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STORAGE BATTERY TECHNOLOGIES   81




                                   AC




                              TRANSFORMER
                                RECTIFIER


                             +
                                   –

                                   DC
                                                              Figure 3.41   Lead-acid battery
                                                              current flow.


                       Company laboratory in Parma, Ohio. Alkaline batteries are capable of storing higher
                       energy within the same package size than comparable conventional dry batteries.

                       Zinc–mercuric oxide alkaline batteries In 1950, Samuel Ruben invented the
                       zinc–mercuric oxide alkaline battery, which was licensed to the P. R. Mallory Company.
                       The company later became Duracell, International. Mercury compounds have since
                       been eliminated from batteries to protect the environment.
                         Deep-discharge batteries used in solar power backup applications in general have
                       lower charging and discharging rate characteristics and are more efficient. A battery
                       rated 4 Ah over 6 hours might be rated at 220 Ah at the 20-hour rate and 260 Ah at the
                       48-hour rate. The typical efficiency of a lead-acid battery is 85–95 percent, and that of
                       alkaline and nickel-cadmium (NiCd) batteries is about 65 percent.
                         Practically all batteries used in PV systems and in all but the smallest backup
                       systems are lead-acid batteries. Even after over a century of use, they still offer the
                       best price-to-power ratio. Systems that use NiCd batteries are not recommended to use
                       them in extremely cold temperatures below −50°F.
                         NiCd batteries are expensive to buy and very expensive to dispose of owing to
                       the hazardous nature of cadmium. I have had almost no direct experience with these
                       (alkaline) batteries, but from what I have learned from others, I do not recommend
                       them—one major disadvantage is that there is a large voltage difference between the
                       fully charged and discharged states. Another problem is that they are very inefficient—
                       there is a 30–40 percent heat loss just during charging and discharging. Figure 3.42
                       shows various types of alkaline batteries.
                         It is important to note that all batteries commonly used in deep-cycle applications are
                       lead-acid batteries. This includes the standard flooded (wet), gelled, and absorbed glass
                       mat (AGM) batteries. They all use the same chemistry, although the actual construction
                       of the plates and so forth can vary considerably. NiCd, nickel-iron, and other types of
                       batteries are found in some systems but are not common owing to their expense and/or
                       poor efficiency.
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