Charge Controller Basics:
A charge controller is needed in virtually all renewable energy power systems that utilize batteries. Charge controllers regulate the power coming from your solar panels, wind turbine or other source to your batteries to prevent overcharging. Overcharging batteries will at the least significantly reduce battery life and at worst damage the batteries to the point that they are unusable, so it is important to control battery charging to protect battery life.
Types of Charge Controllers
The most basic charge controller simply monitors battery voltage and opens the circuit, stopping the charging when the battery voltage rises to a certain level. More advanced charge controllers, Pulse Width Modulation or PWM, slowly lower the amount of power applied to the batteries as the batteries approach a full charge. This type of controller allows the batteries to be more fully charged with less stress on the battery, extending battery life. It can also keep batteries in a fully charged state (called “float”) indefinitely. The newest and best charge controller technology is called maximum power point tracking or MPPT. MPPT controllers convert excess voltage into amperage, increasing the amount of power you can get from your renewable energy system.
PWM or MPPT Charge Controller – Which one is right for me?
PWM charge controllers reduce the voltage from your solar or wind system to that of your battery bank resulting in a decrease in efficiency. The efficiency loss depends on the size of the voltage mismatch between your system input and batteries. PWM charge controllers are less expensive than MPPT charge controllers, but are best only for smaller systems or where the additional power provided by an MPPT charge controller is not really needed.
MPPT charge controllers are the most efficient type on the market; they are also the most expensive. They allow your solar panels (or wind turbine) to operate at their optimum voltage levels in varying conditions; summer, winter, morning, noon, cloudy, etc., by taking the voltage output of your system and comparing it to the battery voltage. It then calculates the best voltage to get maximum AMPS into the battery. Remember it’s the AMPS that matter. Put another way, the MPPT controller tracks the best voltage level and then converts it to the voltage of your system (12, 24, 48VDC). This “tracking” of the optimal power point improves performance by as much as 30%.
Another benefit of MPPT charge controllers is that many can accept high input voltages (up to 120+V DC) and efficiently convert the DC voltage to that of your system so that you aren’t losing any generated power and you are able to use what you generate more efficiently. Additionally, using a higher DC voltage on the input side allows you to use thinner wire, decreasing your wire cost and making installation easier.
MPPT and PWM technologies are not mutually exclusive; you do not have to use one or the other. Several charge controllers use both technologies, but adding MPPT circuitry increases the cost of a charge controller. When selecting the right charge controller be sure to consider how many panels you are using, what your battery bank power storage capacity will be and how much load your power system will create. You should also consider how long you will need to run the system (years). The annual power gains from an MPPT charge controller can offset the cost savings of a controller only using PWM in many situations, and thus make it worth the extra cost.