Civil Engineering Reference
In-Depth Information
cell stores electrochemical energy at low electrical potentials, typically a few
volts. The cell capacity, denoted by C, is measured in Ampere-hours (Ah),
meaning it can deliver C amperes for one hour or C/n amperes for n hours.
The battery is made of numerous electrochemical cells connected in a
series-parallel combination to obtain the desired operating voltage and cur-
rent. The higher the battery voltage, the higher the number of cells required
in series. The battery rating is stated in terms of the average voltage during
discharge and the Ah capacity it can deliver before the voltage drops below
the specified limit. The product of the voltage and the Ah forms the Wh
energy rating it can deliver to a load from the fully-charged condition. The
battery charge and discharge rates are stated in unit of its capacity in Ah.
For example, charging a 100 Ah battery at C/10 rate means charging at 10
A rate. Discharging that battery at C/2 rate means draining 50 A, at which
rate the battery will be fully discharged in 2 hours. The State of Charge (SOC)
of the battery at any time is defined as the following:
Ah
capacity remaining in the battery
Rated Ah capacity
SOC
=
10.2
Types of Batteries
There are at least six major rechargeable electrochemistries available today.
They are as follows:
•
lead-acid (Pb-acid).
•
nickel-cadmium (NiCd).
•
nickel-metal hydride (NiMH).
•
lithium-ion (Li-ion).
•
lithium-polymer (Li-poly).
•
zinc-air.
New electrochemistries are being developed by the United States Advance
Battery Consortium for a variety of applications, such as electric vehicles,
spacecraft, utility load leveling and, of course, for renewable power systems.
1
The average voltage during discharge depends on the electrochemistry, as
listed in
Table 10-1
.
The energy density of various batteries, as measured by
Wh capacity per unit mass and per unit volume, are compared in
Figure 10-2
.
The selection of the electrochemistry for a given application is a matter of
performance and cost optimization.
Some construction and operating features of the above electrochemistries
are presented in the proceeding sections.
