What is a solar charge controller and why are they important?

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Solar charge controller
Charge controllers regulate electricity generated by off-grid solar panels. With an MPPT charge controller, it can send DC electricity directly to a load, like a light bulb, or a battery for storage.

A solar charge controller is one of the key components of a solar panel system with a battery, such as an off-grid system. Their main job is to act as a charge regulator for the electricity going into the battery bank from a solar array.

For instance, the charge controller ensures that batteries are not overcharged during the day and that they do not send their stored power back to solar panels at night.

One important distinction is that if you have a storage battery installed with your grid-tie home system, you do not need a charge controller. Typically, your solar inverter will do the job of maintaining your battery health. Charge controllers are best for off-grid use such as for an RV or a small, off-grid cabin.

In this article, we will discuss the difference between a PWM and MPPT charge controller, how to determine which charge controller you might need, and the best types of charge controllers for your off-grid lifestyle.

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    How do charge controllers work?

    One key thing to know is that electricity flows from high voltage to low voltage. So for instance, solar panels produce the most energy in the middle of the day so their voltage is high while the battery bank's voltage is low. Excess electricity is then sent to be stored in the battery.

    But at night, the battery has a higher voltage than the solar panels because they are all charged up from the day. To stop this high voltage from flowing to the solar panels, the solar charge controller steps in to keep the electricity in place.

    When the battery charger is empty or close to it, the charge controller directs a great deal of power to the battery and charges it quickly. Once the battery is close to being full or is in use, the charge controller sends a smaller amount of power to the battery via a trickle charge to keep the battery always ready to supply power.

    Additionally, charge controllers automatically disconnect non-critical loads from the battery bank when the voltage falls below a certain threshold. This is a process called low voltage disconnect (LVD) and it prevents the over-discharging of the battery's energy.

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    What is the difference between PWM and MPPT charge controllers?

    There are two main types of charge controllers, an MPPT(maximum power point tracking) and a PWM (pulse width modulation) charge controller. There are a few key differences between the controller types, which we will review below.

    Before we dive into the differences between PWM and MPPT charge controllers, there are a few acronyms that need to be addressed:

    VPM: Voltage at Maximum Power

    VPM, or Voltage at Maximum Power, is the voltage that occurs when the solar module is operating at peak performance. Under perfect conditions, this is the voltage a solar panel produces, which is listed on a module specification sheet.

    The below example is from the spec sheet of a Panasonic EverVolt solar panel. We will look at the EVP 370W as our reference and that VPM is listed at 37.4V.

    Panasonic EverVolt spec sheet

    Image source: Panasonic

    VOC: Voltage at open circuit or open circuit voltage

    Under STC, when the module is not connected to any load, i.e. no device is taking energy from the solar panel, the VOC is read by a multimeter and is listed on the module's specification sheet. This is how much the voltage decreases for every one-degree increase in celsius, or how to refer to when the voltage drops as solar panels heat up, measured by the temperature coefficient.

    In the example above, you can see that the VOC is -.24%. For every increase in degrees celsius, the solar panel efficiency reduces by .24%, meaning the voltage also reduces by .24%.

    The current (amp) at maximum power is when the power output is the largest. When connected to an MPPT charge controller under standard test conditions, the IMP is the amperage that occurs.

    Both PWM and MPPT charge controllers are rated by the amount of amps that they output to batteries. Using our above example, the IMP for one panel is 9.90A, or 9.90 amp-hours at maximum power. You need to be sure that the amps that your solar panels produce can be handled by and sent to your batteries safely by your charge controller.

    To do this, match the amount of amps your solar system is to the amount of amps the charge controller can manage.

    Now that we have those acronyms sorted out, we can dig into why they matter when speaking about PWM and MPPT charge controllers.

    PWM solar charge controller

    A PWM charge controller connects your solar array directly with a battery bank. While the battery charges, the voltage of the battery increases, leading to an increase in the solar panel voltage that is charging the battery. You can only use solar panels that are made to work with the voltage of the battery and charge controller.

    With a PWM charge controller, a 12-volt solar panel is needed to charge a 12-volt battery and so forth. If you used a PWM charge controller with a 12V battery but with 48V solar panels, the PWM will not harvest the extra energy produced, it will only harvest 12V.

    Buying a PWM charge controller is not necessarily a bad choice even though it is less efficient than an MPPT. The best charge controller for you is a high-quality one that works for the situation you need it for, which we will cover below to help you decide.

