The care and feeding of your embedded design's Li-ion battery subsystem

Fran Hoffart

June 24, 2008

Fran Hoffart

Much emphasis has been put on increasing Li-ion battery capacity to provide the longest product runtime in the smallest physical size. But there are instances where a longer battery life, an increased number of charge cycles or a safer battery is more important than battery capacity.

This article presents methods relating to charging and discharging Liion batteries that can considerably increase battery life.

Rechargeable Li-ion, including Li-ion polymer batteries, can be found in practically every high performance portable product and the reason for this is well justified. Compared to other rechargeable batteries, Li-ion batteries have a higher energy density, higher cell voltage, low self-discharge, very good cycle life, are environmentally friendly and are simple to charge and maintain.

In addition, because of their relatively high voltage (2.9V to 4.2V) many portable products can operate from a single cell, thereby simplifying an overall product design.

The basics
Before covering the battery charger's role in extending battery life, a quick review of the Li-ion battery is necessary. Lithium - -one of the lightest and most reactive metals - -has the highest electrochemical potential, making it the ideal material for a battery.

A Liion battery contains no lithium in a metallic state, but instead uses lithium ions that shuttle back and forth between the cathode and anode of the battery during charge and discharge.

Although there are many different types of Li-ion batteries, the most popular chemistries presently in production can be narrowed down to three, all relating to the cathode materials used in the battery.

The lithium cobalt chemistry has become more popular in laptops, cameras and cell phones mainly because of its greater charge capacity. Other chemistries are used based on the need for high discharge currents, improved safety, or where cost is the driving factor.

Also, new hybrid Li-ion batteries are in development, based on a combination of cathode materials incorporating the best features of each chemistry.

Table 1: The advantages and disadvantages offered by Li-ion batteries based on cathode material used are listed.

Unlike some other battery chemistries, Li-ion battery technology is not yet mature. Research is ongoing with new types of batteries that have even higher capacities, longer life and improved performance than present day batteries. Table 1 above highlights some important characteristics of each battery type.

A Li-ion polymer battery is charged, discharged and has characteristics similar to a standard Li-ion battery. The main difference between the two is that a solid ion conductive polymer replaces the liquid electrolyte used in a standard Li-ion battery, although most polymer batteries also contain an electrolyte paste to lower the internal cell resistance.

Eliminating the liquid electrolyte allows the polymer battery to be housed in a foil pouch rather than the heavy metal case required for standard Li-ion batteries. Li-ion polymer batteries are gaining popularity based on their cost-effectiveness to produce and their flexibility for fabricating in many different shapes, including very thin.

Life expectancy
All rechargeable batteries wear out, and Li-ion cells are no exception. Battery manufacturers usually consider end-of-life for a battery to be when the battery capacity drops to 80 percent of the rated capacity. However, batteries can still deliver usable power below 80 percent charge capacity, even though runtime is shortened.

The number of charge/discharge cycles is commonly used when referring to battery life, but cycle life and battery life (or service life) can be different lengths of time. Charging and discharging will eventually reduce the battery's active material and cause other chemical changes, resulting in increased internal resistance and permanent capacity loss. But permanent capacity loss also occurs even when the battery is not in use.

<>Permanent capacity loss is greatest at elevated temperatures with the battery voltage maintained at 4.2V (fully charged). For maximum storage life, batteries should be stored with a 40 percent charge (3.6V) at 40°F (refrigerator). Perhaps one of the worst locations for a Li-ion battery is in a laptop computer when used daily on a desktop with the charger connected.

Laptops and portable devices typically run warm or even hot, raising the battery temperature, and the charger is maintaining the battery near 100 percent charge. Both of these conditions shorten battery life, which could be as short as 6 months to a year.

If possible, remove the battery and use the AC adapter for powering the laptop when the computer is used on a desktop. A properly cared for laptop battery can have a service life of two to four years, or more.