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6-2. Controllers. Why have a Charge Controller. Open cell batteries can lose water quickly if overcharged. If electrolyte falls below the top of the plates damage occurs. Sealed batteries may be ruined if frequently overcharged. Why have a Discharge Control?.
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Why have a Charge Controller • Open cell batteries can lose water quickly if overcharged. If electrolyte falls below the top of the plates damage occurs. • Sealed batteries may be ruined if frequently overcharged
Why have a Discharge Control? • To prevent damage to batteries from excessively deep discharge
Most Important Controller Characteristic • Assuming the controller turns the charging and discharging on and off at the right voltages, by far the most important characteristic of a controller is reliability. When a controller fails completely it may be days or weeks before the problem is reported and then weeks or months before a replacement is installed. The battery is usually damaged as a result. More subtle controller failures (changes in set points, high internal losses, etc.) are difficult to troubleshoot and may not be found at all. Deterioration of the battery is usually rapid and battery failure is what shows that the controller is not working correctly. • A highly reliable controller that does the basic on-off switching of charging and discharging currents with reasonable accuracy is much better than a controller that is less reliable but has highly accurate voltage settings and uses more effective charging methods such as tapered charging or PWM charging. For open cell batteries that can be a relay type controller. For sealed batteries, a well designed and accurate voltage semiconductor controller with excellent lightning protection is best.
Shunt (Parallel) Controller Placed between the panels and the battery To prevent overcharge, the output from the panel is shorted by the controller using a semiconductor switch Because the panel wires go to the battery, a blocking diode has to be installed or shorting the panel output would also short the battery
Advantages of the Shunt Controller • Simple and cheap. Lends itself to local production • Less likely to be damaged by excess current flow than series switching control.
Disadvantages of Shunt Control • All the panel power is shorted through the controller and converted to heat. So large panels generate a great deal of heat and it is a problem to get rid of it. Shunt controls are therefore best suited to small (under 50Wp) PV systems. • Sensitive to lightning • Requires a blocking diode with its attendant power loss
Series (switching) Charge Control • Placed between the panels and the battery • A transistor switch or a relay is used to disconnect the panel from the battery so overcharging cannot occur
Advantages of Relay Type Switching Controller • Relay switching is very resistant to lightning strike damage • Very low voltage drop and internal losses • There is no heat dissipation problem so it can be used with any size of PV system without problem. • Typically has lower power losses than the shunt controller since there is no blocking diode • Simple circuitry that can usually be repaired locally • No microchips required so less vulnerable to moisture and corrosion • No high frequency circuitry so less vulnerable to dirt and salt
Disadvantages of relay type controllers • Usually more expensive than semiconductor controllers • Difficult to incorporate other than on/off type of switching. Semiconductor switches can do high frequency pulse charging, tapered charging and other more sophisticated charging methods. • May draw more current for its internal operation than semiconductor circuits though good designs minimize this potential problem.
Relay type controller High reliability relay type controller circuitry designed by S.P.I.R.E. and constructed in Kiribati by the Kiribati Solar Energy Co.
Disadvantages of Semiconductor type Switching Controllers • Semiconductor switching type of controller is sensitive to lightning damage (not the case for relay types) • Easy to damage by excess current flow through the semiconductor controller (not true for relay types) • Higher internal voltage drop than relay controllers • In the Pacific, the experience has been that the operating life of relay controllers is around twice that of all but the best semiconductor controllers
Pulse Width Modulation (PWM) • In charging a battery, the charging current and rate of charge has a lot to do with how quickly and efficiently charging can be accomplished. By charging using pulses of energy, high current can be used without overheating the battery. The rate of the charging can be adjusted by changing the ratio between the amount of time the pulse occurs and the amount of time between pulses. Pulse Width Modulation or PWM is a charging approach that can be used by semiconductor controllers to “punch” in the charging at a fairly high current but to avoid the high heat that is generated from continuous charging. The charge rate is adjusted by changing the width of the pulses. Wide pulses with little time between them provides high charging rate. Narrow pulses with much time between them a low rate.
Changing PWM charging rate Red area represents energy going to battery for charging
SHS Semiconductor Controller • ProStar controller • 12/20/40 Amperes • Up to 48V battery • Lightning protection included • “On-Off” type controller
Large semiconductor Controller • Morningstar TS style semiconductor controller • Remote data port • Manually controlled equalization • 12, 24 or 48V operation • Up to 60A • Both PWM and “on-off” charge control available
Discharge controller • Placed between the battery and the loads • All are of the series switching type. Often use a relay even if the charge controller uses semiconductors but many use a semiconductor switch
Maximum Power Point Tracking Controller (MPPT controller) • A complex but effective electronic controller that always takes energy from the panels at its maximum power point increasing energy delivery for charging a battery by 10-15% or more over standard controllers plus allowing higher voltage strings of panels for lowered wiring losses. • Cost effective for battery charging only on large installations (typically 1kWp and larger). • Often used for direct connection of pumps to solar panels thereby avoiding batteries while still maintaining efficient use of the solar energy • Difficult to troubleshoot and usually impossible to repair locally.
