2. Chapter 7 Charge Controllers
Charge Controller Features • Charge Controller Types • Charge Controller Setpoints • Charge Controller Installation

3. Charge controllers manage interactions and energy flows between a PV array, battery bank, and electrical load.
Charge controllers orchestrate the interaction between major PV system components. They manage energy flow between the array, battery bank, and electrical loads. See Figure 7-1. After system sizing, charge controllers have the greatest impact on overall system performance. Some charge controllers also include load control and other energy management features, and may indicate system status through LEDs or LCD displays.

4. Single-stage battery charging is simpler, but multistage battery charging brings batteries to a higher state of charge more efficiently.
Charge controller designs use various methods to regulate current and voltage during battery charging. Single-stage charging processes are simpler, but relatively inefficient. Multistage charging changes the applied voltage and/or current in steps to bring the battery to a higher state of charge. This process is more sophisticated, requiring additional electronics, but it greatly improves charge acceptance. See Figure 7-2. Multistage methods can charge batteries faster and limit excessive overcharge and gassing.

5. Charge controllers protect batteries from overcharge by terminating or limiting charging current as they approach a full state of charge.
During high insolation periods, energy generated by an array may exceed the electrical load demand. The excess may charge a battery, but if it is already at a high state of charge, then the charging current must be limited to avoid overcharging. In this case, overcharge is the condition of a fully charged battery continuing to receive a significant charging current. A charge controller protects a battery from overcharge through charge regulation. This involves either interrupting or limiting the current flow from the array when the battery approaches a full state of charge. See Figure 7-3.

6. Charge controllers protect batteries from overdischarge by disconnecting loads at low battery voltage, which indicates a low state of charge.
A charge controller protects a battery from overdischarge through load control, disconnecting electrical loads when the battery reaches a low voltage (low state of charge) condition. See Figure 7-4. Overdischarge protection limits battery depth of discharge, which prevents damage to the battery. For example, under cold conditions, overdischarge protection prevents a battery from freezing. Since some loads will operate improperly, or not at all, at lower than normal voltages, this feature also protects them. Once the battery is charged to a certain level, the loads are reconnected to the battery.

7. Many charge controllers include displays or LEDs to indicate battery voltage, charging current, state of charge, and/or present operating mode.
Most charge controllers provide information on the operational state of the system. For charge regulation, this may include digital displays or LED lights to indicate when the battery is charging and when it reaches full charge. Other indicators may be used to alert operators of a low battery voltage or low state of charge condition, such as a red light or an audible alarm. See Figure 7-5. More advanced controllers may include meters to monitor battery and array currents and voltages or track the amount of energy supplied to and from a battery. This system status information can be extremely useful for improving load management and optimizing system performance.

8. Shunt charge controllers regulate charging current by short-circuiting the array.
Unlike batteries, PV devices are current-limited by nature, so PV modules and arrays can be short-circuited without any harm. A shunt charge controller is a charge controller that limits charging current to a battery system by short-circuiting the array. See Figure 7-6. The array is short-circuited through a shunt element inside the charge controller, moving the array’s operating point on the I-V curve very near the short-circuit condition and limiting the power output. All shunt controllers must also include a blocking diode in series between the battery and the shunt element to prevent the battery from short-circuiting.

9. Series charge controllers regulate charging current by opening the circuit from the array.
A series charge controller is a charge controller that limits charging current to a battery system by open-circuiting the array. See Figure 7-7. As the battery reaches full state of charge, a switching element inside the controller opens, moving the array’s operating point on the I-V curve to the open-circuit condition and limiting the current output. This method works in series between the array and battery, rather than in parallel as for the shunt controller.

10. Pulse-width modulation (PWM) simulates a lower average current level by pulsing a higher current level ON and OFF for short intervals.
A series-interrupting, pulse-width-modulated (PWM) charge controller is a charge controller that simulates a variable charging current by switching a series element ON and OFF at high frequency and for variable lengths of time. A PWM charge controller pulses the full charging current and varies the width of the pulses over time to regulate the amount of charge current flowing into the battery. See Figure 7-8. The frequency of the pulses is several hundred hertz and the pulses may last only a couple of milliseconds. When the battery is partially charged, the current pulse is essentially ON all the time. To simulate a lower charging current as the voltage rises, the pulse width is decreased. For example, if the pulses switch the full charging current so that it is ON half the time and OFF half the time, the resulting current effectively simulates a charge current at 50% of the full current.

