Charge controllers are required in most PV systems using a battery to protect against battery overcharging and overdischarging. There are different types of charge controller design, and their specifications dictate their intended operating limits and applications. Reference: Photovoltaic Systems, p. 175 References: Photovoltaic Systems, Chap. 7 National Electrical Code (NEC), Articles 110, 690 7-1 7-2
A charge controller is equipment used to regulate the charging of a battery, by limiting the voltage and/or current from the charging source such as a PV array. Charge control is required for most PV systems that use batteries. Charge control may not be required for very small systems, where the PV array has been carefully matched to the voltage and current charging requirements of the battery, and the maximum charge current multiplied by one hour is less than 3 percent of the manufacturer s rated ampere-hour capacity. For example, a 200 ampere-hour battery charged by a PV array producing a maximum charging current of 6 amps or higher requires charge control. Charge controllers used in PV system vary widely in their size, functions and features. Suggested Exercise: Review charge controller manufacturer s specifications and installation instructions. Reference: Photovoltaic Systems, p. 175-180 References: NEC 690.72(A), 690.2 Photovoltaic Systems, p. 175-180 7-3 7-4
The primary function of charge controllers is to prevent battery overcharge. Many charge controllers also provide overdischarge protection for the battery by disconnecting DC loads at low voltage and low state-of-charge condition. Additional functions may also be performed by charge controllers, including controlling lighting loads or multiple energy sources. Special controllers are also available that regulate battery charge by diverting excess power to auxiliary loads. Reference: Photovoltaic Systems, p. 177-178 References: Photovoltaic Systems, p. 175-180 See video clip on CD-ROM 7-5 7-6
Charge controllers are also used to protect a battery from excessively deep discharges that can damage the battery and shorten its life. Typically, the controller disconnects DC loads at a predetermined battery voltage, corresponding to low state-of-charge conditions. High-current DC utilization equipment like inverters are usually connected directly to batteries rather than through charge controllers, and have integral low-voltage disconnect means. A charge controller may protect a battery from both overcharge and/or overdischarge with the same unit or separate equipment. Reference: Photovoltaic Systems, p. 179-180 Charge controllers are classified by the way they regulate the PV array current and battery charging process. The charging current from a PV array can be controlled by either short-circuiting (shunt) or by open-circuiting (series) the PV array. A charge controller algorithm is the method of charge regulation. Simple interrupting type controllers regulate the PV array current in an on-off manner, while more advanced algorithms use multi-stage methods. Solid-state switching devices can be used to regulate array current at high-frequencies or in a linear manner to gradually reduce current applied to the battery as it reaches full state-of-charge. Some charge controllers also provide array maximum power point tracking. Reference: Photovoltaic Systems, p. 179-180 7-7 7-8
A shunt charge controller regulates battery charging by short-circuiting the PV array. Since PV arrays are current-limited, this does not harm the array. However, a blocking diode is required to prevent short-circuiting the battery. The shunt device can be a power transistor, Zener diode or a linear solid-state device that shortcircuits the PV array or decreases the shunt resistance as the battery reaches full charge. Shunt-type controllers are usually limited to 20 amps due to their heat dissipation requirements. Reference: Photovoltaic Systems, p. 181-182 A Zener diode can be used as the short-circuiting device for a shunt-type controller. The Zener diode (D1) conducts current in the reverse direction once the diode breakdown voltage is reached, regulating the array current while holding the battery at constant voltage. The value for the Resistor R1 is determined by voltage drop across R1 divided by the circuit current. Assuming the circuit current is 400 ma, and the voltage drop is 5 volts, then the value of R1 would be 5 V/0.4 A = 21.5 ohms, and the minimum power rating for the resistor should be 0.4 A x 5 V = 2 watts. The Zener diode should have a Zener (breakdown) voltage of approximately 14 volts for a nominal 12 V lead-acid battery, and be rated for at least 156% of the PV module short-circuit current. The minimum power rating for the Zener diode should be at least the Zener voltage times the circuit current, or 14 V x 0.4 A = 5.6 watts. Diode D2 may be an inexpensive silicon diode, with a current rating of at least 156% of the maximum PV module short circuit current, and a voltage rating 125% of the PV module open-circuit voltage. This diode prevents reverse current flow from the battery to the PV module at night and when the Zener diode is regulating. Suggested Exercise: Build and test a simple Zener diode shunt charge controller using small components. 7-9 7-10
