The purpose of a battery is to
store energy and release it at the appropriate time in a controlled
manner. In this section we examine the discharge under different C-rates
and evaluate the depth to which a battery can safely be discharged. We
also observe how deep discharges affect battery life.
What is C-rate?
The charge and discharge
current of a battery is measured in C-rate. Most portable batteries are
rated at 1C. This means that a 1000mAh battery would provide 1000mA for
one hour if discharged at 1C rate. The same battery discharged at 0.5C
would provide 500mA for two hours. At 2C, the 1000mAh battery would
deliver 2000mA for 30 minutes. 1C is often referred to as a one-hour
discharge; a 0.5C would be a two-hour, and a 0.1C a 10-hour discharge.
The capacity of a battery is commonly measured with a battery analyzer.
If the analyzer's capacity readout is displayed in percentage of the
nominal rating, 100% is shown if a 1000mAh battery can provide this
current for one hour. If the battery only lasts for 30 minutes before
cut-off, 50% is indicated. A new battery sometimes provides more than
When discharging a battery
with a battery analyzer that allows the setting of different discharge
C-rates, a higher capacity reading is observed if the battery is
discharged at a lower C-rate and vice versa. By discharging the 1000mAh
battery at 2C, or 2000mA, the analyzer is scaled to derive the full
capacity in 30 minutes. Theoretically, the capacity reading should be
the same as with a slower discharge, since the identical amount of
energy is dispensed, only over a shorter time. Due to internal energy
losses and a voltage drop that causes the battery to reach the low-end
voltage cut-off sooner, the capacity reading may be lowered to 95%.
Discharging the same battery at 0.5C, or 500mA over two hours may
increase the capacity reading to about 105%. The discrepancy in capacity
readings with different C-rates is related to the internal resistance of
One battery that does not
perform well at a 1C discharge rate is the portable sealed lead-acid. To
obtain a reasonably good capacity reading, manufacturers commonly rate
these batteries at 0.05C or 20 hour discharge. Even at this slow
discharge rate, a 100% capacity is hard to attain. To compensate for
different readings at various discharge currents, manufacturers offer a
capacity offset. Applying the offset to correct the capacity readout
does not improve battery performance; it merely adjusts the capacity
calculation if discharged at a higher or lower C-rate than specified.
batteries are electronically protected against high load currents.
Depending on battery type, the discharge is limited to between 1C and
2C. This protection makes the lithium ion unsuitable for biomedical
equipment and power tools demanding high inrush currents.
Depth of discharge
end-of-discharge voltage for nickel-based batteries is 1V/cell. At that
voltage level, roughly 99% of the energy is spent and the voltage starts
to drop rapidly if the discharge continued. 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,
also known as cell reversal, will occur across a weaker cell if the
discharge is allowed to continue uncontrolled. The more cells that are
connected in series, the greater the likelihood of cell reversal
Nickel-cadmium can tolerate
some 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 on the
positive electrode leads to pressure build-up and possible cell venting.
If the cell is pushed further into voltage reversal, the polarity of
both electrodes is being reversed and the cell produces an electrical
short. Such a fault cannot be corrected.
Some battery analyzers
apply a secondary discharge (recondition) that discharges the battery
voltage to a very low voltage cut-off point. These instruments control
the discharge current to assure that the maximum allowable current,
while in sub-discharge range, does not exceed a safe limit. Should cell
reversal develop, the current would be low enough not to cause damage.
Cell breakdown through recondition is possible on a weak or aged pack.
If the battery is
discharged at a rate higher than 1C, the end-of-discharge point of a
nickel-based battery is typically lowered to 0.9V/cell. This compensates
for the voltage drop induced by the internal resistance of the cells,
wiring, protection devices and contacts. A lower cut-off point also
produces better capacity readings when discharging a battery at cold
Among battery chemistries,
nickel-cadmium is least affected by repeated full discharge cycles.
Several thousand charge/discharge cycles are possible. This is why
nickel-cadmium performs well on power tools and two-way radios that are
in constant use. nickel-metal-hydride is less durable in respect to
repeated deep cycling.
discharges to 3.0V/cell. The spinel and coke versions can be discharged
to 2.5V/cell to gain a few extra percentage points. Since the equipment
manufacturers do not specify the battery type, most equipment is
designed for a 3-volt cut-off.
A discharge below 2.5V/cell
may put the battery's protection circuit to sleep, preventing a recharge
with a regular charger. These batteries can be restored with the Boost
program available on the Cadex C7000 Series battery analyzers.
Some lithium-ion batteries
feature an ultra-low voltage cut-off that permanently disconnects the
pack if a cell dips below 1.5V. 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.
Manufacturers rate the
lithium-ion battery at an 80% depth of discharge. Repeated full (100%)
discharges would lower the specified cycle count. It is therefore
recommended to charge lithium-ion more often rather than letting it
discharge down too low. Periodic full discharges are not needed because
lithium-ion is not affected by memory.
end-of-discharge voltage for lead-acid is 1.75V/cell. The discharge does
not follow the preferred flat curve of nickel and lithium-based
chemistries. Instead, Lead-acid has a gradual voltage drop with a rapid
drop towards the end of discharge.
The cycle life of sealed
lead-acid is directly related to the depth of discharge. The typical
number of discharge/charge cycles at 25ºC with respect to the
depth of discharge is:
150 - 200 cycles with 100%
depth of discharge (full discharge)
400 - 500 cycles with 50%
depth of discharge (partial discharge)
1000 and more cycles with
30% depth of discharge (shallow discharge)
The lead-acid battery should not be discharged beyond 1.75V per cell,
nor should it be stored in a discharged state. The cells of a discharged
lead-acid sulfate, a condition that renders the battery useless if left
in that state for a few days. Always keep the open terminal voltage at
2.10V and higher.
What constitutes a
There are no standard
definitions that constitute a discharge cycle. Smart batteries that keep
track of discharge cycles commonly use a depth-of-discharge of 70% to
define a discharge cycle. Anything less than 70% does not count. The
reason of the cycle count is to estimate the end-of-battery life.
A battery often receives
many short discharges with subsequent recharges. With the smart battery,
these cycles do not count because they stress the battery very little.
On satellites, the depth-of-discharge is only about 10%. Such minute
discharge cycles put the least amount of stress on the batteries in
space. With shallow discharges, however, nickel-based batteries require
a periodic deep discharge to eliminate memory.
Lithium and lead-based
batteries do not require a periodic full discharge. In fact, it is
better not to discharge them too deeply but charge them more often.
Using a larger battery is one way to reduce the stress on a battery.