How to Prolong Lithium-based Batteries
What Causes Lithium-ion to Age?
The lithium-ion
battery works on ion movement between the positive and negative
electrodes. In theory, such a mechanism should work forever, but shelf
life, cycling and temperature affect the performance. Because batteries
are used under many demanding environmental conditions, manufacturers
take a conservative approach and specify a battery life between 300 and
500 discharge/charge cycles. Life cycle testing is easy to measure and
is well understood by the user. Some organizations also add a date stamp
of three to five years; however, this method is less reliable because
it does not include the type of use.
Figure 1 illustrates
the capacity drop of 11 Li-polymer batteries that have been cycled at a
Cadex laboratory. The 1500mAh pouch cells were first charged to
4.20V/cell at 1C rate (1500mA) and allowed to saturate to 0.05C (75mA)
as part of 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 1500mA Li-ionbatteries for smart phone 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
Designed for a smart
phone, the packs were already a few months old at time of testing and
none of the batteries made it to 100 percent. It is common to see lower
than specified capacities and shelf life may have contributed to this.
Manufacturers tend to overrate their batteries; they know that very few
customers would complain. In our test, the expected capacity loss was
uniform over the 250 cycles. All sample 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 smaller the
depth of discharge, the longer the battery will last. If at all
possible, avoid frequent full discharges and charge more often between
uses. If full discharges cannot be avoided, try utilizing a larger
battery. Partial discharge on Li-ion is fine; there is no memory and the
battery does not need periodic full discharge cycles other than to
calibrate the fuel gauge on a smart battery.
Table 2 compares the
number of discharge/charge cycles a battery can deliver at various DoD
levels before lithium-ion is worn out. We assume end of life when the
battery capacity drops to 70 percent. This is an arbitrary threshold
that is application based.
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Depth of discharge
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Discharge cycles
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Table 2: Cycle life and depth of discharge
A partial discharge reduces stress and prolongs battery life.
Elevated temperature and high currents also affect cycle life.
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100% DoD
50% DoD
25% DoD
10% DoD
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500
1500
2500
4700
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Specifying battery
life by the number of discharge cycles is not complete by itself;
equally if not more important are temperature conditions and charging
voltages. Lithium-ion suffers stress when exposed to heat and kept at a
high charge voltage.
Elevated temperature
is anything that dwells above 30°C (86°F), and a high voltage is higher
than 4.10V/cell. When estimating longevity, these conditions are
difficult to assess because the battery state is in constant flux, and
so is the temperature in which it operates. Exposing the battery to high
temperature and being at full state-of-charge for an extended time can
be more damaging than cycling. Manufacturers do not like to talk about
these environmental conditions and release information only in
confidence when so requested.
In this essay we do
not depend on the manufacturer’s specifications alone but also listen to
the comments of users. BatteryUniversity.com is 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.
Let’s look at
real-life situations and examine what stress a lithium-ion battery
encounters. Most packs last three to five years, less if exposed to high
heat and if kept at a full charge. Table 3 illustrates capacity loss as
a function of temperature and state-of-charge. One can clearly see a
performance drop of recoverable capacity caused by environmental
conditions and not cycling. The worst condition is keeping a fully
charged battery at elevated temperatures, which is the case when running
a laptop on the power grid. Under these circumstances the battery will
typically last for about two years, whether cycled or not. The pack does
not die suddenly but will produce decreasing runtimes as part of aging.
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Battery Temperature
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Permanent capacity loss when
stored at 40% state-of-charge
(recommended storage charge level)
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Permanent capacity loss when
stored at 100% state-of-charge
(typical user charge level)
|
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0°C
25°C
40°C
60°C
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2% loss in 1 year; 98% remaining
4% loss in 1 year; 96% remaining
15% loss in 1 year; 85% remaining
25% loss in 1 year 75%; remaining
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6% loss in 1 year; 94% remaining
20% loss in 1 year; 80% remaining
35% loss in 1 year; 65% remaining
40% loss in 3 months
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Table 3: Permanent capacity loss of lithium‑ion as a function of temperature and charge level. High charge levels and elevated temperatures hasten permanent capacity loss. Newer designs may show improved results.
Batteries are also
exposed to elevated temperature when charging with wireless chargers.
The energy transfer from a charging mat to the portable device is 70 to
80 percent and the remaining 20 to 30 percent is lost mostly in heat.
Placing a cellular phone on the heat generating charging mat stresses
the battery more than if charged on a designated charger. We keep in
mind that the mat will cool down once the battery is fully charged. Read
more: Charging without wires.
