Charge Controllers

The primary function of a charge controller in a stand-alone 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/1OO 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.

Source for information up to this point: Polar Powe Inc.

Charge Controller Design

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

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 shortcircuited, 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.

Shunt-Interrupting Design

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.

Shunt-Linear Design

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

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