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 laptop 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.
Depth of discharge
|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.
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.
Permanent capacity loss when
Permanent capacity loss when
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
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
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 HP 484170-001 battery is fully charged.
Equally stressful is leaving a laptop computer 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.
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)
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 battery 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.
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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 laptop battery 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 Acer Aspire 5520 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.
* 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.