Page 107 - Alternative Energy Systems in Building Design

STORAGE BATTERY TECHNOLOGIES 83

batteries used in PV systems are lead-acid batteries. Alkaline-type batteries are used only
in exceptionally low-temperature conditions of below −50°F. Alkaline batteries are
expensive to buy and, because of their hazardous contents, very expensive to dispose of.

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 10–12 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 storage uncharged in a
hot area. Even dry charged batteries have a maximum shelf life 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 current for a very short time. Regular automotive 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. Automotive batteries generally will fail after 30–150 deep cycles if deep-
cycled, whereas they may last for thousands of cycles in normal starting conditions.
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 a regular battery is that
the plates in a deep-cycle battery are made from solid lead and are not impregnated
with lead oxide paste. Figure 3.43 shows a typical solar battery bank system.
Stored energy in batteries in general is discharged rapidly. For example, short bursts
of power are needed when starting an automobile on a cold morning, which results
in high amounts of current being rushed from the battery to the starter. The standard
unit for energy or work is the joule (J), which is defined as 1 Ws of mechanical work
performed by a force of 1 N or 0.227 lb pushing or moving a distance of 1 m. Since
1 hour has 3600 seconds, 1 Wh is equal to 3600 J. The stored energy in batteries is
measured in either milliampere-hours (mAh) if small or ampere-hours (Ah) if large.
Battery ratings are converted to energy if their average voltages are known during
discharge. In other words, the average voltage of the battery is maintained relatively
unchanged during the discharge cycle. The value in joules also can be converted into
various other energy values as follows:

Joules divided by 3,600,000 yields kilowatthours.

Joules divided by 1.356 yields English units of energy foot pounds.
Joules divided by 1055 yields British thermal units.
Joules divided by 4184 yields calories.