A series charge controller regulates battery charging by open-circuiting the PV array. The series element can be a power transistor or a linear solid-state device that open-circuits the PV array or increases the series resistance as the battery reaches full charge. Reference: Photovoltaic Systems, p. 182-183 Pulse-Width-Modulation (PWM) charge controllers regulate battery charge by modulating the charging current on a high-frequency carrier signal. When the battery is at lower state-of-charge, the width of the current pulses is wider, applying the full charge current to the battery for a longer interval. When the battery approaches full charge, the width of the current pulses narrows, limiting the current to the battery. PWM controllers can be either shunt or series types. Reference: Photovoltaic Systems, p. 183 7-11 7-12
MPPT charge controllers operate PV arrays at maximum power under all operating conditions independent of battery voltage. Typically, the PV array is configured at higher voltages than the battery, and DC to DC power conversion circuits in the controller automatically provides a lower voltage and higher current output to the battery. MPPT controllers can improve array energy utilization and allow nonstandard and higher array operating voltages, requiring smaller conductors and fewer source circuits. A diversionary charge controller diverts excess PV array power to auxiliary loads when the primary battery system is fully charged, allowing a greater utilization of PV array energy. Whenever a diversionary charge controller is used, a second independent charge controller is required to prevent battery overcharge in the event the diversion loads are unavailable or the diversion charge controller fails. The additional charge controller uses a higher regulation voltage, and permits the diversionary charge controller to operate as the primary control. Reference: Photovoltaic Systems, p. 184 References: NEC 690.72(B)(1) Photovoltaic Systems, p. 184-185 See video clip on CD-ROM 7-13 7-14
Several requirements apply to PV systems using DC diversionary loads and DC diversion charge controllers. These requirements are intended to help prevent hazardous conditions and protect the battery if the diversion controller fails or the DC loads are unavailable. References: NEC 690.72(B)(2) Photovoltaic Systems, p. 184-185 Some interactive PV systems use battery-based inverters as a backup power source when the utility is de-energized. These systems must also have a second independent charge controller to prevent battery overcharge when the grid or loads are not available to divert excess power. Normally, these systems regulate the battery charge by diverting excess PV array DC power through the inverter to produce AC power to feed site loads or the grid. When the grid de-energizes, an automatic transfer switch disconnects loads from utility network and the system operates in stand-alone mode. If all loads have been met and the grid is not available, the battery can be overcharged. References: NEC 690.72(B)(3) Photovoltaic Systems, p. 107-108 7-15 7-16
Reference: Photovoltaic Systems, p. 187-192 Set points are the battery voltage levels at which a charge controller performs regulation or control functions. The regulation voltage (VR) is the maximum voltage set point the controller allows the battery to reach before the array current is disconnected or limited. For interrupting type controllers, the array reconnect voltage (ARV) is the voltage set point at which the array is again reconnected to charge the battery. The regulation voltage hysteresis (VRH) is difference or span between the VR and ARV set points. PWM and constant-voltage type controllers do not have a definable ARV. The proper VR and VRH set points are critical for optimal battery charging. The low-voltage disconnect (LVD) is the battery voltage set point at which the controller disconnects the system loads to prevent overdischarge. The LVD defines the maximum battery depth-of-discharge at the given discharge rate. The load reconnect voltage (LRV) set point is the voltage that load are permitted to be reconnected to the battery. The low-voltage disconnect hysteresis (LVDH) is the voltage difference between the LVD and LRV. A higher LRV and LVDH allow a battery to receive more charge before loads are reconnected to the battery. 7-17 7-17
Reference: Photovoltaic Systems, p. 187-191 Optimal charge regulation set points depend on the type of battery and control method used. Higher charge regulation voltages are required for all types of batteries using interrupting type controllers, compared to more effective constantvoltage, PWM or linear designs. References: Photovoltaic Systems, p. 187-188 Charge controller and battery manufacturer s specifications. 7-18 7-19
References: Photovoltaic Systems, p. 189-190 Charge controller and battery manufacturer s specifications. Some charge controllers provide the capability for manual or automatic or equalization charging. The equalization charge settings may be programmable, or set with jumpers on the controller circuit board. Equalization charging is performed on flooded, open-vent batteries to help minimize differences and restore consistency between individual cells, and can help reduce sulfation and stratification. Flooded lead-acid batteries are normally equalized at approximately 2.6 volts per cell (VPC) at 25 C for 1-3 hour periods once or twice a month. Reference: Photovoltaic Systems, p. 191 7-20 7-21
Charge controller set point adjustments may be fixed, programmable or manually adjusted by the installer. Because improper charge control settings may create a hazardous condition for batteries, any adjusting means for a charge controller must only be accessible to qualified persons. Suggested Exercise: Use a min/max recording voltmeter the measure and adjust the charge controller set points in a small stand-alone PV system. References: NEC 690.72(A) Photovoltaic Systems, p. 187-193 Temperature compensation is a feature of charge controllers that automatically adjusts the charge regulation voltage for battery temperature changes. Where battery temperatures vary seasonally more than 10 C, compensation of the charge regulation set point is normally used. Temperature compensation is also recommended for all types of sealed batteries, which are much more sensitive to overcharging than flooded types. Temperature compensation helps to fully charge a battery during colder conditions, and helps protect it from overcharge during warmer conditions. Reference: Photovoltaic Systems, p. 192-193 7-22 7-23
