What is a charge controller?

A charge controller is a regulator that prevents batteries from overcharging. Almost all battery based systems use charge controllers as they are a critical component to higher voltage PV arrays. Charge controllers regulate the electric current that flows in and out from the battery bank, motor or pump. By controlling the rate of electric current, charge controllers prevent overcharging and can protect against over-voltage which could damage the battery or motor. A charge controller can also prevent totally draining ("deep discharging") a battery, or perform controlled discharges, depending on the battery technology, to protect battery life.

MPPT and PWM: Which one Is For Me?

There are several different sizes and styles of charge controllers. You can drill down the one you need by reviewing the two main types of charge controllers, MPPT and PWM. Pulse Width Modulation (PWM) charge controllers are the most basic charge controllers. These units regulate the flow of energy into the battery by turning "on" and "off". This cycle will continue until the voltage charge set-point is reached and will proceed to trickle charge your battery. These types of charge controllers are popular for smaller, lower voltage systems. (Example: 12V nominal solar panel charging a 12V battery bank.) When the input voltage from your PV array is close to that of your battery bank, using a rudimentary charge controller will be a very cost effective way to charge your batteries safely. Keep in mind that if you want to use a higher voltage panel to charge your battery bank, the "on"/ "off" function of the PWM style controller could reduce the life of your battery bank by charging it constantly with a significantly higher voltage. This is where MPPT charge controllers come in. Maximum Power Point Tracking (MPPT) charge controllers track the voltage curve as the sun moves along the sky using an algorithm to ensure that at any point during the day, you achieve maximum energy harvest. As a rule of thumb, anytime your solar panel voltage is twice that of your battery bank, an MPPT charge controller is the best choice. While more expensive, these units will consistently charge your battery bank to ensure the maximum charge rate (about 30% higher than PWM style charge controllers) without overcharging your batteries. MPPT charge controllers also allow for larger and higher voltage solar arrays. Most PWM charge controllers reach a 60 amp maximum output and around 120V solar panel voltage input. MPPT charge controllers can be up to 100 amps. MPPT charge controllers also come in higher voltage options like 150V, 200V, 250V, 300V and 600VDC configurations. For example, if you need to mount your solar panels a few hundred feet away from your battery bank, looking into a higher voltage input MPPT charge controller will help you minimize wire sizing and voltage drop.

How do I size my charge controller?

There are several factors that you need to consider when sizing your charge controller for your system. You can start by first determining your system voltage. Will this be a 12V, 24V or a 48V system? If you are starting from scratch and need a gauge to help determine your system voltage, a good rule of thumb is:
12V systems - generally under 1200 watts
24V systems - 1200 watts - 3000 watts
48V systems - above 3000 watts

MPPT Solar Charge Controller Sizing Tools from the Manufacturers

• Morningstar Sizing Tools
• OutBack Power String Sizing Tool
• Midnite Solar String Sizing Tool
• Blue Sky Energy Sizing Tools

For more information on overall system design you can reference our blog post "Charge Controller Sizing – PWM and MPPT."