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.
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.
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.