    MPPT solar charge controller

    MPPT charge controllers are more sophisticated than PWM charge controllers. They work by measuring the VMP input voltage of the solar panel and then down-converts the PV voltage to match the battery voltage.

    Essentially, it takes the high voltage from the panel and reduces it so that it matches the battery voltage, which is less.

    When using an MPPT solar charge controller, you can use a voltage solar array that is higher than your battery system because your charge controller will reduce the voltage so that it does not overwhelm your battery.

    If you want better efficiency and are tired of worrying about the difference between your battery and solar panel voltage, invest in an MPPT charge controller. You just need to make sure that the VOC of your panels is under the permitted voltage of your charge controller.

    What type of charge controller do I need?

    Whichever option you choose, getting a high-quality charge controller for battery charging is your best bet. The charge controller is extremely important for your solar system to function properly, and you do not want to lose quality by trying to save money.

    The type of charge controller that you need, PWM vs. MPPT, will depend on what you are using it for. First, follow the chart below to ensure your charge controller is the right fit.

    Specification PWM Charge Controller MPPT charge controller
    Battery voltage Operates at battery voltage Operates above battery voltage to provide a boost to a low battery
    Array voltage Battery voltage and PV array voltage must match PV array voltage can be higher than battery voltage
    System size Small system Works best for systems 150 wattage or higher

    If you still are not sure, pick a PWM charge controller if you need it for:

    1. Small systems - like for your RV with AGM deep cycle batteries.
    2. When efficiency is not a necessity, such as for trickle charging your boat battery.
    3. For solar panels with a VMP of up to 18V that will charge a 12V battery.
    4. To spend less money - PWM charge controllers are cheaper.

    Go for an MPPT charger for:

    1. For larger systems, like your off-grid cabin where efficiency matters.
    2. In situations where the solar array has a higher voltage than the battery, for instance using your home's solar panel system to charge a 12V. An MPPT charge controller will charge this safely.
    3. MPPT charge controllers have the ability to monitor the state of charge and update their method based on situations, i.e., can add a boost of power when the battery is low.

    Once you settle on which charge controller option is best for you, these are the brands we recommend:

    EPEVER MPPT Charge Controller XTRA Series 10A/20A/30A/40A

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    EPEVER MPPT Charge Controller XTRA Series 10A/20A/30A/40A

    Image source: EP Solar

    The EPEVER charge controller is a high-quality MPPT charge controller with high efficiency and the ability to manage multiple loads. This is a great choice for your boat battery or RV.

    Key features:

    • >99.5% MPPT tracking efficiency
    • Supports lead-acid batteries and lithium batteries
    • Includes LCD display with real-time numbers
    • Can handle 10amps, 20amps, 30amps, and 40amps

    Victron Energy SmartSolar MPPT 100/30 & 100/50

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    Victron Energy SmartSolar MPPT 100/30 & 100/50

    Image source: Victron Energy

    The smart solar charge controller can charge even severely depleted batteries and comes in 12 volt or 24-volt models. This is a great option for your RV and potentially an off-grid home.

    Key features:

    • Built-in Bluetooth ability to monitor via smartphone
    • Automatic battery voltage recognition and temperature sensor
    • Advanced Maximum Power Point Detection in the event of shaded conditions
    • Can handle up to 100amps

    Renogy Wanderer 10A PWM Charge Controller

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    Renogy Wanderer 10A PWM Charge Controller

    Image source: Renogy

    The Renogy Wanderer is built for off-grid use for 12V and 24V batteries, perfect for charging 12V batteries on your boat or for your car.

    Key features:

    • Offers intelligent protection against overcharging
    • Can be used with flooded and lithium-ion batteries
    • Its compact size allows for easy installation
    • Can handle 10amps

    Elevate your off-grid experience with your new charge controller

    Now that you have decided on a charge controller, you can add it to your off-grid solar installation to manage your battery. A charge controller will maintain your battery by not overcharging it, this will help make sure that your battery remains viable for many years.

    Although there are a lot of intimidating acronyms and phrases containing the word “voltage,” all you need to do when buying a charge controller is to pick a high-quality brand, set it up and let it do its job. If generating electricity with solar power is appealing to you, adding solar panels to your home can help you create power and save money.

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     - Author of Solar Reviews

    Ana Almerini

    Web Content Specialist

    Ana is a web content specialist at SolarReviews. She has 5 years of marketing experience blended with 2 years of experience in climate communications and holds a master of arts degree in climate and society from Columbia University. Ana frequently volunteers for environmental causes ranging from oyster reef restoration in NJ to expanding bike sharing in Naples, Italy.

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