The primary function of a charge controller in a stand-alone photovoltaic (PV) system is to protect the battery from overcharge and over discharge. Any system that has unpredictable loads, user intervention, optimized or undersized battery storage (to minimize initial cost), or any characteristics that would allow excessive battery overcharging or over discharging requires a charge controller and/or low-voltage load disconnect. Lack of a controller may result in shortened battery lifetime and decreased load availability. Systems with small, predictable, and continuous loads may be designed to operate without a battery charge controller. If system designs incorporate oversized battery storage and battery charging currents are limited to safe finishing charge rates (C/SO flooded or C/100 sealed) at an appropriate voltage for the battery technology, a charge controller may not be required in the PV system. Proper operation of a charge controller should prevent overcharge or over discharge of a battery regardless of the system sizing/design and seasonal changes in the load profile and operating temperatures. The algorithm or control strategy of a battery charge controller determines the effectiveness of battery charging and PV array utilization, and ultimately the ability of the system to meet the load demands. Additional features such as temperature compensation, alarms, and special algorithms can enhance the ability of a charge controller to maintain the health, maximize capacity, and extend the lifetime of a battery.
Basics of Charge Controller Theory
While the specific control method and algorithm vary among charge controllers, all have basic parameters and characteristics. Manufacturer's data generally provides the limits of controller application such as PV and load currents, operating temperatures, losses, set points, and set point hysteresis values. In some cases the set points may be intentionally dependent upon the temperature of the battery and/or controller, and the magnitude of the battery current. A discussion of the four basic charge controller set points follows:
- Regulation set point (VR): This set point is the maximum voltage a controller allows the battery to reach. At this point a controller will either discontinue battery charging or begin to regulate the amount of current delivered to the battery. Proper selection of this set point depends on the specific battery chemistry and operating temperature.
- Regulation hysteresis (VRH): The set point is voltage span or difference between the VR set point and the voltage when the full array current is reapplied. The greater this voltage span, the longer the array current is interrupted from charging the battery. If the VRH is too small, then the control element will oscillate, inducing noise and possibly harming the switching element. The VRH is an important factor in determining the charging effectiveness of a controller.
- Low voltage disconnect (LVD): The set point is voltage at which the load is disconnected from the battery to prevent over discharge. The LVD defines the actual allowable maximum depth-of-discharge and available capacity of the battery. The available capacity must be carefully estimated in the system design and sizing process. Typically, the LVD does not need to be temperature compensated unless the batteries operate below 0°C on a frequent basis. The proper LVD set point will maintain good battery health while providing the maximum available battery capacity to the system.
- Low voltage disconnect hysteresis (LVDH): This set point is the voltage span or difference between the LVD set point and the voltage at which the load is reconnected to the battery. If the LVDH is too small, the load may cycle on and off rapidly at low battery state-of-charge, possibly damaging the load and/or controller. If the LVDH is too large, the load may remain off for extended periods until the array fully recharges the battery. With a large LVDH, battery health may be improved due to reduced battery cycling, but this will reduce load availability. The proper LVDH selection will depend on the battery chemistry, battery capacity, and PV and load currents.
Charge Controller Designs
Two basic methods exist for controlling or regulating the charging of a battery from a PV module or array - shunt and series regulation. While both of these methods are effectively used, each method may incorporate a number of variations that alter their basic performance and applicability. Simple designs interrupt or disconnect the array from the battery at regulation, while more sophisticated designs limit the current to the battery in a linear manner that maintains a high battery voltage. The algorithm or control strategy of a battery charge controller determines the effectiveness of battery charging and PV array utilization, and ultimately the ability of the system to meet the electrical load demands. Most importantly, the controller algorithm defines the way in which PV array power is applied to the battery in the system. In general, interrupting on-off type controllers require a higher regulation set point to bring batteries up to full state of charge than controllers that limit the array current in a gradual manner. Some of the more common design approaches for charge controllers are described in this section.
