rough rule is that if the panel puts out about 2W or less for each 50Ah, then you don’t need one.

For example, a standard flooded golf car battery is around 210 amp-hours. So to keep up a series pair of them (12 volts) just for maintenance or storage, you would want a panel that is around 4.2 watts. The popular 5 watt panels are close enough, and will not need a controller. If you are maintaining AGM deep cycle batteries, such as the Concorde Sun Xtender then you can use a smaller 2 to 2 watt panel.

Solar Charge Controller Feature

There is a wide variety of features that are optional on some, but not all controllers. In most cases a display does not automatically come with the controller, but can be added separately for a 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 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.

Most systems do not need the LVD function – it can drive only smaller loads. Depending on the rating of the controller, this may be from 6 to 60 amps. You cannot run any but the smallest inverter from the LVD output. On some controllers, such as the Morningstar SS series, the load output can be used to drive a heavy duty relay for load control, gen start etc. The LVD output is most often used in RV & remote systems, such as camera, monitor, and cell phone sites where the load is small and the site is unattended.

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

Solar Charge Controller Types

Solar charge controller come with 3 main types:

Shunt Charge Controller just turn the flow to the batteries on or off are rarely used anymore, so we won’t go into them.

The 2 main types you’ll find these days are PWM and MPPT. You can get more detail in my another article: PWM vs. MPPT: Which type of solar charge controller is the best choice for your solar system?

Most controllers come with some kind of indicator, either a simple LED, a series of LED’s, or digital meters. Many newer ones, such as the Outback Power, Midnite Classic, Morningstar MPPT, and others now have built in computer interfaces for monitoring and control.  The simplest usually have only a couple of small LED lamps, which show that you have power and that you are getting some kind of charge. Most of those with meters will show both voltage and the current coming from the panels and the battery voltage. Some also show how much current is being pulled from the LOAD terminals.

All of the charge controllers that we stock are 3 stage PWM types, and the MPPT units. (in reality, “4-stage” is somewhat advertising hype – it used to be called equalize, but someone decided that 4 stage was better than 3).

Let talk about 4-stage charging.

4-stage Charging

  • Bulk Charging, when the battery is low, it will accept all the current provided by the solar array.
  • Absorption, the battery has reached the regulation voltage, the controller begins to hold the voltage constant. This is to avoid over-heating and over-gassing the battery. The current will taper down to safe levels as the battery becomes more fully charged.
  • Float charge is when the battery is fully recharged, the charging voltage is reduced to prevent further heating or gassing of the battery.
  • Equalization is done with flooded batteries, not sealed batteries like AGM and Gel. Many batteries benefit from a periodic high voltage boost charge to stir the electrolyte, level the cell voltages, and complete the chemical reactions. Your battery specs will tell you how often and at what rates it wants to be equalized.

How to choose the perfect solar charge controller

Selecting a relevant charge controller is the key to the longevity and efficiency of your entire PV system. You’ll need to choose an option that is scalable and appropriate for your power needs. Your solar charge controller is an item well worth investing in and researching as you design your system.

So, what do you need to do? The answer is:

Calculating the solar array current (amps) and the solar system voltage (push)

Solar charge controllers are rated and sized by the solar module array current and system voltage. Most common are 12, 24, and 48-volt controllers. Amperage ratings normally run from 1 amp to 80 amps, voltages from 6-600 volts.

For example, if one module in your 48-volt system produces 8.05 amps and two parallel strings of modules are used, your system will produce 16.1 amps at 48 volts. Certain factors such as light reflection or cloud effect at irregular intervals can increase current levels.  This is quite common.

Therefore, we increase the charge controller amperage by a margin of 25% bringing our minimum controller amperage to 20.13. We migrate over to our catalog and we find a 30-amp controller, which is a very close match. There is no problem going with a larger controller, other than the additional cost. This would allow you to expand the size of your system later on down the road if your load demands change or you find you need a little more power.

These day, MPPT charge controller is the most common and can gain up to 30% more power than PWM controller. It also allows the strings of panels to be connected in series for higher voltages, keep Amp lower and wire size smaller, especially for long-wire runs to PV array.

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