Controllers, or charge regulators, prevent excessive overcharge of the batteries within a remote power system. Two different methods of charge control are generally used, Series and Shunt type, though both typically use battery voltage (set point) to determine when charging should be reduced or stopped completely.
Unlike other types of generators, solar modules can be short circuited or open circuited without causing damage to them. In a Series type controller the current flowing into a battery will occasionally be broken by opening the circuit between the array and the battery. In a Shunt type controller, this same array current is directed to a resistor of some type effectively short circuiting the solar modules. We generally do not recommend the use of shunt controllers in our solar installations since partial shading of any cells may cause overheating even if bypass diodes are used.
Series relay controllers
Depending upon the size of the solar array single or multiple relay controllers can be used. These controllers simply open one or more relays depending upon battery voltage to stop or reduce the current flowing to a battery and are suitable for array currents up to 40 - 50 amps. These relays open and close based upon preset high and low voltages. In multiple series controllers one relay may always remain closed to maintain a full charge, by passing some current to the battery via a linear current device or power transistor.
For systems with output currents exceeding 50 amps, controllers should contain more than one (mercury) relay connected to one or more individual controllers offering the benefits of individual or stepped voltage setting and failsafe operation.
Solid state switching controllers (Pulse Width Modulation)
These controllers use MOSFET or power transistors at very high frequencies to pulse the charge current on and off in order to maintain a constant battery voltage. Used in a series or shunt configuration the duration of the cycle will vary depending upon the battery voltage and the available charge current. These controllers offer excellent charging characteristics for solar arrays of up to 40 amps, however, due to their high switching frequency may cause noise on some telecommunications equipment.
The relays used within these controllers offer some limitations which affect performance and useful life. To extend life, relay's on/off range will be widened to reduce their cycling though this results in less efficient battery charging. Where currents are high, these relays may have a bypass circuit to temporarily handle the current during the cycle. Solid state switching devices will cycle almost endlessly without damage to them, however, the resultant voltage drop will lead to some heat generation.
Maximum Power Point Trackers
MPPTs are smart DC to DC converters that optimize the match between the solar array and the battery bank. While gains of 50% in solar module output are possible the typical wattage gain using an MPPT is 10 15% vs. a PWM controller. For example, Shell Solars SP75 is rated at 4.4 amps @ 17 volts that is 4.4 times 17 = 74.8 watts. But this 75 watts does NOT equal 75 watts of charging capacity since your battery is only charging near 13.5 V. The output of a solar module is characterized by a performance curve of voltage versus current known as its I-V curve. For crystalline modules, the current remains fairly constant as the voltage changes relative to the voltage of battery it is charging. A battery charging at 13 V is only using 57.2 watts of power not the full 75 watts a loss of about 24%. In an extreme case, such as a fully discharged battery at 10.5 volts, you would get nearly 7 amps at 10.5 volts from the MPPT into the battery! MPPT's are most effective under these conditions: Cloudy or hazy days - when the extra power is needed the most. Cold weather - solar module output increases in cold temperatures during the winter when sun hours are low and you need the most power. Low battery charge - the lower the state of charge in your battery, the more current a MPPT puts into them - another time when the extra power is needed the most. With higher voltage solar arrays of 300 Wp or more. Below this size, it may be more cost effective to simply add another solar module.
Diode Protection only
Self regulating systems
Solar systems may be operated without a regulator, using only a blocking diode to prevent reverse current flow, under the following conditions:
• The load on the battery coincides with the solar array's output, including, allowances for temperature, throughout the period of use.
• Temperature is relatively constant.
• The battery is at least 30 times greater than the maximum short circuit array current.
• The solar array`s open output voltage is no more than 18.0 V. A blocking diode of sufficient current capacity provides reverse current protection.
Early, small PV systems used Zenor diodes to shunt array current. This means of control is used in limited applications stemming from variations in their performance due to temperature and difficulty matching diodes to the specific voltage required for battery charging.
Choosing a Voltage Regulator or Charge Controller
The choice of a regulator is contingent upon three main factors - charge voltage, current and ambient temperature. Regarding current capability, the controller should be sized 30% higher than the short circuit current of the solar array (ie) If Isc is 4.8A the controller must have rated capacity of 6.24 A.
A number of options are available:
* Adjustable voltage setpoints or voltage by battery type.
* Temperature compensation should be used only unless both the battery and charge controller will remain within a few degrees of the 25 C STC. Typically, the controllers output voltage will vary 5 mV/cell/C.
* Low voltage disconnect (LVD) is used to prevent the battery from excessive discharge via a prefixed or adjustable low voltage set point.
* Auto equalization is available on certain controllers. Set at about 14 V on 12 V nominal systems, this feature injects a higher than normal voltage to the battery to avoid stratification and reduce battery sulfation. Equalization should not be applied to batteries with gelled electrolyte.
* LED indicators for charge and/or load status.
* Digital monitoring to indicate array voltage and charge current, battery voltage and/or load current.
* Remote system monitoring and communications for remote sites.
* Reverse polarity protection.
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The information contained herein is subject to change without notice.