Civil Engineering Reference
In-Depth Information
value before discharging rated capacity at room temperature. The low value
is generally taken as a 1.0 volt in cells with nominal voltage of 1.2 V. This is
a very conservative definition of a battery failure. In practice, if one cell
shows less than 1.0 V, other cells can make up without detecting the failure
at the battery level. Even if all cells show stable voltage below 1.0 V at full
load, the load can be reduced to maintain the desired voltage for some time
until the voltage degrades further.
The cell can fail open, short, or somewhere in between (a soft short). A
short that starts soft eventually develops into a hard short. In a low voltage
battery, any attempt to charge with a shorted cell may result in physical
damage to the battery and/or the charge regulator. On the other hand, the
shorted cell in a high voltage battery with numerous series connected cells
may work forever. It, however, loses the voltage and Ah capacity, hence,
would work as load on the healthy cells. An open cell, on the other hand,
disables the entire battery of series-connected cells.
In a system having two parallel batteries, if one cell in one battery fails
short, the two batteries would have different terminal characteristics. Charg-
ing or discharging such batteries as a group can result in highly uneven
current sharing, subsequently overheating one of the batteries. Two remedies
are available to avoid this. One is to charge and discharge both batteries
with individual current controls such that they both draw their rated share
of the load. The other is to replace the failed cell immediately, which can
sometimes be impractical. In general, the individual charge/discharge con-
trol for each battery is the best strategy. It may also allow replacement of
any one battery with different electrochemistry or different age, which would
have different load sharing characteristics. Batteries are usually replaced
several times during the economic life of the plant.
10.4.11
Wear-Out Failure
In addition to the random failure, the battery has the wear out failure mode.
It is associated with the electrode wear due to repeated charge/discharge
cycles. The number of times the battery can be discharged and recharged
before the electrodes wear out depends on the electrochemistry. The battery
life is measured in the number of charge/discharge cycles it can deliver
before failure. The life depends strongly on the depth of discharge and the
temperature as shown in
Figure 10-13
for high quality NiCd batteries. The
life also depends, to a lesser degree, on the electrolyte concentration and the
electrode porosity and thickness. The former factors are application related,
whereas the latter are construction related.
It is noteworthy from
Figure 10-14
that the life at a given temperature is
an inverse function of the depth of discharge. If the life is 100 units at
50 percent DoD, then it would be 200 units at 25 percent DoD. This makes
the product of the cycles to fail and the DoD remain constant. This product
decreases with increasing temperature. Such is true for most batteries. This
