Page 93 - Solar Power in Building Design The Engineer's Complete Design Resource

STORAGE BATTERY TECHNOLOGIES 63

Figure 3.11 Deep-cycle battery bank system. Courtesy of Solar Integrated
Technologies.

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 watt-second of mechanical
work performed by a force of 1 newton (N) or 0.227 pound (lb) pushing or moving a
distance of 1 meter (m). Since 1 hour has 3600 seconds, 1 watt-hour (Wh) is equal to
3600 J. The stored energy in batteries is either measured in milliampere-hours if small
or ampere-hours if large. Battery ratings are converted to energy if their average volt-
ages 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 can
also be converted into various other energy values as follows:

Joules divided by 3,600,000 yields kilowatt-hours.
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.

BATTERY POWER OUTPUT
In each instance when power is discharged from a battery, the battery’s energy is
drained. The total quantity of energy drained equals the amount of power multiplied