05. Discharge Methods 3

GUIDE: Batteries in a portable world. 5. Discharge Methods 3

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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.

The Li-ion typically discharges to 3.0V/cell. The spinel and coke versions can be discharged to 2.5V/cell. The lower end-of-discharge voltage gains a few extra percentage points. Since the equipment manufacturers cannot specify which battery type may be used, most equipment is designed for a three-volt cut-off.

Caution should be exercised not to discharge a lithium-based battery too low. Discharging a lithium-based battery below 2.5V may cut off the battery’s protection circuit. Not all chargers accommodate a recharge on batteries that have gone to sleep because of low voltage.

Some Li-ion batteries feature an ultra-low voltage cut-off that permanently disconnects the pack if a cell dips below 1.5V. This precaution prohibits recharge if a battery has dwelled in an illegal voltage state. A very deep discharge may cause the formation of copper shunt, which can lead to a partial or total electrical short. The same occurs if the cell is driven into negative polarity and is kept in that state for a while. A fully discharged battery should be charged at 0.1C. Charging a battery with a copper shunt at the 1C rate would cause excessive heat. Such a battery should be removed from service.

Discharging a battery too deeply is one problem; equipment that cuts off before the energy is consumed is another. Some portable devices are not properly tuned to harvest the optimal energy stored in a battery. Valuable energy may be left behind if the voltage cut-off-point is set too high.

Digital devices are especially demanding on a battery. Momentary pulsed loads cause a brief voltage drop, which may push the voltage into the cut-off region. Batteries with high internal resistance are particularly vulnerable to premature cut-off. If such a battery is removed from the equipment and discharged to the appropriate cut-off point with a battery analyzer on DC load, a high level of residual capacity can still be obtained.

Most rechargeable batteries prefer a partial rather than a full discharge. Repeated full discharge robs the battery of its capacity. The battery chemistry which is most affected by repeat deep discharge is lead acid. Additives to the deep-cycle version of the lead acid battery compensate for some of the cycling strain.

Similar to the lead acid battery, the Li-ion battery prefers shallow over repetitive deep discharge cycles. Up to 1000 cycles can be achieved if the battery is only partially discharged. Besides cycling, the performance of the Li-ion is also affected by aging. Capacity loss through aging is independent of use. However, in daily use, there is a combination of both.

The NiCd battery is least affected by repeated full discharge cycles. Several thousand charge/discharge cycles can be obtained with this battery system. This is the reason why the NiCd performs well on power tools and two-way radios that are in constant use. The NiMH is more delicate with respect to repeated deep cycling.

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