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Rechargeable Li-ion and Li-ion-polymer batteries are ubiquitous, and the reason is well justified. Compared to other rechargeable batteries, Li-ion batteries have a higher energy density, higher cell voltage, low self-discharge and very good cycle life, and are environmentally friendly as well as simple to charge and maintain. Also, because of their relatively high voltage (2.9 V to 4.2 V), many portable products can operate from a single cell, thereby simplifying an overall product design.
Depending on the application, there can be an argument as to what is the most important battery characteristic. Too much emphasis has been put on increasing Li-ion battery capacity to provide the longest product run-time in the smallest physical size. There are times when a longer battery life, an increased number of charge cycles or a safer battery is more important than battery capacity.
Before covering the battery charger's role in extending battery life, let's review the Li-ion battery's characteristics. Lithium is one of the lightest metals, is one of the most reactive and has the highest electrochemical potential, making it the ideal material for a battery. A Li-ion 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, respectively.
Although there are many different types of Li-ion batteries, the most popular chemistries now in production can be narrowed down to three, all relating to their cathode materials. Lithium-cobalt chemistry has become more popular in laptops, cameras and cell phones mainly because of its greater charge capacity. Other chemistries depend on the need for high discharge currents or 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.
Unlike 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. The table highlights some important characteristics of each battery type.
Li-Ion Polymer Batteries
With characteristics similar to a standard Li-ion battery, you can charge and discharge a Li-ion polymer battery in a similar manner. 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 because of their cost-effective manufacturing flexibility, which allows them to be fabricated in many different shapes, including very thin.
All rechargeable batteries wear out, and Li-ion cells are no exception. Battery manufacturers usually consider the end of life for a battery to be when the battery capacity drops to 80% of the rated capacity. However, batteries can still deliver usable power below 80% charge capacity, although they will produce shorter run-times.
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. Charging and discharging will eventually reduce the battery's active material and cause other chemistry 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 4.2 V (fully charged).
For maximum storage life, batteries should be stored with a 40% charge (3.6 V) 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 typically run warm or even hot, raising the battery temperature, and the charger is maintaining the battery near 100% charge. Both of these conditions shorten battery life, which could be as short as six months to a year. If possible, the user should be instructed to remove the battery and use the ac adapter for powering the laptop when used as a desktop computer. A properly cared for laptop battery can have a service life of two to four years or more.
There are two types of battery capacity losses: recoverable loss and permanent loss. After a full charge, a Li-ion battery will typically lose about 5% capacity in the first 24 hours, then approximately 3% per month because of self-discharge and an additional 3% per month if the battery pack has pack-protection circuitry. These self-discharge losses occur when the battery remains around 20°C, but will increase considerably with higher temperature and also as the battery ages. This capacity loss can be recovered by recharging the battery.
Permanent capacity loss, as the name implies, refers to permanent loss that is not recoverable by charging. Permanent capacity loss is mainly due to the number of full charge/discharge cycles, battery voltage and temperature. The more time the battery remains at 4.2 V or 100% charge level (or 3.6 V for Li-ion phosphate), the faster the capacity loss occurs. This is true whether the battery is being charged or just in a fully charged condition with the voltage near 4.2 V. Always maintaining a Li-ion battery in a fully charged condition will shorten its lifetime. The chemical changes that shorten the battery lifetime begin when it is manufactured, and these changes are accelerated by high float voltage and high temperature. Permanent capacity loss is unavoidable, but it can be held to a minimum by observing good battery practices when charging, discharging or simply storing the battery. Using partial-discharge cycles can greatly increase cycle life, and charging to less than 100% capacity can increase battery life even further.
The letter “C” is a battery term used to indicate the battery manufacturers stated battery discharge capacity, measured in milliamp-hours. For example, a 2000-mAhr rated battery can supply a 2000-mA load for one hour before the cell voltage drops to its zero-capacity voltage. In the same example, charging the battery at a C/2 rate would mean charging at 1000 mA (1 A). C is important in battery chargers because it determines the correct charge current required and the length of time needed to fully charge a battery. When discussing minimum charge-current termination methods, a 2000-mAhr battery using C/10 termination would end the charge cycle when the charge current drops below 200 mA.