Charge Controllers
A Charge Controller is one of the primary components in a battery-based renewable electric system. Though charge controllers can be used with wind and hydro systems, the vast majority control photovoltaic solar panel arrays. In the most basic sense, the charge controller is a voltage regulator that is intended to efficiently charge a battery and prevent overcharging. Most controllers provide a varying degree of more sophisticated functions, and many include fully programmable software that greatly increases their versatility.
Virtually all modern charge controllers use PWM (pulse width modulation) to control the charging process. PWM technology switches electricity from the solar array ON and OFF as needed to regulate the charge going into the battery. The rate and duration of the pulse is used to regulate the charging energy as needed to meet the parameters of the charge cycle. PWM is both reliable and efficient.
MPPT (maximum power point tracking) is an important technology that is available on more advanced charge controllers. MPPT continuously monitors the voltage and current (amps) from the solar array in relation to battery voltage. The controller is able to calculate the optimal incoming voltage and adjust it as required to produce higher battery charging rates. Depending on conditions, MPPT technology can deliver 30% more charging energy to the battery as compared to standard PWM controllers. This increased performance easily justifies the higher initial cost of the controller when solar arrays are about 500 watts or more. MPPT is genuinely worth the money. Some MPPT controllers have "voltage matching" capability, meaning that they can be connected to a high-voltage solar array and reduce that voltage to match the battery. High voltage arrays can allow the use of smaller wire from the array to controller, thereby reducing installation costs and energy loss. High voltage arrays generally require less wiring and fewer breakers at the array, further reducing costs.
Battery temperature compensation is an important charge controller feature when battery temperature varies significantly from 77 degrees Fahrenheit. As battery temperature falls, the internal chemical reactions are suppressed and charge voltages must be increased to achieve a full charge. Conversely, when a battery is hot the charging voltages must be reduced to prevent overcharging, overheating, and excessive gassing which depletes electrolyte. Many small charge controllers have on-board temperature sensing to adjust their charge voltages, but this solution may not work well if the battery temperature is significantly different than the controller temperature. Some controllers offer the option of using a remote sensor that can be attached to the battery for precise monitoring.
Most charge controllers use a 3-stage process to bring the battery to a full charge:
BULK charging is the initial stage of the charging process. The charge controller delivers the solar array's maximum available current (amps) to the partially discharged battery for rapid charging. During the BULK stage, the battery voltage gradually rises until it reaches the ABSORB voltage set point. The battery will be about 80% charged at this point. This is a "constant current" phase because the current is maintained at maximum while the voltage changes (rises).
ABSORB charging begins when the battery voltage reaches the ABSORB set point. The charge controller then begins to regulate the current delivered to the battery to keep the battery voltage steady. As the battery continues to "absorb" energy, the current needed to maintain absorb voltage gradually decreases. The battery can be considered to be fully charged when the current (amps) required to keep the battery at absorb voltage decreases to about 2% of total battery amp-hour capacity. For instance, an 850 amp-hour battery would reach full charge when the charge current decreases to about 17 amps, while the battery is at absorb voltage. At this point, ABSORB charging terminates and the FLOAT stage begins. Absorb charging is considered to be a "constant voltage" stage because the voltage is maintained at a constant level while the current changes (decreases).
FLOAT charging is a maintenance cycle intended to keep the battery at a full charge without overcharging, overheating, or causing the battery to lose water by excessive gassing. For a 12-volt flooded lead-acid battery, a typical float voltage is 13.2 volts. This will maintain the battery at a high state of charge until the end of the solar day, and the battery will usually continue to accumulate a small level of charge that "finishes" the charging cycle.
EQUALIZE charging is sometimes considered to be a 4th stage. In truth, equalizing is not part of a daily battery charging cycle, but rather a specific maintenance operation that is performed according to a regular schedule or as a response to an unfavorable battery condition. Simply, equalizing is a controlled overcharge of the battery that is intended to bring all of the cells of the battery to an equal state of charge. Equalizing is commonly performed on a monthly schedule as a preventative measure, or as needed to correct cell inbalance. Charge controllers with equalizing capability may initiate and terminate the cycle manually or automatically, and the voltage and cycle frequency may be adjustable according to the unit's sophistication. Flooded and AGM batteries may be equalized, but gel batteries should not.
We separate charge controllers into three simple categories:
Standard Charge Controllers use PWM technology for efficient charging. They may be simple 2-stage units with no adjustability or controls, or fully adjustable 3-stage models with digital displays and many features. Some models may have networking capability, or programming control through a personal computer. They almost never have full programming capability through on-board controls. Standard PWM units are often built for a specific task such as outdoor lighting, but most PWM controllers are best suited for small PV systems under 500 watts.
MPPT Charge Controllers offer the advantages of increased energy harvest with maximum power point tracking. They range widely in sophistication from smaller units that offer limited adjustability, to more versatile models with digital displays and advanced features. MPPT charge controllers are a good choice for increased performance from arrays that approach 500 watts or more, when the more complex programming and features of the deluxe MPPT units are not required. Most controllers in this class have interfaces for limited networking and/or remote control, which may also include battery monitoring. Some of these controllers have a limited capability to manage high voltage solar arrays.
Deluxe MPPT Charge Controllers are separated from other controllers by their sophisticated menu-driven programming with on-board controls. This means that their functions can be easily adjusted in almost every parameter without the need for a computer or additional programming device. Deluxe controllers typically have auxiliary relays that can control external devices for a variety of specialized uses. Network capability is a standard feature, allowing them to interface with other components from the same manufacturer. All deluxe MPPT controllers have full voltage-matching capability for use with high voltage arrays. Finally, these units generally have the highest current capacity of all charge controllers, making them the clear choice for nearly any system with over 1000 watts of solar power.