How to Prolong Lithium-based Batteries
The lithium-ion battery works on ion movement between the positive and
negative electrodes. In theory such a mechanism should work forever, but
cycling, elevated temperature and aging decrease the performance over
time. Since batteries are used in demanding environmental conditions,
manufacturers take a conservative approach and specify the life of most
Li-ion between 300 and 500 discharge/charge cycles.
Counting cycles is not conclusive because a discharge may vary in depth
and there are no clearly defined standards of what constitutes a cycle.
Read more about
What Constitutes a Discharge Cycle?.
In lieu of cycle count, some batteries in industrial instruments are
date-stamped, but this method is not reliable either because it ignores
environmental conditions. A battery may fail within the allotted time
due to heavy use or unfavorable temperature conditions, but most quality
packs will last considerably longer than what the stamp indicates.
The performance of a battery is measured in capacity, a leading health
indicator. Internal resistance and self-discharge also play a role but
with modern Li-ion these carry lower significance in predicting the
end-of-battery-life. Figure 1 illustrates the capacity drop of 11
Li-polymer batteries that have been cycled at a Cadex laboratory. The
1500mAh pouch cells for smartphones were first charged at a current of
1500mA (1C) to 4.20V/cell and allowed to saturate to 0.05C (75mA) as
part of the full charge procedure. The batteries were then discharged at
1500mA to 3.0V/cell, and the cycle was repeated.
|
|
Figure 1: Capacity drop as part of cycling
A pool of new 1500mAh Li-ionbatteries for smartphones istested on a
Cadex C7400 battery analyzer. All 11 pouch packs show a starting
capacity of 88–94 percent and decrease in capacity to 73–84 percent
after 250 full discharge cycles (2010).
Courtesy of Cadex
|
Although a battery should deliver 100 percent capacity during the first
year of service, it is common to see lower than specified capacities,
and shelf life may have contributed to this loss. In addition,
manufacturers tend to overrate their batteries; knowing that very few
customers would complain. In our test, the expected capacity loss of
Li-ion batteries was uniform over the 250 cycles and the batteries
performed as expected.
Similar to a mechanical device that wears out faster with heavy use, so
also does the depth of discharge (DoD) determine the cycle count. The
shorter the discharge (low DoD), the longer the battery will last. If at
all possible, avoid full discharges and charge the battery more often
between uses. Partial discharge on Li-ion is fine; there is no memory
and the battery does not need periodic full discharge cycles to prolong
life, other than to calibrate the fuel gauge on a smart battery once in a
while. Read more about
Battery Calibration.
Table 2 compares the number of discharge/charge cycles Li-ion can
deliver at various DoD levels before the battery capacity drops to 70
percent. The number of discharge cycles depends on many conditions and
includes charge voltage, temperature and load currents. Not all Li-ion
systems behave the same.
|
Depth of discharge
|
Discharge cycles
|
Table 2: Cycle life as a function of
depth of discharge
A partial discharge reduces stress and prolongs battery life.
Elevated temperature and high currents also affect cycle life.
|
|
100% DoD
50% DoD
25% DoD
10% DoD
|
300 – 500
1,200 – 1,500
2,000 – 2,500
3,750 – 4,700
|
Lithium-ion suffers from stress when exposed to heat, so does keeping a
cell at a high charge voltage. A battery dwelling above 30°C (86°F) is
considered
elevated temperature and for most Li-ion, a voltage above 4.10V/cell is deemed as
high voltage.
Exposing the battery to high temperature and dwelling in a full
state-of-charge for an extended time can be more stressful than cycling.
Table 3 demonstrates capacity loss as a function of temperature and
SoC.
|
Temperature
|
40% charge
|
100% charge
|
Table 3: Estimated recoverable capacity when storing Li-ion for one year at various temperatures
Elevated temperature hastens capacity loss. The capacity cannot be restored. Not all Li-ion systems behave the same.
|
|
0°C
25°C
40°C
60°C
|
98%
96%
85%
75%
|
94%
80%
65%
60%
(after 3 months)
|
Most Li-ions are charged to 4.20V/cell and every reduction of
0.10V/cell is said to double cycle life. For example, a lithium-ion
cell charged to 4.20V/cell typically delivers 300–500 cycles. If charged
to only 4.10V/cell, the life can be prolonged to 600–1,000 cycles;
4.00V/cell should deliver 1,200–2,000 and 3.90V/cell 2,400–4,000 cycles.
Table 4 summarizes these results. The values are estimate and depend on
the type of li-ion-ion battery.
|
Charge level (V/cell)
|
Discharge cycles
|
Capacity at full charge
|
Table 4: Discharge cycles and capacity
as a function of charge
Every 0.1V drop below 4.20V/cell doubles the cycle; the retained
capacity drops accordingly. Raising the voltage above 4.20V/cell
stresses the battery and compromises safety.
|
|
[4.30]
4.20
4.10
4.00
3.92
|
[150 – 250]
300 – 500
600 – 1,000
1,200 – 2,000
2,400 – 4,000
|
[110%]
100%
90%
70%
50%
|
For safety reasons, lithium-ion cannot exceed 4.20V/cell. While a
higher voltage would boost capacity, over-voltage shortens service life
and compromises safety. Figure 5 demonstrates cycle count as a function
of charge voltage. At 4.35V, the cycle count is cut in half.
|
Figure 5: Effects on cycle life at elevated charge voltages
Higher charge voltages boost capacity but lowers cycle life and compromises safety.