Equally 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 unit with an appropriate cut-off.
Users should not worry too much about the state-of-charge; a cool and
dry place is more important.
The voltage level to
which the cells are charged also plays a role in extending longevity.
For safety reasons, most lithium-ion cannot exceed 4.20V/cell. While a
higher voltage would boost capacity, over-voltage shortens service life.
Figure 4 demonstrates the increased capacity but shorter cycle life if
Li-ion were allowed to exceed the 4.20V/cell limit. At 4.35V, the
capacity would increase by 10 to 15 percent, but the cycle count would
be cut in half. More critical than the extra capacity is reduced safety,
which would be the results of a higher charge voltage.
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Figure 4: Effects on cycle life at elevated charge voltages
Higher charge voltages boost capacity but lower cycle life and compromise safety.
Source: Choi et al. (2002)
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Chargers for
cellular phones, laptops and digital cameras bring the Li-ion battery to
4.20V/cell. This allows maximum runtime, and the consumer wants nothing
less than optimal use of the battery capacity. The industry, on the
other hand, is more concerned with longevity and prefers lower voltage
thresholds. Satellites and electric vehicles are examples where
longevity is important.
We have limited
information by how much lower charge voltages prolong battery life; this
depends on many conditions, as we have learned. What we do know,
however, is the capacities. At a charge to 4.10V/cell, the battery holds
a capacity that is about 10 percent less than going all the way to
4.20V/cell. In terms of optimal longevity, a charge voltage limit of
3.92V/cell works best but the capacity would be low. Besides selecting
the best-suited voltage thresholds, it is also important that the
battery does not stay in the high-voltage stage for a long time and is
allowed to drop after full charge has been reached.
The voltage
threshold of commercial chargers cannot be changed, and making it
adjustable would have advantages, especially for laptops as a means of
prolonging battery life. When running on extended AC mode, the user
would select the “long life” mode and the battery would charge to only,
say, 4.05V/cell. This would get a capacity of about 80 percent. Before
traveling the user would apply the “full charge mode” to bring the
charge to 4.20V/cell. This saturation charge would take about an hour
and would fill the battery to 100 percent capacity.
Realizing the stress
on the battery, some laptop and cellular phone manufacturers choose an
end-of-charge voltage that is less than 4.20V/cell. A slightly larger
pack compensates for the reduced runtime. Another option to extend
battery life is removing the pack from the laptop when running on the
power grid. The Consumer Product Safety Commissionadvises the
public to do this out of concern for overheating and causing a fire.
Removing the battery has the disadvantage of losing unsaved work on
power failure.
Heat buildup is
always a concern and running a laptop in bed or on a pillow may
contribute to this by restricting airflow. Not only will heat stress
electronic components, elevated temperature causes the electrodes in the
battery to react with the electrolyte and this will permanently lower
the capacity. Placing a ruler or other object under the laptop to
increase floor clearance improves air circulation around the enclosure
and keeps the unit cooler.
The question is
often asked: Should I disconnect my laptop from the power grid when not
in use? Under normal circumstances this should not be necessary because
once the lithium-ion battery is full, a correctly functioning charger
will discontinue the charge and will only engage when the battery
voltage drops to a low level. 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 and use it in a way that is least
stressful. This is not always feasible. Sometimes we need to run the
battery in environments that are not conducive to optimal service life.
As a doctor cannot predict how long a person will live based on diet and
activity alone, so also does the life of a battery vary, and it can
always be cut short by an unexpected failure. Batteries and humans share
the same volatility.
To get a better
understanding of what causes irreversible capacity loss in Li-ion
batteries, several research laboratories* are performing forensic tests.
Scientist 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
example, a phenomenon that is irreversible. Knowing the reason for such
capacity loss might enable battery manufacturers to produce future
batteries with longer life spans.
Power loss through Protection Circuit
Besides common
aging, a Li-ion battery can also fail because of undercharge. This
occurs if a Li-ion pack is stored in a discharged condition.
Self-discharge gradually lowers the voltage of the already discharged
battery and the protection circuit cuts off between 2.20 and 2.90V/cell.
Some chargers and battery analyzers (including those from Cadex)
provide a wake-up feature, or “boost,” to re-energize and recharge these
seemingly dead Li-ion batteries.
.
On the computer
Removing
the CMOS battery like the one shown in the picture to the right will cause the
system to lose all CMOS settings including the password. To do this locate and remove the
Anyway,
open up the computer case using a screw driver and locate the flat,
circular and metallic CMOS battery. It should look something like the
picture to the right. Some computers have this part standing upright.