Temperature compensation sensors are used to monitor battery temperature remotely from a charge controller. This is particularly important where the controller and battery are located in different enclosures or thermal environments. Sensors are typically resistive thermal devices (RTDs) that vary resistance with temperature. Some sensors are intended to be permanently affixed to the sides of the internal battery bank, while others are connected to the negative battery terminal. Reference: Photovoltaic Systems, p. 192-193 Reference: Photovoltaic Systems, p. 192-193 7-24 7-25
These two charts illustrate the daily operating characteristics for a small stand-alone PV lighting system operating with a shunt-interrupting charge controller. Beginning at the left on the charts (midnight), the light is operating and discharging the battery at about 3 amps (Ibat), while the battery voltage decreases from about 12.1 volts to 11.9 volts. Just after 0400 hours, the load is disconnected by the charge controller load timing circuit, the battery current (Ibat) goes to zero, and the battery voltage (Vbat) rises to about 12.3 volts. At sunrise (0700 hours), the battery voltage increases as the PV array begins to recharge the battery. Until about noon time (1200 hours), the charging current (Ipv) and the battery voltage (Vbat) increase steadily with increasing solar irradiance as the battery is being recharged. Note that during this period, the battery charge controller is not regulating and the PV array current is approximately the same as the battery current. At about noon, the battery voltage reaches the charge controller regulation voltage (about 14.1 volts), and the controller begins to regulate the battery current in an on-off manner. Because the shunt controller short-circuits the PV array, the average PV array current follows the solar irradiance while the average PV array voltage reduces to about 5 volts, indicating regulation. With the onset of regulation, the minimum and maximum battery voltages are distinguished from the six-minute average voltage, and show the approximate charge controller set points. After regulation, the maximum battery voltage is between 14.3 and 14.5 volts and corresponds to the voltage regulation set point. The minimum battery voltage is about 13.7 volts, corresponding to the point at which the charge controller reconnects the array to the battery to resume charging. Towards the end of the sunlight hours (1600-1700 hours), the PV array current output reduces to a low enough value, in this case about 2.5 amps, where regulation is not required to limit the battery voltage below the charge regulation set point, and the PV array remains connected to the battery for the remainder of the day. Once the sun sets (about 1800 hours), the battery voltage begins a gradual decrease to its steady-state voltage. Note how the battery voltage at this time is higher than in the morning before the battery was recharged. At about 2030 hours, the 3 amp lighting load is again connected and the battery voltage begins to steadily decrease. These two charts illustrate the daily operating characteristics for a small stand-alone PV lighting system operating with a series-interrupting charge controller. Beginning at the left on the charts (midnight), the light is operating and discharging the battery at about 3 amps (Ibat), while the battery voltage decreases from about 11.9 volts to 11.7 volts. Just after 0400 hours, the load is disconnected by the charge controller load timing circuit, the battery current (Ibat) goes to zero, and the battery voltage (Vbat) rises to about 12.1 volts. At sunrise (0700 hours), the battery voltage increases as the PV array begins to recharge the battery. Until about noon time (1200 hours), the charging current (Ipv) and the battery voltage (Vbat) increase steadily with increasing solar irradiance as the battery is being recharged. Note that during this period, the battery charge controller is not regulating and the PV array current is approximately the same as the battery current. At about noon, the battery voltage reaches the charge controller regulation voltage (about 14.1 volts), and the controller begins to regulate the battery current in an on-off manner. Because the series controller open-circuits the PV array, the average PV array current decreases while the average PV array voltage approaches the array open-circuit voltage. With the onset of regulation, the minimum and maximum battery voltages are distinguished from the six-minute average voltage, and show the approximate charge controller set points. After regulation, the maximum battery voltage is about 14.1 volts and corresponds to the voltage regulation set point. The minimum battery voltage is between 13.2 and 13.4 volts, corresponding to the point at which the charge controller reconnects the array to the battery to resume charging. Once the sun sets (about 1800 hours), the battery voltage begins a gradual decrease to its steadystate voltage. Note how the battery voltage at this time is higher than in the morning before the battery 7-26 7-27
