DEPTH OF DISCHARGE


The commonly used end-of-discharge voltage for the NiCd and NiMH is one volt per cell. At that voltage level, most of the energy is spent and the voltage starts to drop rapidly. Discharging a battery further could damage the battery through cell reversal. Caution should be exercised when discharging a battery too deeply under heavy load.

Since the cells in a battery pack cannot be perfectly matched, a negative voltage potential (cell reversal) across a weak cell may occur if the discharge is allowed to continue below one volt per cell. A NiCd can tolerate a limited amount of cell reversal. However, if a high current is drawn at the same time, the affected cell will likely develop an electrical short. (Mismatched Cells). On battery analyzers that apply a secondary discharge (recondition), the current is carefully controlled to make certain that the maximum allowable current through the reversed cell is not exceeded while discharging below one volt per cell.

The recommended end-of-discharge voltage for the SLA is 1.75V/cell. Unlike the rather flat discharge curve of the NiCd, the SLA has a more steady voltage drop with a rapid drop towards the end of discharge (see figure below). In fact, the voltage level of the SLA can be used to estimate the charge state of the battery. To be reasonably accurate, the load current and ambient temperature need to be taken into account. Because of the relatively low impedance of the SLA, an increase in load current causes a noticeable voltage drop. In addition, a warmer temperature depresses the voltage and a cool temperature raises it, a phenomenon that applies to most rechargeable batteries.



Discharge characteristics of NiCd, NiMH and SLA Batteries


The cycle life of an SLA is directly related to the depth of discharge. The typical number of discharge/charge cycles with respect to the depth of discharge is:

150 cycles with 100% discharge (full discharge)
400 cycles with 50% discharge (partial discharge)
1000 cycles with 30% discharge (shallow discharge)

The SLA should not be discharged beyond 1.75V per cell, nor can it be stored in a discharged state. The cells of a discharged SLA sulfate, a condition that renders the battery useless if left in that state for a few days. ( charging the SLA battery ).

The end-of-discharge voltage of the Li-ion needs to be carefully controlled and must, under no circumstances, go below 2.5V per cell. Protection circuit intrinsic to the Li-ion pack prevent the battery from being discharged below the safe limit. If allowed to self-discharge below 2.5V, unrecoverable capacity loss occurs when stored in that condition for three months or longer. Similar to the NiMH and SLA, more cycles can be obtained by partially, rather than fully discharging the Li-ion battery.

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