Shunt Controller Designs
Since photovoltaic cells are current-limited by design (unlike batteries), PV modules and arrays can be short-circuited without any harm. The ability to short-circuit modules or an array is the basis of operation for shunt controllers. The shunt controller regulates the charging of a battery from the PV array by short-circuiting the array internal to the controller. All shunt controllers must have a blocking diode in series between the battery and the shunt element to prevent the battery from short-circuiting when the array is regulating. Because there is some voltage drop between the array and controller and due to wiring and resistance of the shunt element, the array is never entirely short circuited, resulting in some power dissipation within the controller. For this reason, most shunt controllers require a heat sink to dissipate power, and are generally limited to use in PV systems with array currents less than 20 amps. The regulation element in shunt controllers is typically a power transistor or MOSFET, depending on the specific design. There are a couple of variations of the shunt controller design. The first is a simple interrupting, or on-off type controller design. The second type limits the array current in a gradual manner, by increasing the resistance of the shunt element as the battery reaches full state of charge. These two variations of the shunt controller are discussed next.
The shunt-interrupting controller completely disconnects the array current in an interrupting or on-off fashion when the battery reaches the voltage regulation set point. When the battery decreases to the array reconnect voltage, the controller connects the array to resume charging the battery. This cycling between the regulation voltage and array reconnect voltage is why these controllers are often called ‘on-off’ or ‘pulsing’ controllers. Shunt-interrupting controllers are widely available and are low cost, however they are generally limited to use in systems with array currents less than 20 amps due to heat dissipation requirements. In general, on-off shunt controllers consume less power than series type controllers that use relays (discussed later), so they are best suited for small systems where even minor parasitic losses become a significant part of the system load. Shunt-interrupting charge controllers can be used on all battery types, however the way in which they apply power to the battery may not be optimal for all battery designs. In general, constant-voltage, PWM or linear controller designs are recommended by manufacturers of gelled and AGM lead-acid batteries. However, shunt-interrupting controllers are simple, low cost and perform well in most small stand-alone PV systems.
Once a battery becomes nearly fully charged, a shunt-linear controller maintains the battery at near a fixed voltage by gradually shunting the array through a semiconductor regulation element. In some designs, a comparator circuit in the controller senses the battery voltage, and makes corresponding adjustments to the impedance of the shunt element, thus regulating the array current. In other designs, simple Zener power diodes are used, which are the limiting factor in the cost and power ratings for these controllers. There is generally more heat dissipation in a shunt-linear controllers than in shunt-interrupting types.Shunt-linear controllers are popular for use with sealed VRLA batteries. This algorithm applies power to the battery in a preferential method for these types of batteries, by limiting the current while holding the battery at the regulation voltage.
Series Controller Designs
As the name implies, this type of controller works in series between the array and battery, rather than in parallel as for the shunt controller. There are several variations to the series type controller, all of which use some type of control or regulation element in series between the array and the battery. While this type of controller is commonly used in small PV systems, it is also the practical choice for larger systems due to the current limitations of shunt controllers.In a series controller design, a relay or solid-state switch either opens the circuit between the array and the battery to discontinuing charging, or limits the current in a series-linear manner to hold the battery voltage at a high value. In the simpler series interrupting design, the controller reconnects the array to the battery once the battery falls to the array reconnect voltage set point. As these on-off charge cycles continue, the ‘on’ time becoming shorter and shorter as the battery becomes fully charged. Because the series controller open-circuits rather than short-circuits the array as in shunt-controllers, no blocking diode is needed to prevent the battery from short-circuiting when the controller regulates.
The most simple series controller is the series-interrupting type, involving a one-step control, turning the array charging current either on or off. The charge controller constantly monitors battery voltage, and disconnects or open-circuits the array in series once the battery reaches the regulation voltage set point. After a pre-set period of time, or when battery voltage drops to the array reconnect voltage set point, the array and battery are reconnected, and the cycle repeats. As the battery becomes more fully charged, the time for the battery voltage to reach the regulation voltage becomes shorter each cycle, so the amount of array current passed through to the battery becomes less each time. In this way, full charge is approached gradually in small steps or pulses, similar in operation to the shunt-interrupting type controller. The principle difference is the series or shunt mode by which the array is regulated.Similar to the shunt-interrupting type controller, the series-interrupting type designs are best suited for use with flooded batteries rather than the sealed VRLA types due to the way power is applied to the battery.
