What is the difference between MPPT and PWM charge controllers?


Charge controllers are a critical component in every solar installation. Although not the first thing people think of when talking about going solar, controllers ensure your system runs efficiently and safely for years to come. There are many changing variables that influence how much power is being generated, such as level of sunlight, temperature, and a battery’s state of charge. Charge controllers ensure your batteries are supplied with a steady and optimum level of power.



What are solar charge controllers?

The solar charge controller sits between the energy source and storage and prevents overcharging of batteries by limiting the amount and rate of charge to your batteries. They also prevent battery drainage by shutting down the system if stored power falls below 50 percent capacity and charge the batteries at the correct voltage level. This helps preserve the life and health of the batteries.

They also offer some other important functions:

Overload protection: If the current flowing into your batteries is much higher than what the circuit can deal with, you system may overload. This can lead to overheating and cause fires. Charge controller provide the important function of overload protection. In larger systems, we also recommend a double safety protection from circuit breakers or fuses.

Low voltage disconnects: This works as an automatic disconnect of non-critical loads from the battery when the voltage falls below a predefined threshold. It will automatically reconnect to the battery when it is being charged. This will prevent an over-discharge and protects equipment from operating at dangerously low voltages.

Block Reverse Currents: Solar panels pump current through your battery in one direction. At night, panels may naturally pass some of that current in the reverse direction without a charge controller to limit this. This can cause a slight discharge from the battery. Charge controllers prevent this from happening by acting as a valve.



Do I always need a charge controller?

Typically, yes. You don’t need a charge controller with smaller 1 to 5 watt panels. If a panel puts out 2 watts or less for each 50 battery amp hours, you probably don’t need a charge controller. Anything beyond that and you do.



Different types of charge controllers

There are two types of charge controllers to consider: Pulse Width Modulation (PWM) controllers and Maximum Power Point Tracking (MPPT) controllers. PWM charge controllers are an older technology and are cheaper, but less efficient than MPPT charge controllers. Both are widely used and perform similar functions of preserving the life of your batteries. Also, it’s important to point out that it’s not a matter of which is best all-around, but often what is best for your unique case. Also, we highly recommend purchasing a high quality charge controller since controllers only account for a small portion of the total system cost.

Both pulse width modulation and maximum power point tracking charge controllers have a lifespan of about 15 years, although that will vary based on the specific controller.



What impacts your decision-making process when selecting a charge controller

The following factors should be considered when buying a charge controller:

  • Your budget
  • Lifespan of the technology
  • Climate where your system will be installed
  • How many solar panels you have and how high your energy needs are
  • Size, number, and type of batteries you’re using in your system
Pulse Width Modulation Charge Controllers

The pulse width modulation controller was the original charge controller used and are simpler and less expensive than MPPT controllers. PWM controllers regulate the flow of energy to the battery by reducing the current gradually, called "pulse width modulation". When batteries are full, PWM charge controllers continue to supply a tiny amount of power to keep batteries full. PWM controllers are best for small scale applications because the solar panel system and batteries have to have matching voltages. This becomes a much more difficult with larger installations.

Cost: $20-$60

Pros:

  • Cheaper than MPPT controllers
  • Best for smaller systems where the efficiency is not as critical
  • Typically longer lifespan due to less components that may break
  • Best for warm sunny weather
  • Performs best when the battery is near the full state of charge

Cons:

  • Less Efficient than MPPT controllers
  • Because solar panels and batteries have to have matching voltages with these controllers, they are not ideal for larger, complex systems

Best for: Those with smaller systems (vans, RVs, tiny homes), those living in warmer climates




Maximum Power Point Tracking Charge Controller

Maximum Power Point Tracking controllers are efficient at using the full power of your solar panels to charge your batteries. They limit their output to ensure batteries don’t get overcharged. MPPT controllers will monitor and adjust their input to regulate your solar system’s current. MPPT controllers step down the voltage and boost the current. The overall output will increase as a result and you can expect efficiency ratings of 90% or higher. MPPT charge controllers are more common nowadays

For example, if it becomes cloudy, your MPPT charge controller will decrease the amount of current drawn in order to maintain a desirable voltage at the output of the panel. When it becomes sunny again, the MPPT controller will allow more current from the solar panel once again.

Cost: $100-$729

Pros:

  • Highly efficient
  • Best for larger systems where the additional energy production is valuable
  • Ideal for situations where the solar array voltage is higher than the battery voltage
  • Best in colder, cloudier environments
  • Performs best when the battery is in a low state of charge


Cons:

  • More expensive than PWM controllers
  • Typically shorter lifespan due to more components


Best for: Those with larger systems (cabins, homes, cottages), those living in colder climates



How to size your charge controller

Overall, charge controller sizing is not as difficult as you may think. Charge controllers are rated and sized depending on your solar array's current and the solar system’s voltage. You typically want to make sure you have a charge controller that is large enough to handle the amount of power and current produced by your panels.

Typically, charge controllers come in 12, 24 and 48 volts. Amperage ratings can be between one and 60 amps and voltage ratings from six to 60 volts.

If your solar system's volts were 12 and your amps were 14, you would need a solar charge controller that had at least 14 amps. However due to factors such as light reflection, sporadic increased current levels can occur, you need to factor in an additional 25% bringing the minimum amps that our solar charger controller must have to 17.5 amps. We’ll round up in this case, so in the end, you would need a 12 volt, 20 amp solar charge controller.

When it comes to charge controller sizing, you also have to take into consideration whether you’re using a PWM or MPPT controller. An improperly selected charge controller can result in up to a 50% loss of the solar generated power.

What to consider with MPPT charge controllers: Because MPPT controllers limit their output, you can make an array as large as you want and a controller will limit that output. However, this means your system isn’t as efficient as it could be since you have panels that aren’t being properly utilized. MPPT controllers will have an amp reading for it, for example a 40 Amp MPPT Controller. Even if your panels have the potential to produce 80A of current, an MPPT charge controller will only produce 40A of current, no matter what.

What to consider with PWM charge controllers: PWM controllers are unable to limit their current output. They simply use the array current. Therefore, if the solar array can produce 40A of current and the charge controller you’re using is only rated to 30A, then the controller could be damaged. It’s crucial to ensure your charge controller is matched, compatible with, and properly sized for your panels.


What is the upper voltage limit?

All charge controllers have an upper voltage limit. This refers to the maximum amount of voltage the controllers can safely handle. Make sure you know what the upper voltage limit of your controllers is.. Otherwise you may end up burning out your solar charge controller or creating other safety risks.



Common Charge Controller Mistakes and Errors

Because of all the different components of a solar installation, it can be easy to make a misstep in the installation process. Here are a few commonly made mistakes when it comes to solar charge controllers.

  • Do not connect AC loads to the charge controller. Only DC loads should be connected to the charge controller’s output.
  • Certain low-voltage appliances must be connected directly to the battery.
  • The charge controller should always be mounted close to the battery since precise measurement of the battery voltage is an important part of the functions of a solar charge controller.

Conclusion

By doing your research and weighing all the factors unique to your installation, you should be able to accurately choose what type and size charge controller is best for your system. Whether you’re living on the wide open road or cozied up in a cabin in the woods, charge controllers play an essential part of your solar installation. Selecting the right solar charge controller that’s right for you and your system will ensure your batteries stay healthy and system runs efficiently and safely for years.