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
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