was recharged. At about 2030 hours, the 3 amp lighting load is again connected and the battery voltage begins to steadily decrease. These two charts illustrate the daily operating characteristics for a small stand-alone PV lighting system operating with a series pulse-width-modulated charge controller. Beginning at the left of the two graphs (midnight), the load is operating and battery voltage decreases steadily from about 12.2 volts to 11.9 volts while being discharged at about 3 amps. At about 0430 hours, the load is disconnected, the battery current goes to zero, and there is a sharp rise in the battery voltage as it approaches steady-state of about 12.3 volts. At sunrise (about 0700 hours), the battery voltage begins to increase as the PV array current charges the battery. Before noon, the PV array current and battery voltage increase steadily with increasing solar irradiance as the battery is being recharged. At noon, the battery voltage reaches the regulation voltage set point for the charge controller (about 14.5 volts), and the controller begins to regulate the PV array current for the remainder of the day. The series switching characteristic can be seen by the fact that once regulation begins, the average PV array current also decreases, while the average PV array voltage approaches the open-circuit array voltage. The PWM controller regulates the array by decreasing current pulses to the battery, which are modulated on a high-frequency carrier signal. When the regulation voltage is reached, the current pulses are gradually reduced to hold the battery voltage at the regulation set point, as long as array current is available. 7-27 7-28
Advanced charge controllers use a multi-stage charging process, and have multiple set points. Reference: Photovoltaic Systems, p. 156, 178 Multiple charge controllers may be used on individual subarrays for larger systems. One subarray may be directly connected to battery without charge control if the charge current multiplied by one hour is less than 3% of the battery capacity, and may improve battery finishing charge. Reference: Photovoltaic Systems, p. 195-196 7-29 7-30
Voltage drop in the circuit from a charge controller to a battery can result in charge regulation occurring at lower battery voltages than the controller set points. Some battery charge controllers have separate connections to measure battery voltage independent of the conductors that carry the charging current. These additional sensing leads measure voltage directly at the battery terminals, and eliminate the affects of voltage drop on the charge regulation set points. Reference: Photovoltaic Systems, p. 194-195 Self-regulating PV systems are stand-alone PV systems using batteries that are designed to operate without charge control. Charge control may only be eliminated under special circumstances when the load is well-defined and the battery is oversized with respect to the charging source. The U.S. Coast Guard commonly uses self-regulating PV systems for navigational aids. Suggested Exercise: Discuss the design requirements for self-regulating standalone PV systems. Reference: Photovoltaic Systems, p. 197-199 7-31 7-32
Special lower-voltage PV modules are used in self-regulating systems. Typically, 36 series-connected silicon solar cells are required to provide adequate charging voltage for nominal 12-volt batteries in PV systems using charge control. For selfregulating systems, PV modules with only 29 or 30 series-connected cells are commonly used. The intention in self-regulating systems is to operate the PV array to the right of the maximum power point on its IV curve, limiting the array current as the battery voltage increases to the regulation voltage. The temperature effects on PV arrays and batteries work in conjunction to make self-regulating system designs possible. As temperature decreases, both the optimal battery charge regulation voltage and the PV array maximum power voltage both increase. Reference: Photovoltaic Systems, p. 197-199 Charge controllers are rated for certain electrical parameters, that are used to properly select and size the controller for specific system applications. The nominal system voltage is the voltage at which the battery, charge controller and DC loads operate in a PV system. Most charge controllers are designed for operation at a specific voltage, while others may be configured for different standard voltages, such as 12, 24 or 48-volts. For larger systems, higher system (battery) voltages are generally used to lower the peak operating currents, reducing the size and ratings of conductors, overcurrent protection, disconnect means and charge controllers required. MPPT charge controllers will have different voltage ratings for their input and output circuits, and specify the range of array voltages at which it will track maximum power. Charge controllers are also rated for the maximum PV current they can handle. Charge controllers used in PV systems must be listed to UL 1741: Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources. Reference: Photovoltaic Systems, p. 193-194 7-33 7-34
Charge controllers, conductors and overcurrent protection devices must normally be sized for at least 125% of the maximum PV output circuit current. Since the maximum PV output circuit current is 125% of the short-circuit current, the combined factors equal 156%. Reference: Photovoltaic Systems, p. 200-202 Some charge controllers may be listed for continuous operation at 100% of their maximum current, and are permitted to be utilized at 100% of their rating. This requires the controller to be sized for only 125% of the array short-circuit current. Reference: NEC 690.8 7-35 7-36
Reference: Photovoltaic Systems, p. 200-202 7-37