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The typical end-of-discharge voltage for nickel-based
batteries is 1V/cell. At that voltage level, about 99 percent
of the energy is spent and the voltage starts to drop rapidly
if the discharge continues. Discharging beyond the cut-off
voltage must be avoided, especially under heavy load.
Since the cells in a battery pack cannot be perfectly
matched, a negative voltage potential (cell reversal) across
a weaker cell occurs if the discharge is allowed to continue
beyond the cut-off point. The larger the number of cells connected
in series, the greater the likelihood of this occurring.
A NiCd battery can tolerate a limited amount
of cell reversal, which is typically about 0.2V. During that
time, the polarity of the positive electrode is reversed.
Such a condition can only be sustained for a brief moment
because hydrogen evolution occurs on the positive electrode.
This leads to pressure build-up and cell venting.
If the cell is pushed further into voltage reversal,
the polarity of both electrodes is being reversed, resulting
in an electrical short. Such a fault cannot be corrected and
the pack will need to be replaced.
On battery analyzers that apply a secondary discharge
(recondition), the current is controlled to assure that the
maximum allowable current, while in sub-discharge range, does
not exceed a safe limit. Should a cell reversal develop, the
current would be low enough as not to cause damage. A cell
breakdown through recondition is possible on a weak or aged pack.
If the battery is discharged at a rate higher
than 1C, the more common end-of-discharge point of a nickel-based
battery is 0.9V/cell. This is done to compensate for the voltage
drop induced by the internal resistance of the cell, the wiring,
protection devices and contacts of the pack. A lower cut-off
point also delivers better battery performance at cold temperatures.
The recommended end-of-discharge voltage for
the SLA is 1.75V/cell. Unlike the preferred flat discharge
curve of the NiCd, the SLA has a gradual voltage drop with
a rapid drop towards the end of discharge (see Figure 5-1).
Although this steady decrease in voltage is a disadvantage,
it has a benefit because the voltage level can be utilized
to display the state-of-charge (SoC) of a battery. However,
the voltage readings fluctuate with load and the SoC readings
are inaccurate.
Figure 5-1:
Discharge characteristics of NiCd, NiMH and SLA batteries.
While voltage readings to measure
the SoC are not practical on nickel-based batteries, the SLA
enables some level of indication as to the SoC.
°C (77°F) with respect
to the depth of discharge is:
- 150 – 200 cycles with 100 percent depth of discharge
(full discharge)
- 400 – 500 cycles with 50 percent depth of discharge
(partial discharge)
- 1000 and more cycles with 30 percent depth of discharge
(shallow discharge)
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