PWM vs MPPT: solar charge controllers compared (plus recommended models)
If you want to use solar panels to charge a mobile battery bank, you need a charge controller. Here's a guide to the different types and how to choose the right one for you.
If you have a battery bank in your RV, boat, or other off-grid setting that you want to charge with solar panels, you need a charge controller.
In a system where solar panels charge a bank of batteries - whether lead-acid or lithium-ion - a charge controller sits between them and regulates the flow of electricity into the batteries.
While grid-connected solar panel systems also use charge controllers, this article will focus on smaller scale applications, such as 12 volt systems for RVs, boats, and cabins. To learn more about solar batteries for the home, you can read this article.
What is a solar charge controller?
A solar charge controller performs the following functions:
- It regulates the flow of electricity into a battery bank. When a battery is depleted, you want a high flow of electricity to charge the battery quickly. When the battery is close to full, that flow should be decreased. And when a battery is full, you only want a trickle of electricity to keep the battery topped up.
- It prevents the backflow of electricity when the sun isn’t shining. Electricity behaves like water: it wants to flow from high to low potential. In a solar panel system with a battery, when the panels aren’t generating power, there will be a tendency for electricity to flow from the battery into the solar panels. While panels should have a blocking diode to prevent this, some cheap panels may not. Your charge controller will only allow electricity to flow from the panels into the batteries, preventing them from being drained when the sun isn’t shining.
- System monitoring. A good charge controller will tell you the state of the battery (voltage) and the load on the system. Some controllers have a small built-in display, while others have a smartphone app that let you talk to the controller over Bluetooth or WiFi.
- Load control. Some charge controllers have the ability to automatically turn on a connected load - such as a set of lights - when the solar panels stop supplying power, or according to a schedule.
- (MPPT controllers) Step-down or step-up panel voltage to match the battery voltage. Normally, if you want to charge a 12v battery, you need a 12v solar panel. While you can use a higher voltage panel, it will be less efficient to do so. On the other hand, if you have a 12v solar panel and want to use that to charge a 24v battery bank, it won’t work. The panel must have a higher voltage than the battery. However, more advanced Maximum Power Point Tracking (MPPT) charge controllers can step-up or step-down the solar panel voltage to match the battery.
As you can see, solar charge controllers provide important functionality and safety features. While it is possible to charge a battery by connecting solar panels to them directly, it is potentially dangerous to do so.
What are the different types of solar charge controllers?
There are two basic types:
- Pulse Width Modulation (PWM) charge controllers. These are less expensive controllers that nonetheless still provide basic battery charging, monitoring, and safety features. If you have a smaller solar system and don’t care about extracting the maximum electricity out of your panels, a PWM charge controller can be a perfectly good choice.
- Maximum Power Point Tracking (MPPT) charge controllers. This type of controller adds several features over more basic PWM models: higher efficiency under varying light and temperature conditions, and the ability to step-up or step-down the incoming voltage to better match your battery.
It’s worth taking a minute to fully explain the advantages of MPPT controllers.
What are the advantages of an MPPT charge controller?
The main reason to choose an MPPT charge controller is to maximize the electricity generation from your solar panels. If you have limited space - such as the rooftop of your RV - but have a large battery bank that you want to keep fully charged, going with an MPPT unit will allow you to get the maximum power out of the panels that you have.
On the other hand, if you have light power demands - maybe a few lights, a radio, and a fan - you might not care about maximizing your power generation. If that’s the case, you can save a few bucks and go with a cheaper PWM controller.
MPPT charge controllers deal with shade better
Partial shading can have a big impact on the electricity production of a solar panel, unless you have the electronics to compensate for it. For example, with a home solar system, additional MPPT units inside string inverters and power optimizers are ways to improve electricity production under difficult shading conditions.
An MPPT charge controller provides a similar advantage. Many websites claim that an MPPT controller will generate an average of 30% more electricity compared to a more basic PWM controller, but this seems like a dubious claim. The actual improvement will depend on the conditions under which you use your panels. If you use solar panels on a bright, sunny, and cool day, then the benefits of an MPPT controller will be minimized, and you could get by with a cheaper PWM model.
MPPT controllers let you use series wiring to increase the voltage for longer runs
Solar panels output low voltage direct current (DC) power. One characteristic of low voltage DC is that electric current is lost to resistance over long runs of wire. This might not be a problem if you have extra panels to compensate, or its a short distance (say, 10 feet) from your solar panels to your batteries. But if it’s a long run of wire (maybe 25 feet or longer), you could start to see a noticable drop is power output.
To compensate for that, you could use a thicker gauge of wire, such as 10 AWG. But thicker wire is more expensive.
An MPPT charge controller offers you a workaround. If you connect your solar panels in series, you increase the voltage of your system. Let’s say you have two 12v solar panels. If you connect them in parallel, the voltage of the system will be 12v. But if you connect them in series, the voltage of the panel is added together, so will you end up with a 24v system.
The advantage of higher voltage is that you lower your resistive losses in the wire. This means that you can continue to use a skinnier wire or longer runs without having to upgrade to a thicker wire.
What does this have to do with MPPT charge controllers? If your input voltage is much higher than the battery voltage, not all of the electrical current can be used to charge your batteries: some electricity effectively is lost.
An MPPT charge controller can help this by stepping down the input voltage to match the battery voltage. Thus, with our panels wired in series that produces 24v electricity, an MPPT controller will step that voltage down to 12v so that it can be efficiently charge a 12v battery.
Keep in mind that series wiring has a disadvantage, which is that it makes the system more suspectible to shading. To learn more, read our article about solar panels for RVs.
Avoid dirt cheap charge controllers on Amazon
If you take a look on Amazon, you can find numerous dirt-cheap charge controllers, some priced lower than twenty bucks. If you take a close look at one, it might seem to have all the right specs. At the same time, another device with similar specs might cost 10 times more. Why would you pay more?
While it’s possible that a cheap device would work just fine, remember that you might have just spent $1,000 or more on your batteries and solar panels. You want a high quality device that doesn’t risk damaging your equipment or cause a safety hazard.
One way to help ensure that you have a higher quality device is to look for a UL Listing label. Underwriters Laboratories performs safety testing of devices, and only devices that have passed are allowed to carry the UL label.
It’s an expensive process for a company, which is why dirt-cheap brands made overseas typically don’t have them.
You can also look for a CSA mark of the Canadian Standards Association, which performs a similar role in Canada.
Two product recommendations
Both Morningstar and Victron (note: Amazon affiliate links) make high quality PWM and MPPT charge controllers. The products from both these companies carry UL and CSA certifications and have 5 year warranties.