Series-Interrupting, 2-step, Constant-Current Design
This type of controller is similar to the series-interrupting type, however when the voltage regulation set point is reached, instead of totally interrupting the array current, a limited constant current remains applied to the battery. This ‘trickle charging’ continues either for a pre-set period of time, or until the voltage drops to the array reconnect voltage due to load demand. Then full array current is once again allowed to flow, and the cycle repeats. Full charge is approached in a continuous fashion, instead of smaller steps as described above for the on-off type controllers. Some two-stage controls increase array current immediately as battery voltage is pulled down by a load. Others keep the current at the small trickle charge level until the battery voltage has been pulled down below some intermediate value (usually 12.5-12.8 volts) before they allow full array current to resume.
Series-Interrupting, 2-Step, Dual Set Point Design
This type of controller operates similar to the series-interrupting type, however there are two distinct voltage regulation set points. During the first charge cycle of the day, the controller uses a higher regulation voltage provide some equalization charge to the battery. Once the array is disconnected from the battery at the higher regulation set point, the voltage drops to the array reconnect voltage and the array is again connected to the battery. However, on the second and subsequent cycles of the day, a lower regulation voltage set point is used to limit battery overcharge and gassing.This type of regulation strategy can be effective at maintaining high battery state of charge while minimizing battery gassing and water loss for flooded lead-acid types. The designer must make sure that the dual regulation set points are properly adjusted for the battery type used. For example, typical set point values (at 25°C) for this type of controller used with a flooded lead-antimony battery might be 15.0 to 15.3 volts for the higher regulation voltage, and between 14.2 and 14.4 volts for the lower regulation voltage.
Series-Linear, Constant-Voltage Design
In a series-linear, constant-voltage controller design, the controller maintains the battery voltage at the voltage regulation set point. The series regulation element acts like a variable resistor, controlled by the controller battery voltage sensing circuit of the controller. The series element dissipates the balance of the power that is not used to charge the battery, and generally requires heat sinking. The current is inherently controlled by the series element and the voltage drop across it.Series-linear, constant-voltage controllers can be used on all types of batteries. Because they apply power to the battery in a controlled manner, they are generally more effective at fully charging batteries than on-off type controllers. These designs, along with PWM types are recommended over on-off type controllers for sealed VRLA type batteries.
Series-Interrupting, Pulse Width Modulated (PWM) Design
This algorithm uses a semiconductor switching element between the array and battery which is switched on/off at a variable frequency with a variable duty cycle to maintain the battery at or very close to the voltage regulation set point. Although a series type PWM design is discussed here, shunt-type PWM designs are also popular and perform battery charging in similar ways. Similar to the series-linear, constant-voltage algorithm in performance, power dissipation within the controller is considerably lower in the series interrupting PWM design.By electronically controlling the high speed switching or regulation element, the PWM controller breaks the array current into pulses at some constant frequency, and varies the width and time of the pulses to regulate the amount of charge flowing into the battery as shown in Figure 12-8. When the battery is discharged, the current pulse width is practically fully on all the time. As the battery voltage rises, the pulse width is decreased, effectively reducing the magnitude of the charge current. The PWM design allows greater control over exactly how a battery approaches full charge and generates less heat. PWM type controllers can be used with all battery types, however the controlled manner in which power is applied to the battery makes them preferential for use with sealed VRLA type batteries over on-off type controls. To limit overcharge and gassing, the voltage regulation set points for PWM and constant voltage controllers are generally specified lower than those for on-off type controllers. For example, a PWM controller operating with a nominal 12 volt flooded lead-antimony battery might use a VR set point of 14.4 to 14.6 volts at 25°C, while an on-off controller used with the same battery might require a VR set point of between 14.7 and 15.0 volts to fully recharge the battery on a typical day.
For further information on charge controllers in solar home systems see
- James P. Dunlop, Florida Solar Energy Center for Sandia National Laboratories: Batteries and Charge Control in Stand-Alone Photovoltaic Systems. Fundamentals and Application, 1997