Sample MPPT controller • Outback MX-60 MPPT controller • Includes data logger and remote monitoring • Programmable equalization • Input voltage up to 140 DC to charge a battery ranging from 12V-60V • Claims up to 30% more energy than non MPPT controllers but more likely 10%-15% if panel voltage is matched to that of batteries.
Charging methods • On/Off charging. Panel power turned off when full charge is reached. • Controlled “taper” charge. Amperes are gradually cut back by the controller when full charge is approached and a trickle charge current always flows while the sun is shining. • Pulse charging. Requires semiconductor switch. • Maximum power point tracking controller electronically channels the maximum energy to the battery under changing conditions. Only semiconductor type can do this. Usually only used for high Wp installations.
Programmable controllers • Using a small micro-processor, the controller can be programmed to match the needs of different types of batteries, panels and loads in order to optimize charging conditions. • MPPT controllers are usually programmable • Can be complicated. Tends to be sensitive to heat and salt air. • Sensitive to nearby lightning strikes unless special lightning protection is included in the controller
Charge Set Point • For SHS, controllers depend on battery voltage to determine whether or not full charge (or maximum discharge) has been reached. • Usually a specific voltage is chosen as the cut off point for charging. For open cell batteries it is typically 14.4 volts. For sealed batteries typically 13.8 volts. This is the charge cutoff “set point”
Discharge Control Set Point • A discharge control set-point voltage is established. When the battery voltage falls to the set-point, usually called the LVD (Low Voltage Disconnect) the discharge controller disconnects the load. According to the type of battery used and the system design, that set point may vary from essentially full discharge to 50% discharge or higher for the disconnect.
Reset conditions • After cutting off the charging, charge controllers usually do not re-establish full charging flow until the battery voltage falls one volt or more. Necessary because the battery voltage falls as soon as the charging current is stopped. Without the “dead band” between charge cutoff voltage and reset voltage, when the charge cut off the voltage would drop and turn charging on. Then the voltage would rise above the charge cutoff voltage causing the controller to oscillate between on and off. • After cutting off the load, discharge controllers usually do not re-connect the load until the battery has recharged somewhat as determined by a rise in battery voltage of one to two volts.
Equalizing Charge • When cells in a battery have significant differences in specific gravity, the implication is that sulfation is progressing. To reverse the early effects of sulfation, a controlled overcharge can be applied that will bring all the cells back into full service causing their SG to become about equal. This is an equalizing charge. • Programmable controllers usually can be set to automatically do an equalizing charge at given times or under specific sets of conditions (e.g. an LVD has occurred) • Manual equalization can be done by connecting panels directly to the battery and not connecting a load until the battery voltage equals or exceeds 2.6V per cell. Water level in the cells should be checked each day and lost water replaced but never overfilling a cell.
Causes of Controller Failure • Nearby lightning strikes. The controller switch is directly in the path of induced voltages in the long wire connecting the panel and the battery. • High temperatures caused by blocking the ventilation around the controller or placing it in a hot location • Damage by technicians and users • Insect damage • Corrosion of circuit boards due to poor design associated with salt exposure, high humidity and high temperatures
Choosing a controller • Reliability is the result of a simple design • Avoid complex extra “features” such as many indicator lights, LCD screens, micro-processor controls • Choose a control that fits the type of battery being used especially if it is for a sealed battery since they require specific controller features for maximum life. • Be sure there is adequate current capacity for both charging and for operating the loads • Low power loss with small internal voltage drop and low internal energy use. • Housing prevents insect entry and water entry • Good lightning protection • Proper voltage of operation
Important Installation Note • There should NEVER be more than 2 meters (6 feet) of wire connecting the controller to the battery and if possible it should be shorter than that. The wire should be the same size as the panel wire or larger. • Longer wires between the battery and controller result in inaccurate sensing of battery voltage by the controller and improper operation.
Integrated controllers • Some solar refrigerators and some inverters designed for solar use have integrated controllers and a separate controller need not be installed if the refrigerator or inverter having an integrated controller is the only thing connected to the panels and battery. • It usually is not a technical problem to include a separate controller even if the refrigerator or inverter has its own controller too though of course that inreases the cost and may add some more internal loss to the solar system.