Source: Choi et al. (2002)
|
Chargers for cellular phones, laptops, tablets and digital cameras
bring the Li-ion battery to 4.20V/cell. This allows maximum capacity,
because the consumer wants nothing less than optimal runtime. Industry,
on the other hand, is more concerned about longevity and may choose
lower voltage thresholds. Satellites and electric vehicles are examples
where longevity is more important than capacity.
Charging to 4.10V/cell the battery holds about 10 percent less capacity
than going all the way to 4.20V. In terms of optimal longevity, a
voltage limit of 3.92V/cell works best but the capacity would only be
about half compared to a 4.20V/cell charge (3.92V/cell is said to
eliminate all voltage-related stresses).
Besides selecting the best-suited voltage thresholds for a given
application, Li-ion should not remain at the high-voltage ceiling of
4.20V/cell for an extended time. When fully charged, remove the battery
and allow to voltage to revert to a more natural level like relaxing
after exercise. Although a properly functioning Li-ion charger will
terminate charge when the battery is full, some chargers apply a topping
charge if the battery terminal voltage drops to a given level. Read
more about
Charging Lithium-ion.
What the user can do
The author of this essay does not depend on the manufacturer’s
specifications alone but also listens to user comments.
BatteryUniversity.comis an excellent sounding board to connect with the
public and learn about reality. This approach might be unscientific, but
it is genuine. When the critical mass speaks, the manufacturers listen.
The voice of the multitude is in some ways stronger than laboratory
tests performed in sheltered environments.
Tables 2, 3 and 4 look at cycle life as a function of discharge,
temperature and charge level. A summary table should be added that also
states the
Optimal Battery Energy Factor Over Life. While this
would help in selecting the optimal battery, battery makers are hesitant
to release such a specification freely, and for good reason. A battery
is in constant flux and capturing all of its data is exhaustive. A
further criterion is price. Batteries can be built to perform better but
this comes at a cost.
Let’s look at real-life situations and examine what stresses
lithium-ion batteries encounter. Most packs last three to five years.
Environmental conditions, and not cycling alone, are a key ingredient to
longevity, and the worst situation is keeping a fully charged battery
at elevated temperatures. This is the case when running a laptop off the
power grid. Under these conditions, a battery will typically last for
about two years, whether cycled or not. The pack does not die suddenly
but will give lower runtimes with aging.
Even more stressful is leaving a battery in a hot car, especially if
exposed to the sun. When not in use, store the battery in a cool place.
For long-term storage, manufacturers recommend a 40 percent charge. This
allows for some self-discharge while still retaining sufficient charge
to keep the protection circuit active. Finding the ideal state-of-charge
is not easy; this would require a discharge with appropriate cut-off.
Do not worry too much about the state-of-charge; a cool and dry place is
more important than SoC. Read more about
How to Store Batteries.
Batteries are also exposed to elevated temperature when charging on
wireless chargers. The energy transfer from a charging mat to a portable
device is 70 to 80 percent and the remaining 20 to 30 percent is lost
mostly in heat that is transferred to the battery through the mat. We
keep in mind that the mat will cool down once the battery is fully
charged. Read more about
Charging Without Wires.
Avoid charging a battery faster than 1C; a more moderate charge rate of
0.7C is preferred. Manufacturers of electric powertrains are concerned
about super-fast charging of 20 minutes and less. Similarly, harsh
discharges should be avoided as also this also adds to battery stress.
Read more about
Charging Lithium-ion and
Ultra-fast Chargers.
Commercial chargers do not allow changing the charge voltage limit.
Adding this feature would have advantages, especially for laptops as a
means to prolong battery life. When running on extended AC mode, the
user could select the “long life” mode and the battery would charge to
4.00V/cell for a standby capacity of about 70 percent. Before traveling,
the user would apply the “full charge mode” to bring the charge to
100%. Some laptop manufacturers may offer this feature but often only
computer geeks discover them.
Another way to extend battery life is to remove the pack from the laptop when running off the power grid. The
Consumer Product Safety Commissionadvises
to do this out of concern for overheating and causing a fire. Removing
the battery has the disadvantage of losing unsaved work if a power
failure occurs. Heat buildup is also a concern when operating a laptop
in bed or on a pillow, as this may restrict airflow. Placing a ruler or
other object under the laptop will improve air circulation and keep the
device cooler.
“Should I disconnect my laptop from the power grid when not in use?” many
ask. Under normal circumstances this should not be necessary because
once the lithium-ion battery is full the charger discontinues charge and
only engages when the battery voltage drops. Most users do not remove
the AC power and I like to believe that this practice is safe.
Everyone wants to keep the battery as long as possible, but a battery
must often operate in environments that are not conducive to optimal
service life. Furthermore, the life of a battery may be cut short by an
unexpected failure, and in this respect the battery shares human
volatility.
To get a better understanding of what causes irreversible capacity loss
in Li-ion batteries, several research laboratories* are performing
forensic tests. Scientists dissected failed batteries to find suspected
problem areas on the electrodes. Examining an unrolled 1.5-meter-long
strip (5 feet) of metal tape coated with oxide reveals that the finely
structured nanomaterials have coarsened. Further studies revealed that
the lithium ions responsible to shuttle electric charge between the
electrodes had diminished on the cathode and had permanently settled on
the anode. This results in the cathode having a lower lithium
concentration than a new cell, a phenomenon that is irreversible.
Knowing the reason for such capacity loss might enable battery
manufacturers to prolong battery life in the future.
* Research is performed by the Center for Automotive Research at the
Ohio State University in collaboration with Oak Ridge National
Laboratory and the National Institute of Standards Technology.
