Comparison: MPPT vs PWM Charge Controllers

At first glance, a charge controller may not seem so important. Sure, charging batteries is a big part of an off-grid system, but can't you just wire your renewable energy source straight into your batteries and be done with it? The answer is yes, but that you definitely shouldn't. Batteries are the most expensive component of your off-grid system and charge controllers actually protect them while charging.

As a lead-acid battery (the current standard for solar and wind applications) charges, the optimal voltage for charging the battery changes (for more details, please click to read our Battery Basics article). A charge controller insures your battery is always charging at or close to this voltage, which extends battery life. Even if you have high voltage solar panels and a low voltage battery bank, the right charge controller will still bring your battery charging voltage to a safe level!

Now that we've established that a charge controller is absolutely essential, let's go into the two types of charge controllers and their advantages or disadvantages.

There are currently two dominant charge controller technologies on the market: MPPT and PWM. While they both match your power source's voltage to your battery voltage, they do so in different ways, which makes them suited for different types of applications. Regardless of the renewable energy source for your power, the benefits and disadvantages of each technology do not change (though the specific controllers you can use may).

The alternate power source and storage batteries are the two principle cost items of an alternate energy system. The battery charge controller based on MPPT or PWM can do one of two things. The charge controller circuit of a MPPT controller (Maximum Power Point Tracking) unit is designed convert the voltage from your solar or wind power sources to the voltage optimum for maximum transfer of power to the battery or grid via a grid tie system. A PWM charge controller does not operate to optimize power transfer to the batteries; it is optimized to extend the life of the batteries. By applying higher voltage in short bursts a PWM controllers essentially clean off the lead plates that are a central part of the battery storage system. How to charge controller� selection depends on if it makes sense to optimize power transfer, with an MPPT controller, or to optimize battery life, with the PWM charge controllers.

To make this tradeoff, we need to understand the relative cost difference between solar panels and batteries. Solar panels have long life- most manufacturers offer guarantees for output good for 20 years or more. Battery life, however, is rated by number of deep cycles and the number is typically rated in hundreds of deep cycles. Since a standalone solar or wind system might cycle once a day, life extension of batteries can be an important goal. To understand the balanced lifecycle cost of the generation + storage system we need to understand batteries will be replaced many times before the solar panels or wind turbine needs to be replaced. If we look at a very rough cut with solar panel life equal to 20 years and battery life equal to 1 year, we can estimate one dollar of battery is equal to twenty dollars of solar panel. How to select a charge controller is how we make this tradeoff.

First, consider solar battery charge controller systems that could be categorized as battery powered with solar charging. A good example might be a sailboat used for weekend trips or a country cabin. Such a boat might spend a week on a mooring or at the dock, and then used on a trip of a night or two, with deep discharge of the battery expected with every use. A cabin may sit in the sun for a week, and then deliver power to the limits of the battery capacity for when visited. For both of these, there is plenty of time to charge the battery, and in fact the boat might be able to use marina supplied grid power to do the charging. For both of these, deep discharge of the battery and consequential wear and tear on the battery is to be expected. In a system like this, the critical variable is battery life, suggesting the use of a PWM charge controller. Efficiency of conversion of solar power to battery power is not the key variable since there is plenty of time to charge the battery, or charging from grid is possible and included in marine ship rental.

PWM Charge Controllers

Because they are less complex, PWM controllers are less expensive than MPPT systems. They are a great choice for systems where your battery and panel voltages are matched.

Xantrex PWM Charge Controllers

A few years ago, Schneider Electric bought out Xantrex's line of charge controllers. These controllers remain an excellent choice for your off-grid system.

Morningstar PWM Charge Controllers

Low cost Morningstar PWM charge controllers offer a wide selection of PWM devices to match any battery voltage and power range. The 12 volt charge controllers are very common for PWM systems on mobile homes, boats and cabins. The Duo even allows you to charge two separate battery banks!

Not all systems face the same challenges as a sailboat or country cabin. The typical home solar power system has access to the grid for backup power and can even sell excess power back to the grid using a grid-tied inverter system. If there are batteries, they are used only for short term, emergency backup when both solar and grid power are unavailable. For this type of system, maximum power transfer from the solar panel or wind turbine to the battery or grid is important. This is where MPPT technology comes in play.

MPPT Charge Controllers

MPPT solar controllers are considerably more elaborate than PWM controllers. The optimum voltage for battery charging changes as the battery's state of charge changes, and the panel's output varies depending on temperature, available sunlight, and more. The MPPT charge controller matches these two voltages and can result in up to 30% more efficient charging. This means a good MPPT solar charge controller can delivery the same level of charge with fewer panels, a tremendous benefit. While only a small part of system price, a MPPT solar panel charge controller can leverage the rest of the system to better performance.

Blue Sky MPPT Charge Controllers

Blue Sky was the first to develop MPPT controllers; they've even patented their technology! Their Solar Boost MPPT controllers include the most options for voltage and power.

Outback MPPT Charge Controllers

Outback offers a pair of excellent MPPT charge controllers as well. While these aren't as varied as the options offered by Blue Sky, they do an excellent job of fulfilling the market's need for larger controllers.

We at EcoDirect hope this article has helped to clarify the different options on the market so you can make an informed decision when going off-grid. If you have any questions or would like a hand sizing your controller, please give our technical sales team a call at 888-899-3509 ext. 1.