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What We Know About Them How to Use Them
of the high charge or discharge currents acting through the internal resistance of the battery which also changes as the battery state-of-charge changes.
Batteries are designed to operate most efficiently at 80°F. If the average temperature is higher, they can deliver higher than rated capacities, but cycle life is shortened. If the average temperature is lower, the capacity will be lower and the life a little longer. Cold batteries in PV systems are like cold batteries in cars. Significant amounts of energy are required to warm the battery to a normal operating temperature, and not much of the early charging current ends up as stored electrons. Temperature compensated charge controllers address this cold-weather problem to some extent by increasing the cut-off set points as the temperature decreases, but they do not change the cold temperature chemical reactions nor eliminate the problem entirely.
While amp-hour efficiencies can reach 90% under ideal conditions, amp-hours cannot do work. Only energy measured in watt-hours can do work, and at the very best, batteries can return only 75% of the energy stored in them. To achieve even these efficiencies, batteries must be subject to ideal conditions. They must be at 80°F. They must be charged and discharged very slowly. They must be charged and discharged by smooth DC currents. The electrolyte must be uniform throughout the battery, and the battery must not be allowed to sulfate.
When the cell voltage rises above 2.38 volts per cell (14.3 volts on a 12-volt battery or 28.6 volts on a 24-volt battery), the electrolyte begins to gas. At this point, charging efficiency goes down since more and more electrons are used to produce gas than are used to charge the battery. Gassing, however, is needed to stir the electrolyte to keep it from stratifying. If the electrolyte stratifies, then the solution in the battery has a higher concentration of sulfuric acid near the bottom of the cells and a weaker concentration of sulfuric acid near the tops of the cells. When this condition exists, the bottom of the cells are more active in the charge- discharge process and the active material (lead) is consumed at a higher rate. This higher than normal consumption of active material results in loss of battery cycle life and eventual loss of battery capacity. Therefore, although gassing is not an efficient use of PV energy to charge a battery, it is a necessary function. Some large industrial batteries have provisions for mechanical or air-bubble stirring and therefore do not need to be gassed.
If batteries are not fully charged on a regular basis, the lead-sulfate crystals that are part of the normal charge-
©1997 John Wiles
Batteries represent a significant initial and continuing cost in a renewable energy (RE) system. The RE owner is sometimes in a quandary when it comes to taking care of the batteries and getting the most bang (amp-hour) for the buck.
There is much information available on battery usage, but this information represents battery use in uninteruptable power systems, stationary float applications, and other systems not related to renewable energy. There are only a few full-fledged, well-documented, RE battery test programs that have been reported in the technical literature. Few PV battery systems have been accurately monitored using data acquisition systems. Nearly every battery “expert” will have different ideas. In most cases, these ideas come from observing different systems in different modes of operation, few of them relate to the small-to-medium sized renewable energy system using flooded lead-acid batteries. The ideas presented are not necessarily mine and have been gathered from a number of sources.
Batteries can be charged and discharged over a wide range of voltages, currents, and temperatures. However, maximum capacity, cycle life, and efficiencies will be strongly affected by how the battery is operated. The following general considerations apply only to flooded lead-acid batteries such as golf-cart and fork-lift batteries and other deep-cycle batteries like the L-16 type. These comments do not apply to any type of maintenance-free batteries, sealed batteries, nickel- cadmium, or nickel-iron batteries.
High charging currents from a large PV array or a battery charger connected to a generator can give rise to battery voltages that are very high (2.5–2.7 volts per cell: 15–16 volts [12-volt system], 30-32 volts [24-volt system]) even when the battery is almost fully discharged. In a similar manner, high discharge currents can show low battery voltages when the battery is near full charge. These voltages are the result
66 Home Power #58 • April / May 1997
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