11. Maximum power point tracking manipulates the load or output voltage of an array in order to maintain operation at or near the maximum power point under changing temperature and irradiance conditions.
Newer charge controllers incorporate the latest in power electronics and microprocessor controls to optimize system performance and allow greater flexibility in system design. A maximum power point tracking (MPPT) charge controller is a charge controller that operates the array at its maximum power point under a range of operating conditions, as well as regulates battery charging. MPPT charge controllers are also known as buck/boost charge controllers. An MPPT circuit monitors array output and dynamically changes its resistance or input voltage to move the operating point on the array’s I-V curve toward the maximum power point. See Figure 7-9. The power is then transformed by a DC-DC converter circuit into another voltage and current required by the load, in this case a battery. Due to the different input and output circuit characteristics, the NECâ requirements for the circuits differ.

12. Diversionary charge controllers regulate charging current by diverting excess power to an auxiliary load when batteries are fully charged.
Since stand-alone PV systems are designed to supply power to the loads and charge the battery system under worst-case conditions, they often have excess energy available during periods of high insolation or low load. Conventional charge controllers waste this excess power by disconnecting or limiting array current. A diversionary charge controller is a charge controller that regulates charging current to a battery system by diverting excess power to an auxiliary load. Instead of wasting excess power, the auxiliary load provides a useful output with the diverted power. See Figure 7-10.

13. Controllers designed for hybrid PV systems must manage multiple current sources simultaneously.
Since the PV array in hybrid systems is not sized to meet all the loads by itself, the auxiliary energy sources are required to provide supplemental energy for loads and battery charging. A hybrid system controller is a controller with advanced features for managing multiple energy sources. See Figure In many cases, hybrid system control functions are integrated into multifunction PCUs.

14. Ampere-hour charge controllers track the cumulative number of ampere-hours applied to a battery bank and discontinue charging at a preset total.
Charge controllers can also regulate charge current based on parameters other than battery voltage. An ampere-hour integrating charge controller is a charge controller that counts the total amount of charge (in ampere-hours) into and out of a battery and regulates charging current based on a preset amount of overcharge. See Figure For example, a lighting load may discharge 72 Ah from the battery each night. To charge the battery, an ampere-hour charge controller set for 115% overcharge would allow 83 Ah (72 Ah × 115% = 83 Ah) of charge to flow back into the battery before terminating charging.

15. Charge regulation setpoints are the voltage levels at which the charge controller interrupts or reconnects the charging current from the array to the battery bank.
Optimal charge regulation setpoints ensure that the battery is maintained at the highest possible state of charge without overcharging and over a range of conditions. At least two charge regulation setpoints are needed in order to regulate the charging function. A higher voltage setpoint is used to disconnect the array from charging the battery and a lower voltage setpoint is used to reconnect the array and resume battery charging. See Figure 7-13.

16. The optimal voltage regulation setpoint depends on the types of battery and charge controller.
The optimal VR setpoint depends on several factors, especially the controller algorithm, temperature, and type of battery. See Figure VR setpoints for PWM and linear-type charge controllers are generally lower than those for interrupting controllers because they are more efficient at charging.

17. Load control setpoints are the voltage levels at which the charge controller disconnects or reconnects the discharging current from the battery bank to the loads.
Load control protects batteries from overdischarge by turning the load circuits ON and OFF as needed. See Figure Load control features are common on charge controllers, or may be added as a separate controller.

18. The equalization setpoint brings the battery voltage to a level that is higher than the normal charge regulation voltage.
Equalization is a controlled overcharge for a few hours, which is only performed on open-vent batteries. Charge controllers may provide an automated or user-activated equalization charge cycle. During an equalization cycle, the VR setpoint is increased to a higher level, where it remains for a predetermined amount of time before returning to its normal level or the slightly lower float voltage. See Figure Controllers that rely on only an array for the equalization charge may require several days or weeks to complete the prescribed time at the equalization voltage, especially during periods of high load or low insolation. With oversized arrays or generator-powered chargers, equalization can be accomplished in shorter periods.

19. Setpoints are adjusted with potentiometers or switches inside the charge controller.
Setpoints may be adjusted with potentiometers that allow a range of settings, or with DIP switches or jumpers for discrete setpoint increments or battery types. See Figure For microprocessor-based charge controllers, including those integrated with inverters, setpoint adjustments are made through software programming.

