Environmental Engineering Reference
In-Depth Information
Table 10.8 Component parameters
Ultra-capacitor
Lithium ion
Product
BCAP1500-P270
TS-LCP200AHA
Manufacturer
Maxwell Technologies
GBS
Ratings
1,500 F, 2.7 V
200 Ah, 3.2 V
0.32 kg and 0.325 dm
3
6.9 kg and 5.11 dm
3
Mass and volume
ESR
0.35 m
W
~1.25 m
W
40
C
65
C
25
C
75
C
Temperature
!þ
!
Deep cycles
1,000,000 at 75% DOD
2,500 at 75% DOD
Number of cells in
series
58
44
Pack energy
78 Wh at BOL
28,200 Wh at BOL
59 MWh at 25
C
53 MWh at 25
C
Cyclable energy
Maximum discharge
rate, 1 s
= 1,060 C
<
3C
Maximum charge
rate, 1 s
= 1,060 C
<
0.5 C
In a series string of capacitors with no voltage sharing, the voltages across
individual cells will not be equally distributed. It is typical for production ultra-
capacitors to have capacitance standard deviation of 1.7% of rated capacitance,
leading to voltage mismatch conditions on the same order. This could lead to ser-
ious cell overvoltage conditions and high stress, causing premature failure if left
unchecked. Figure 10.33 illustrates the origins of cell mismatch in a string. The cell
voltage, or capacitance times charge, can be expressed as the integral of the applied
current flowing through the series string. For example, if cell C
2
has higher capa-
citance than cell C
1
, then after some amount of time when cell C
1
is fully charged,
cell C
2
will be undercharged. Conversely, if cell C
2
were monitored during charge
and the charge source were removed when it registered fully charged, then cell C
1
would be in overvoltage. Cell balancing is generally necessary to avoid conditions
leading to cell overvoltage and their influence on cell service life.
Figure 10.33 illustrates the mechanisms of cell overvoltage. In this figure,
ultra-capacitor C
1
>
C
2
, so that after a constant current charge time of
T
seconds
either cell C
2
will be fully charged and cell C
1
in overvoltage or cell C
2
will be
undercharged and cell C
1
fully charged according to (10.54):
ð
i
ð
t
Þ
dt
1
C
1
U
C
1
ð
t
Þ¼
ð
V
Þ
ð
10
:
54
Þ
ð
i
ð
t
Þ
dt
1
C
2
U
C
2
ð
t
Þ¼
The various cell equalization schemes will now be discussed.
