Page 92 - Solar Power in Building Design The Engineer's Complete Design Resource

62 SOLAR POWER SYSTEM DESIGN CONSIDERATIONS

Common flooded-type batteries are usually equipped with removable caps for
maintenance-free operation. Gelled-type batteries are sealed and equipped with
a small vent valve that maintains a minimal positive pressure. AGM batteries are also
equipped with a sealed regulation-type valve that controls the chamber pressure within
2
4 pounds per square inch (lb/in ).
As described earlier, common automobile batteries are built with electrodes that are
grids of metallic lead containing lead oxides that change in composition during charging
and discharging. The electrolyte is dilute sulfuric acid. Lead-acid batteries, even though
invented nearly a century ago, are still the battery of choice for solar and backup power
systems. With improvements in manufacturing, batteries can last as long as 20 years.
Nickel-cadmium, or alkaline, storage batteries, in which the positive active material
is nickel oxide and the negative material contains cadmium, are generally considered
very hazardous due to the cadmium. The efficiency of alkaline batteries ranges from
65 to 80 percent compared to 85 to 90 percent for lead-acid batteries. Their nonstan-
dard voltage and charging current also make them very difficult to use.
Deep-discharge batteries used in solar power backup applications in general have
lower charging and discharging rate characteristics and are more efficient.
In general, all batteries used in PV systems are lead-acid type batteries. Alkaline-
type batteries are used only in exceptionally low temperature conditions of below
–50°F. Alkaline batteries are expensive to buy and due to the hazardous contents are
very expensive to dispose off.

BATTERY LIFE SPAN
The life span of a battery will vary considerably with how it is used, how it is main-
tained and charged, the temperature, and other factors. In extreme cases, it can be dam-
aged within 10 to 12 months of use when overcharged. On the other hand if the battery
is maintained properly, the life span could be extended over 25 years. Another factor
that can shorten the life expectancy by a significant amount is when the batteries are
stored uncharged in a hot storage area. Even dry charged batteries when sitting on a
shelf have a maximum life span of about 18 months; as a result most are shipped from
the factory with damp plates. As a rule, deep-cycle batteries can be used to start and run
marine engines. In general, when starting, engines require a very large inrush of cur-
rent for a very short time. Regular automotive starting batteries have a large number of
thin plates for maximum surface area. The plates, as described earlier, are constructed
from impregnated lead paste grids similar in appearance to a very fine foam sponge.
This gives a very large surface area, and when deep-cycled, the grid plates quickly
become consumed and fall to the bottom of the cells in the form of sediment. If auto-
motive batteries are deep-cycled, they will generally fail after 30 to 150 deep cycles,
whereas they may last for thousands of cycles in normal starting use discharge condi-
tions. Deep-cycle batteries are designed to be discharged down time after time and are
designed with thicker plates.
The major difference between a true deep-cycle battery and regular batteries is that
the plates in a deep-cycle battery are made from solid lead plates and are not impreg-
nated with lead oxide paste. Figure 3.11 shows a typical solar battery bank system.