Page 91 - Solar Power in Building Design The Engineer's Complete Design Resource

STORAGE BATTERY TECHNOLOGIES 61

Figure 3.10 Alkaline batteries.

and 260 Ah at the 48-hour rate. The typical efficiency of a lead-acid battery is 85 to
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 sys-
tems are lead-acid type batteries. Even after over a century of use, they still offer the
best price-to-power ratio. It is not recommended, however, to use NiCd batteries in
systems that operate in extremely cold temperatures such as at –50°F or below.
NiCd batteries are expensive to buy and very expensive to dispose off due to the
hazardous nature of cadmium. We have had almost no direct experience with these
(alkaline) batteries, but from what we have learned from others we 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 to 40 percent heat loss just during charging and discharging.
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 con-
struction of the plates and so forth can vary considerably.
Nickel-cadmium, nickel-iron, and other types of batteries are found in some systems
but are not common due to their expense and/or poor efficiency.

MAJOR BATTERY TYPES
Solar power backup batteries are divided into two categories based on what they are used
for and how they are constructed. The major applications where batteries are used as
solar backup include automotive systems, marine systems, and deep-cycle discharge
systems.
The major manufactured processes include flooded or wet construction, gelled, and
AGM types. AGM batteries are also referred to as “starved electrolyte” or “dry” type,
because instead of containing wet sulfuric acid solution, the batteries contain a fiber-
glass mat saturated with sulfuric acid, which has no excess liquid.