Civil Engineering Reference
In-Depth Information
and then charged, the lithium is stripped away, a free metal surface is
exposed to the electrolyte and a new film is formed. To compensate, the cell
uses thick electrodes, adding into the cost. Or else, the life would be short-
ened. For this reason, it is more expensive than NiCd.
In operation, the lithium-ion electrochemistry is vulnerable to damage
from overcharging or other shortcomings in the battery management. There-
fore, it requires more elaborate charging circuitry with adequate protection
against overcharging.
10.2.5
Lithium-Polymer
This is a lithium battery with solid polymer electrolytes. It is constructed
with a film of metallic lithium bonded to a thin layer of solid polymer
electrolyte. The solid polymer enhances the cell's specific energy by acting
as both the electrolyte and the separator. Moreover, the metal in solid elec-
trolyte reacts less than it does with liquid electrolyte.
10.2.6
Zinc-Air
The zinc-air battery has a zinc negative electrode, a potassium hydroxide
electrolyte, and a carbon positive electrode, which is exposed to the air.
During discharge, oxygen from the air is reduced at the carbon electrode
(the so-called air cathode), and the zinc electrode is oxidized. During dis-
charge, it absorbs oxygen from the air and converts into oxygen ions for
transport to the zinc anode. During charge, it evolves oxygen. A good air
management is essential for the performance of the zinc-air battery.
10.3
Equivalent Electrical Circuit
For steady-state electrical performance calculations, the battery is repre-
sented by an equivalent circuit shown in
Figure 10-3
.
In its simplest form,
the battery works as a constant voltage source with small internal resistance.
The open-circuit (or electrochemical) voltage E
of the battery decreases
i
, and the internal resistance R
increases
linearly with the Ah of discharge Q
d
i
linearly with Q
. That is, the battery open-circuit voltage is lower and the
internal resistance is higher in a partially discharged state as compared to
the E
d
and R
values in the fully charged state. Quantitatively,
o
o
EEKQ
=−
i
0
1
d
(10-1)
RRKQ
=+
i
0
2
d
where K
and K
are constants to be found by curve-fitting the test data.
1
2