20. Temperature probes are placed between batteries and connect to a charge controller to compensate the charge regulation setpoint.
Charge controllers with temperature compensation use a sensor to measure temperature. For small systems and controls located in the same thermal environment, temperature sensing within the charge controller is generally satisfactory for approximating battery temperature. For larger systems or systems with batteries located in a different thermal environment than the controller, external temperature probes should be used. See Figure Temperature probes must be securely attached to a battery case to ensure the most accurate battery temperature measurement. If temperature probes become detached from a charge controller, the controller should regulate at the nominal setpoints.

21. Many criteria should be considered when selecting charge controllers for PV systems.
Charge controllers used in PV systems must be properly specified, configured, and installed based on the application requirements. They must have appropriate current and voltage ratings, and must be compatible with other equipment in the system. The selection and sizing of charge controllers in PV systems involves consideration of many factors. See Figure 7-19.

22. Long conductors between a charge controller and a battery bank have resistance that causes voltage drops. Voltage drops affect the voltage measured at the charge controller, which triggers overcharge and overdischarge protections too early.
Installing charge controllers close to batteries also minimizes voltage drop. As charging current increases on the conductors between the charge controller and battery terminals, the voltage drop increases. Since many charge controllers sense battery voltage with the conductors used to deliver charging current, the measured voltage is slightly higher than the actual battery voltage. See Figure This prematurely activates charge regulation, causing the battery to be undercharged.

23. Some charge controllers use additional battery voltage sense conductors to avoid the effect of voltage drop on setpoints.
Some controllers, particularly ones designed for higher currents, include additional terminals for conductors to sense battery voltage. See Figure This is called a four-wire or Kelvin measurement. No current flows in these conductors, so there is no voltage drop. Therefore, a precise battery voltage is measured and setpoints are activated at the correct voltages. When a charge controller does not have separate voltage sense leads, voltage drop can be minimized by using oversized conductors and locating the controller close to the batteries.

24. Larger PV systems often use independent charge controllers for each array source circuit.
For large arrays with multiple subarray source circuits, multiple independent charge controllers can be used instead of a single, larger controller. The array charging current is limited in a stepped manner as each controller begins to regulate each subarray. See Figure This configuration improves overall system reliability in the event of the failure of any individual controller. This design can also facilitate later system expansion by allowing for the addition of new array source circuits and charge controllers. As long as the charge rate for that circuit is limited to no more than 3% of the battery bank capacity, one source circuit may be left unregulated for float charging.

25. Separate charge controllers are usually recommended for charging independent battery banks from a single array.
Separate charge controllers are the best method for charging multiple battery banks from a single array. Each controller must be rated to handle the entire array current, and blocking diodes are required between each charge controller and the array to prevent the battery banks from operating at the same voltage. See Figure In this configuration, each charge controller can be set for the specific battery types and charge each battery bank appropriately. Since the battery banks are isolated by the blocking diodes and independent charge controllers, system loads may be connected to either battery bank, but not both.

26. In some cases, a single charge controller may be used to charge independent battery banks.
Some charge controllers can be used alone to charge multiple battery banks if the voltages are sufficiently close, though this is not recommended by most charge controller manufacturers. In this configuration, a resistor is added between the array and battery positive terminals on the charge controller and blocking diodes are added between the positive battery terminal of the charge controller and the positive terminal of each battery. See Figure This allows the charge controller to distribute the appropriate charging current to each battery bank. The batteries are also isolated from one another in the controller output circuit, instead of on the array input side when using separate charge controllers.

27. To balance the charge to and from the battery in a self-regulating PV system, the load must be well defined, the battery must be oversized, and the array current must be self-limiting.
A charge controller is required in nearly all PV systems using batteries. However, some small systems can be specially designed to operate without a charge controller. A self-regulating PV system is a type of stand-alone PV system that uses no active control systems to protect the battery, except through careful design and component sizing. See Figure When maintenance is infrequent or prohibitively expensive, such as for remote applications, eliminating the need for a sensitive electronic charge controller can simplify the system design, lower costs, and improve reliability.

28. Low-voltage modules control current into a battery bank without a charge controller because their current automatically falls as the battery reaches full charge.
Low-voltage modules include 29 or 30 cells in series and have a maximum power voltage of about 15 V. At typical operating temperatures, the maximum power point falls within the range of typical battery voltages. As a battery approaches full state of charge and the module voltage passes the maximum power point, the module current falls sharply as the voltage increases further. See Figure 7-26.