Overview

The useful life of a lithium-ion battery in an application is based on several factors. In more demanding applications of high current draw, these factors can prevent the battery from providing sufficient current draw due to diminishing capacity and increasing internal resistance as the batteries age. This document will examine the factors that cause batteries to age and how those causes can be mitigated to extend their useful life.

Factors that Cause Li-Ion Battery Cells to Age

Aging is a concern with all lithium-ion battery chemistries. As the cell "ages" two things appear to occur from the user's standpoint:

  1. Battery capacity appears to diminish, reducing the time the battery can supply current to the device.
  2. The voltage level, while in use, appears to drop off faster, therefore applying an ever decreasing voltage to the circuit. This is more noticeable for higher current applications as the internal resistance will cause this voltage drop to be magnified.
    Both phenomena cause the battery to appear to the user to have less capacity as its cells age.
    Battery capacity will diminish with age as its internal plates and electrolytes undergo irreversible damage. This damage is caused by corrosion and other irreversible chemical processes. Two factors contribute to this:
  3. Classic aging of the internal battery components. The plates of the cells will corrode similar to steel rusting. As the plates corrode, their surface area will diminish and the electrolyte will undergo chemical changes causing them both to be less chemically reactive. This change reduces the volume of reactive components in the cell causing the charge capacity of the cell to be reduced. It also increases the internal resistance of the cell as these corrosion products inhibit the free flow of electrons through the plates, electrolytes and less efficient chemical reactions due to diluted reaction agents. This deterioration occurs whether or not the battery is being used and will increase under certain environmental conditions which are discussed further below.
  4. Each charge/discharge cycle of a battery also has a similar effect but at an accelerated pace.

So what does this mean to the user? Time and usage are both enemies to the useful battery life.

How to Maximize the Useful Life of a Battery

Some capacity deterioration will occur due to aging whether the battery is in use or not. The battery's capacity and internal resistance will be reduced resulting in continuing deterioration until it is no longer useful. Factors that can reduce this deterioration include:

  1. Ensuring that the battery, when received new, is as "fresh" as possible. Ensure via the date code that it has not been sitting on the shelf in the supply chain for excessive periods. A battery should be less than 3 or 4 months old when you receive it (Li-ion is more susceptible to aging than other chemistries so this is applicable only for Li-ion). Typically a battery will lose approximately 10% of its capacity if stored for 1 year at room temperature.
  2. Proper storage of batteries when they are not in use. The most important environmental factor for slowing aging is the temperature in which batteries are stored. An ideal storage temperature range would be 10° to 15°C, which would reduce the aging effect by roughly half over room temperature and approximately double at 35°C. Do not store batteries at freezing temperatures as other factors may result in decreased life.
  3. Charge level during storage. Battery cells are shipped from the manufacturing plant at a 40% charge level. Higher charge levels are acceptable but pose risk due to the energy levels of the batteries during shipment. Lower levels are not recommended as this will accelerate aging.
  4. Temperature of the batteries during charging and discharging. Most of the corrosion occurs during charge/discharge cycles, which increases at higher temperatures. Many battery manufactures operate in strict temperature ranges during the charge cycles to help minimize this. After usage in a high current consumption application, the batteries should be allowed to cool prior to charging. If possible, charging should be performed in a lower temperature environment (like storage, 15°C is the optimal). The same applies for the discharge environment.
  5. Charging voltages have a significant effect on battery longevity. Charging algorithms used in Psion Teklogix supplied chargers are optimized to provide the correct voltages to the batteries to provide fast charging without sacrificing longevity.
  6. Discharge rate; in general the user has little control on discharge rate as most battery applications are already optimized for battery life. The higher the discharge rate, the higher the internal cells will corrode due to self heating. The rate of corrosion will accelerate as the internal resistance of the battery increases due to aging (the higher resistance creates internal heat in the cell). This will be experienced as a rapid decrease in capacity for older cells.
  7. Discharge depth. This factor may not be as obvious but more frequent battery changes will increase the overall lifespan of the Lithium-ion battery. It will reduce the time in both the charge and discharge cycles where the battery is hotter due to internal resistances and it will keep the battery chemistry in the higher charge states where corrosion rates are less. Less stress is applied to the battery through the charge/discharge cycle.

Lithium-ion works within the discharge temperature limits of -20°C to 60°C (-4°F to 140°F). The performance is temperature based, meaning that the rate capability at or below -20°C is reduced due to the increased impedance of the electrolyte. Discharging at low temperatures does not harm the battery. Lithium-ion may be used down to -30°C (-22°F) with acceptable results. Larger packs will be necessary to compensate for the reduced capacity at these temperatures.

It is not recommended to discharge lithium-ion at temperatures above 60°C. High discharge rates combined with elevated temperatures can cause self-heating, an effect that could permanently damage the separator and electrodes of the cells.

Further Reading

  • Lithium-Ion Battery Life Performance