Proper Care Extends Li-Ion Battery Life

Apr 1, 2008 12:00 PM
By Fran Hoffart, Applications Engineer, Linear Technology, Milpitas, Calif.

Boosting Battery Life

Usually, a combination of several factors increases or decreased battery life. For increased cycle life

• 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.

Charging Methods

The recommended way to charge a Li-ion battery is to provide a ±1% voltage-limited constant current to the battery until it becomes fully charged, and then stop. Methods used to determine when the battery is fully charged include timing the total charge time, monitoring the charge current or a combination of the two.

The first method applies a voltage-limited constant current, ranging from C/2 to 1C for 2.5 to 3 hours, thus bringing the battery up to 100% charge. You also can use a lower-charge current, but it will require more time. The second method is similar, but it requires monitoring the charge current. As the battery charges, the voltage rises, exactly as in the first method. When it reaches the programmed voltage limit, which is also called the float voltage, the charge current begins to drop. When it first begins to drop, the battery is about 50% to 60% charged. The float voltage continues to be applied until the charge current drops to a sufficiently low level (C/10 to C/20), at which time the battery is approximately 92% to 99% charged and the charge cycle ends. Presently, there is no safe method for fast charging (less than one hour) a standard Li-ion battery to 100% capacity.

Applying a continuous voltage to a battery after it is fully charged is not recommended, as it will accelerate permanent capacity loss and may cause internal lithium metal plating. This plating can develop into an internal short circuit, resulting in overheating and making the battery thermally unstable. The length of time required is months.

Some Li-ion battery chargers employ a thermistor to monitor battery temperature. Such a monitor's main purpose is to prevent charging if the battery temperature is outside the recommended window of 0°C to 40°C. Unlike NiCd or NiMH batteries, Li-ion cell temperature rises very little when charging. Fig. 1 shows a typical Li-ion charge profile's charge current, battery voltage and battery capacity versus time.

The main determining factor for float voltage is the electrochemical potential of the active materials used in the battery's cathode, which for lithium is approximately 4 V. The addition of other compounds will raise or lower this voltage. The second factor is a tradeoff between cell capacity, cycle life, battery life and safety. The curves in Fig. 2 show the relationship between cell capacity and cycle life.

Most Li-ion manufacturers have set a 4.2-V float voltage as the best balance between capacity and cycle life. Using 4.2 V as the constant voltage limit (float voltage), a battery can typically deliver about 500 charge/discharge cycles before the battery capacity drops to 80%. One charge cycle consists of a full charge to a full discharge. Multiple shallow discharges add up to one full-charge cycle.

Although charging to a capacity less than 100% using either a reduced float voltage or minimum charge-current termination will result in initial reduced battery capacity, as the number of cycles increases beyond 500, the battery capacity of the lower float voltage can exceed the higher float voltage. Fig. 3 illustrates how the recommended float voltage compares with a reduced float voltage with regard to capacity and the number of charge cycles.

Because of the different Li-ion battery chemistries and other conditions that can affect battery life, the curves shown here are only estimates of the number of charge cycles and battery-capacity levels. Even a similar battery chemistry from different manufacturers can have dramatically different results due to minor differences in battery materials and construction methods.

Battery manufacturers specify a charge method and a float voltage the end user must use to meet the battery specifications for capacity, cycle life and safety. Charging above the recommended float voltage is not recommended. Many batteries include a battery-pack protection circuit, which temporarily opens the battery connection if the maximum battery voltage is exceeded. Once opened, connecting the battery pack to the charger will normally reset the pack protection. Battery packs often have a voltage printed on the battery, such as 3.6 V for a single-cell battery. This voltage is not the float voltage, but rather the average battery voltage when the battery is discharging.