Boosting Battery Life in Lithium batteries
Use partial-discharge cycles. Using only 20% or 30% of the battery capacity before recharging will extend cycle life considerably. As a general rule, 5 to 10 shallow discharge cycles are equal to one full discharge cycle. Although partial- discharge cycles can number in the thousands, keeping the battery in a fully charged state also shortens battery life. Full discharge cycles (down to 2.5 V or 3 V, depending on chemistry) should be avoided if possible.
Avoid charging to 100% capacity. Selecting a lower float voltage can do this. Reducing the float voltage will increase cycle life and service life at the expense of reduced battery capacity. A 100-mV to 300-mV drop in float voltage can increase cycle life from two to five times or more. Li-ion cobalt chemistries are more sensitive to a higher float voltage than other chemistries. Li-ion phosphate cells typically have a lower float voltage than the more common Li-ion batteries.
Select the correct charge termination method. Selecting a charger that uses minimum charge-current termination (C/10 or C/x) can also extend battery life by not charging to 100% capacity. For example, ending a charge cycle when the current drops to C/5 is similar to reducing the float voltage to 4.1 V. In both instances, the battery is only charged to approximately 85% of capacity, which is an important factor in battery life.
Limit the battery temperature. Limiting battery-temperature extremes extends battery life, especially prohibiting charging below 0°C. Charging below 0°C promotes metal plating at the battery anode, which can develop into an internal short, producing heat and making the battery unstable and unsafe. Many battery chargers have provisions for measuring battery temperature to assure charging does not occur at temperature extremes.
Avoid high charge and discharge currents. High charge and discharge currents reduce cycle life. Some chemistries are more suited for higher currents such as Li-ion manganese and Li-ion phosphate. High currents place excessive stress on the battery.
Avoid very deep discharges (below 2 V or 2.5 V). Very deep discharges will quickly, permanently damage a Li-ion battery. Internal metal plating can occur causing a short circuit, making the battery unusable and unsafe. Most Li-ion batteries have protection circuitry within their battery packs that open the battery connection if the battery voltage is less than 2.5 V or exceeds 4.3 V, or if the battery current exceeds a predefined threshold level when charging or discharging.
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.