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The main function of a controller or regulator is to fully charge a battery without permitting overcharge while preventing reverse current flow at night. If a non-self-regulating solar array is connected to lead acid batteries with no overcharge protection, battery life will be compromised. Simple controllers contain a transistor that shunts the PV charging circuit, terminating the charge at a pre-set high voltage and, once a pre-set reconnect is reached, opens the shunt, allowing charging to resume. More sophisticated controllers utilize pulse width modulation (PWM) or maximum power point tracking (MPPT) to assure the battery is being fully charged. The first 70 percent to 80 percent of battery capacity is easily replaced, but the last 20 percent to 30 percent requires more attention and therefore more complexity.
How controllers work and available options
The circuitry in a controller reads the voltage of the batteries to determine the state of charge. Designs and circuits vary, but most controllers read voltage to control the amount of current flowing into the battery as the battery nears full charge. Features of a controller to consider include:
• Reverse current leakage protection - by disconnecting the array or using a blocking diode to prevent current loss into the solar modules at night.
• Low-voltage load disconnect (LVD) - to reduce damage to batteries by avoiding deep discharge.
• System monitoring - analog or digital meters, indicator lights and/or warning alarms.
• Over current protection - with fuses and/or circuit breakers.
• Mounting options - flush mounting, wall mounting, indoor or outdoor enclosures.
• System control - control of other components in the system; standby generator or auxiliary charging system, diverting array power once batteries are charged, transfer to secondary batteries.
• Load control - automatic control of secondary loads, or control of lights, water pumps or other loads with timers or switches.
• Temperature compensation - utilized whenever batteries are placed in a non-climate controlled space. The charging voltage is adjusted to the temperature.
• Pulse Width Modulation (PWM) - an efficient charging method that maintains a battery at its maximum state of charge and minimizes sulfation build-up by pulsing the battery voltage at a high frequency.
• Maximum Power Point Tracking (MPPT) - a new charging method designed to extract the most power possible out of a solar module by altering its operating voltage to maximize the power output.
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