What is a solar charge controller and how to choose one

The main objective of a solar charge controller is to prevent batteries from overcharging. Not only is it detrimental for batteries, if current is too high for batteries to handle, but it can also cause an overload and may even start a fire. In this case the regulator acts as a valve and controls the current so that it isn't dangerous.

At the same time the battery shouldn't operate when it's discharged or at a very low level, so the regulator disconnects these batteries to prevent their usage. Once the battery is being charged again, the regulator reconnects it back. What is more, solar charge controllers keep the energy from flowing back to panels: at night batteries can give out reverse current but the regulator doesn't let it happen.

Solar charge controller is an essential part of your solar panel system. Given the fact that the lifespan of solar panels often exceeds 25 years, it is best to make sure that these years will pass without unhappy accidents. The lifespan of a controller itself is around 15 years, so maybe once you might have to replace it. The warranty for them varies from 2 to 5 years.

There are two predominant types of solar charge controllers on the market: PWM (pulse width modulation) and MPPT (maximum power point tracking) regulators. On paper MPPT is a more complex, modern and more expensive device. It doesn't mean, however, that you should always choose it, even if you've got extra money. First, let's explore the principle behind each type of solar charge controller.

Pulse width modulation controllers are an older version of regulators, but still they are perfectly viable for solar panel systems of a small scale. The current which is drawn out of panels is kept just slightly above the voltage of batteries with PWM solar charge controllers, so they should have matching voltages. The regulator gradually reduces the current and keeps the battery charged up with the series of pulses of energy. This is especially convenient with those systems that are not being used as much. The efficiency rating of PWM controllers varies around 75-80% and their cost ranges from 20$ to 100$.

The PWM controllers are best when:

• you have a small system and the maximum efficiency is not a priority
  
• the maximum power voltage of the solar array is slightly higher than the voltage of batteries.
  

MPPT controllers are a much more modern type of regulators — they first emerged in 1985 and became popular since then. With maximum power point tracking regulators the owner of solar panels is not constrained by voltage of his batteries. MPPT regulators draw the current out of solar panels at maximum power voltage, but transform all of it into current for the recharging process. It means that the voltage of panels doesn't have to match the voltage of batteries. In fact the closer the voltage of panels to the voltage of batteries, the less beneficial MPPT controller is.

In theory you would harvest up to 20-25% more energy with MPPT regulators than with PWM controllers (in practice this number gets slightly lower due to weather changes that affect the voltage of batteries). While MPPT solar charge controllers use the energy of solar panels more efficiently (the efficiency varies around 90-95%), it does come with a cost: their price hardly comes below 100$ and can easily go over 1000$. Keep in mind that sometimes a vendor might try to disguise his regulator as a MPPT controller when in fact it's a PWM regulator.

The MPPT controller is the best choice when:

• the voltage of your solar array is much higher than the voltage of your batteries.
• you have a large-scale system and try to maximize its efficiency.

Let's say you've decided whether you need a PWM or MPPT controller. Now it's time to discuss the size of your controller. The voltage and the amperage are usually the first things that you have to check about a controller. Regulators can have 12, 24 or 48 volts and their amperage varies between 1 and 60. However, it's safer to choose a controller with slightly higher (about 25% more) amperage than the current from solar panels to ensure the safety of the system.

PWM-controllers do not control or limit the output and just use the array current. As a result, you can't be using PWM-regulators, whose amperage is lower than the amperage of your system — it will simply break. Let's set an example. Imagine that you're setting 3 100-watt panels in parallel and their rated voltage is 12V. On a good day one of them produces around 5.5 A. Multiply by 3, you get 16.5 A. Sometimes it gets really hot, and irradiance rises higher than standard 1000W/m2. This is why you need to add these 25%, so 16.5 A * 1.25 = 20.625 A. For this case you need a controller whose amperage is 20 A or better even above that.

We've already talked about how PWM controllers perform well when the voltage of the solar panel system is slightly above the voltage of batteries. PWM regulators will be as efficient (or almost) as MPPT controllers if you are using:

• 30-cells panels and 12 V batteries
  
• 60-cells panels and 24 V batteries
  
• 120-cells panels and 48 V batteries

While the rated voltage of panels in these cases is equal to the voltage of batteries, their open circuit voltage is usually slightly higher. That makes them perfectly suited for a PWM controller, which is significantly cheaper than MPPT, but will perform just as fine.

MPPT controllers, on the other hand, can be considered as smart DC-DC converters, and can handle the current higher than its amperage. However, it means that you are losing the energy you've gathered, because if the amperage of your controller is, let's say, 60 A, it will produce 60 A even if panels give out 100 A altogether.

Another thing to look at while sizing a charge controller is a maximum solar output. Whatever the type, the maximum voltage of the controller must be higher than the voltage of the system. Let's say you have 2 60-cells panels, and their open circuit voltage goes up to 36 V. Then 2 * 36 = 72 V, so the maximum voltage of the controller must be higher than that.

Finally, you need to take a look at battery Amp/hour rating. The amperage of the controller should be 10%-20% of it. For example, a lead-acid battery with 100 Ah rating, a 15 A regulator will do just fine. To generate 10 A you would need two 100 W panels or just one 150-200 W panel. Of course, the size of your batteries, power and number of your panels and therefore the properties of the charge regulator depend on the size of your house and your energy needs.

There are situations when the voltage of your panels is not high enough for your batteries — for example, if you are using 36-cells panels with 24V batteries (or even 48 V batteries). In this case a boost MPPT controller can come in handy — it helps to charge batteries regardless of the voltage. Although it's not a common problem for home installation, such cases can occur when it comes to a solar vehicle or a boat, whose batteries can be especially powerful.

Charge regulators are often equipped with a display that shows you the voltage of panels, the load of the battery and its current state — charge, disconnect, standby. Some models (like Victron SmartSolar Controllers) have Bluetooth, that lets you customize and manage the regulator via an app on your smartphone. If you are living in a rainy area, you can get a waterproof controller to ensure the safety of the system.

We hope that this article gave you an idea of what charge controllers are all about and how to choose the right solar charge controller. A1 Solar Store has a variety of regulators to offer and we'll be glad to give you details on any model, if needed. If you're having questions about solar batteries and their types, here's another article that can help you: "Solar batteries for your solar system: basics to make the right choice".