 | Renewable Energy Systems |
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Batteries - The Heart of a stand alone PV or Hybrid SystemBatteries act as a central reservoir into which electricity is placed and removed. They perform two main functions in a renewable energy system. Batteries store electricity for delivery over an extended time period and provide the ability to deliver large amounts of current for relatively short time periods.
Deep cycle batteries are designed with the intention of having a high percentage of their stored electricity removed on a regular basis and are the most logical choice for renewable power storage. Conversely, shallow cycle batteries are engineered to provide infrequent stand-by service, with only 15-20% of their stored power intended for use. Examples of deep cycle service are golf cart and fork lifts while shallow cycle examples would include hospital and telephone back-up batteries. Battery capacity is expressed in amp-hours at a given discharge rate (X A/hr@Y hour rate) to full discharge. For example, a battery states that it has a capacity of 788 amp hours at the 72 hour rate. Another way to express this would be to say that this battery would be fully depleted if an 11 amp load was continuously connected for 72 hours. Rated battery capacity divided by the hour rate determines the size of load. The number of times a battery can be cycled before it is worn out increases significantly as the percentage depth of discharge (DOD) declines. A battery which is required to deliver 50% of its power will provide up to 30% more cycles than when it is drained to 80% DOD. Cycling - Deep Vs. ShallowA cycle, as defined in the battery world, occurs when you completely discharge a battery and then recharge it to back the same level. The battery is designed to absorb and give up electricity by a reversible electrochemical reaction. How deep a battery is discharged is termed “depth of discharge.” A shallow cycle occurs when the top 20% of less of the battery’s power is discharged and then recharged. Some batteries, like automotive starting batteries, are designed for this type of cycling only. The plates of active material are thin with large overall surface area. This design can give up a large amount of power in very short time. The second type of battery is a deep cycle where up to 80% of the battery capacity is discharged and recharged. Batteries designed for deep cycling are built with thicker plates of active material which have less overall surface area. Because of the decreased availability of surface area for chemical reaction, these batteries yield just as much power relative to there size. This type of battery design is preferred for a PV system because discharging a battery to a deeper level is normal during extended cloudy weather. The depth of cycling has a good deal to do with determining a battery’s useful life. Even batteries that are designed for deep cycling are “used up” faster as the depth of discharge is increased. Temperature Effects The speed of the chemical reaction occurring in a lead-acid battery is determined by temperature. For example, the colder the ambient temperature, the slower the reaction. The warmer the temperature the faster the reaction and the more quickly the charge can be drawn from the battery. The optimum operating temperature for a lead-acid battery is around 77 degrees Fahrenheit. You may have experienced this effect when starting a car on a cold morning; the engine just doesn’t turn over as quickly. Warming that same battery will result in a major improvement in performance. For this reason we like to see batteries placed indoors or in a heated and ventilated space at a maintained temperature between 55 and 80 degrees. If your batteries are installed in an unheated space, the capacity must be increased to compensate for this de-rating. High temperatures can drastically shorten the life of the battery and should be avoided. |
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There are three major types of lead acid batteries: deep cycle, shallow cycle and SLI (starting lighting and ignition). SLI batteries are meant for automotive use and are almost universally unsuitable for solar electric appliances.