e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, Controllers

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, 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

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, Why have a Discharge Control? To prevent damage to batteries from excessively deep discharge

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, 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

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, 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.

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, 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

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, 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

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, 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

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, 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.

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, Relay type controller High reliability, simple relay type controller designed by S.P.I.R.E. and constructed in Kiribati by the Kiribati Solar Energy Co.

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, 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

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, 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

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, 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

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, 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

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, 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.

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, Batteries for Solar Home Systems

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, Types of Batteries Lead-Acid –Cheapest, mature technology, readily available –Easily damaged by improper discharge control, some types require periodic maintenance Nickel-Cadmium –Expensive, mature technology, not readily available –Long life, minimal maintenance

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, Types of Lead-Acid Batteries Open cell automotive starting battery (NO GOOD) “Maintenance Free” starting battery (NO GOOD) “Solar Modified” automotive battery (OK) Traction battery (powers vehicles) (GOOD) Stationary (intended for back up power) (POOR) Tubular cell deep discharge (EXCELLENT) Valve-Regulated sealed battery (GOOD)

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, General Battery Characteristics Nominal voltage (number of 2V cells) Capacity in Ampere hours Open cell or sealed Liquid or Gel electrolyte Cycle life Acceptable repeated depth of discharge “Starting Amps” “Number of Plates” or “Starting Minutes” not useful for solar specifications

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, Charge/Discharge Rate Rate of charge or discharge is based on the capacity of the battery in Ah and the time taken to bring it to full charge or full discharge Designated as C x where C represents the capacity in Ampere Hours and x represents the number of hours to fully charge or discharge the battery at a fixed number of Amperes. Thus C 10 means that the full capacity of the battery is discharged in 10 hours at a fixed rate.

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, The effect of discharge rates The more hours taken to discharge a battery, the more energy can be transferred because with a slow discharge the chemical process that produces electricity is more efficient. So a battery delivers more Ah at C 100 than at C 10 by a quite significant amount. Note that C 100 means that the battery takes 100 hours to discharge fully while C 10 means it only takes 10 hours to discharge fully. A 100Ah battery at C 100 may become a 65 Ah battery at C 10

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, Battery Ah ratings A 100 Ah battery at C 100 may be a 65 Ah battery at C 10. So to compare batteries, the battery Ah rating must include the C x rate for the stated capacity and you must compare at the same C x rate. Manufacturers of solar batteries, particularly those of questionable quality, often give Ah ratings using a C 100 discharge rate. That gives a substantially inflated Ah value but that capacity is never reached in practice. Always compare battery capacities at the same discharge rate, preferably C 10 or C 20 (C 20 represents the typical solar discharge rate for SHS and is best though C 10 comparisons are commonly done and are ok. Just be sure all comparisons are at the same C x rate.)

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, Cycle Life Cycle life is the number of times that a battery can be expected to be fully discharged then immediately fully charged before losing 20% of its rated capacity. Partial cycles add up to full cycles. For example, five days of 20% discharge/charge add up to be the equivalent of 1 full cycle. So with a 20% average daily discharge, a 300 cycle life battery will work 1,500 days (about 4 years) before the cycle life is exceeded.

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, Measuring the level of charge Battery voltage. About 10.5V represents full discharge. Full charge voltage for a battery with no current flow in or out will be around 12.6V. When charging, full charge is reached at about 14.2V Electrolyte specific gravity. This is an indication of acid concentration and is measured using a hydrometer. The higher the value the greater the charge (1.26 to 1.28 is full charge, 1.0 to 1.1 is about fully discharged). Neither battery voltage nor specific gravity is an accurate measure of charge, especially in old batteries.

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, Accuracy of charge estimation The use of voltage to determine level of charge is not very accurate because the rate of charge or discharge affects the voltage too. As the battery gets older the accuracy of voltage readings as an indicator of state of charge during charging or discharging gets less and less because the battery’s internal resistance goes up. The use of specific gravity is accurate when a battery is new but as the battery ages, the hydrometer tends to show a lower charge than is actually present due to increasing sulfation

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, Causes of Battery Failures Sulfation – Most common problem. Sulfation is where part of the cell becomes resistant to charge. Caused by the battery remaining at partial charge for long periods. May be offset by equalizing charges when cells are seen to have unequal specific gravity. Internal corrosion – results in high internal resistance and open circuits. Caused by cheap design, adding acid instead of water and stratification of the acid in some types of batteries Internal shorts – results in one or more cells not producing voltage. May be due to cheap construction, overheating or mechanical damage Loss of active material from plates – caused by excessive depth of discharge and age. Mostly a problem with cheaper batteries.

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, What is Sulfation? When a battery discharges, Lead Sulfate is created. When the battery is recharged, the Lead Sulfate is supposed to dissolve. But if the Lead Sulfate is not dissolved after a week or so because the battery is not fully charged, it tends to form a mass that is very difficult to dissolve when charging does take place. Over time the amount of Lead Sulfate increases and the battery loses its ability to be charged fully. The effect is a loss of capacity. A 100Ah battery may become a 50Ah battery after serious sulfation has occurred.

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, What are the signs of sulfation? When battery voltage indicates a full charge but the hydrometer reading indicates a partial charge, that is a strong indicator that serious sulfation has occurred in the battery. Lead Sulphate is white in color. When looking at the plates in a battery, if the battery has been fully charged and the plates look light in color or have white sections, that is an indication of sulfation

e8 / PPA Solar PV Design Implementation O&M Marshall Islands March 31-April 11, Failure modes of batteries Total loss of power. Zero volts, cannot charge. Caused by an internal open circuit. This may be because of corrosion eating through a cell connector or mechanical damage Gradually decreasing capacity. The time to charge and discharge gets shorter and shorter. Caused by sulfation or loss of active material from the plates or both. Accelerated by deep discharge conditions and operation at partial charge levels for weeks at a time. Reduced voltage at full charge. Cannot get the battery to charge to more than about 10V. Caused by a short in a cell making one cell inoperative. Excessive discharge and mechanical damage are typically the reasons for this mode of failure.