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

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


Much emphasis has been put on increasing Li-ion battery capacity toprovide the longest product runtime in the smallest physical size. Butthere are instances where a longer battery life, an increased number ofcharge cycles or a safer battery is more important than batterycapacity.

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

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

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

The basics
Before covering the battery charger's role in extending battery life, aquick review of the Li-ion battery is necessary. Lithium – -one of the lightest andmost 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 insteaduses lithium ions that shuttle back and forth between the cathode andanode of the battery during charge and discharge.

Although there are many different types of Li-ion batteries, themost popular chemistries presently in production can be narrowed downto 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 dischargecurrents, improved safety, or where cost is the driving factor.

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

Table1: The advantages and disadvantages offered by Li-ion batteries basedon cathode material used are listed.

Unlike some other battery chemistries, Li-ion battery technology isnot yet mature. Research is ongoing with new types of batteries thathave even higher capacities, longer life and improved performance thanpresent day batteries. Table 1 above highlights some important characteristics of each battery type.

A Li-ion polymer battery is charged, discharged and hascharacteristics similar to a standard Li-ion battery. The maindifference between the two is that a solid ion conductive polymerreplaces the liquid electrolyte used in a standard Li-ion battery,although most polymer batteries also contain an electrolyte paste tolower the internal cell resistance.

Eliminating the liquid electrolyte allows the polymer battery to behoused in a foil pouch rather than the heavy metal case required forstandard Li-ion batteries. Li-ion polymer batteries are gainingpopularity based on their cost-effectiveness to produce and theirflexibility for fabricating in many different shapes, including verythin.

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 bewhen the battery capacity drops to 80 percent of the rated capacity.However, batteries can still deliver usable power below 80 percentcharge capacity, even though runtime is shortened.

The number of charge/discharge cycles is commonly used whenreferring to battery life, but cycle life and battery life (or servicelife) can be different lengths of time. Charging and discharging willeventually reduce the battery's active material and cause otherchemical changes, resulting in increased internal resistance andpermanent capacity loss. But permanent capacity loss also occurs evenwhen the battery is not in use.

<>Permanent capacity loss is greatest at elevated temperatureswith thebattery voltage maintained at 4.2V (fully charged). For maximum storagelife, batteries should be stored with a 40 percent charge (3.6V) at40°F (refrigerator). Perhaps one of the worst locations for aLi-ion battery is in a laptop computer when used daily on a desktopwith the charger connected.

Laptops and portable devices typically run warm or even hot, raisingthe battery temperature, and the charger is maintaining the batterynear 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 poweringthe laptop when the computer is used on a desktop. A properly cared forlaptop battery can have a service life of two to four years, or more.

Loss of capacity
There are two types of battery capacity losses, recoverable loss andpermanent loss. After a full charge, a Li-ion battery will typicallylose about 5 percent capacity in the first 24hrs, then approximately 3percent per month because of self-discharge and an additional 3 percentper month if the battery pack has pack protection circuitry.

These self-discharge losses occur when the battery remains around20°C, but will increase considerably with higher temperature andalso as the battery ages. This capacity loss can be recovered byrecharging the battery.

Permanent capacity loss, as the name implies, refers to permanentloss that is not recoverable by charging. This loss is linked tobattery life because when the permanent capacity loss drops toapproximately 80 percent, the battery is considered at the end of itslife.

Permanent capacity loss is mainly due to the number of fullcharge/discharge cycles, the battery voltage and battery temperature.The more time the battery remains at 4.2V or 100 percent charge level(or 3.6V for Li-ion Phosphate) the faster the capacity loss occurs.

This is true whether the battery is being charged or just in a fullycharged condition with the voltage near 4.2V. Always maintaining aLi-ion battery in a fully charged condition will shorten its lifetime.The chemical changes that shorten the battery lifetime begin when it ismanufactured, and these changes are accelerated by high float voltageand high temperature.

Permanent capacity loss is unavoidable, but it can be held to aminimum by observing good battery practices when charging, dischargingor simply storing the battery. Using partial discharge cycles cangreatly increase cycle life and charging to less than 100 percentcapacity can increase battery life even further.

Cycle life extenders
There isn't any one factor that increases or decreases battery life,but it often is a combination of several. For increased cycle life:

1. Use partialdischarge cycles . Using only 20 percent or 30 percent of thebattery capacity before recharging will extend cycle life considerably.As a rule, 5 to 10 shallow discharge cycles are equal to one fulldischarge cycle.

Although partial discharge cycles can number in the thousands,keeping the battery in a fully charged state also shortens batterylife. Full discharge cycles (down to 2.5V or 3V, depending onchemistry) should be avoided if possible.

2. Avoidcharging to 100 percent capacity. Selecting a lower floatvoltage can do this. Reducing the float voltage will increase cyclelife and service life at the expense of reduced battery capacity.

A 100mV to 300mV drop in float voltage can increase cycle life from2 to 5X or more. Li-ion Cobalt chemistries are more sensitive to ahigher float voltage than other chemistries. Li-ion Phosphate cellstypically have a lower float voltage than the more common Li-ionbatteries.

3. Select thecorrect charge termination method. Selecting a charger that usesminimum charge current termination (C/10 or C/x) can also extendbattery life by not charging to 100 percent capacity.

For example, ending a charge cycle when the current drops to C/5 issimilar to reducing the float voltage to 4.1V. In both instances, thebattery is only charged to approximately 85 percent of capacity, whichis an important factor in battery life.

4. LimitBattery temperature. Limiting battery temperature extremesextends battery life, especially prohibiting charging below 0°C.

Charging below 0°C promotes metal plating at the battery anodewhich can develop into an internal short, producing heat and make thebattery unstable and unsafe. Many battery chargers have provisions formeasuring battery temperature to assure charging does not occur attemperature extremes.

5. Avoid high charge anddischarge currents, as they reduce cycle life. Some chemistriesare more suited for higher currents such as Li-ion manganese and Li-ionPhosphate. High currents place excessive stress on the battery.

6. Avoid verydeep discharges below 2V or 2.5V. This will quickly,permanently damage a Li-ion battery. Internal metal plating can occurcausing a short circuit, making the battery unusable and unsafe.

Most Li-ion batteries have electronic circuitry within the batterypack that open the battery connection if the battery voltage is lessthan 2.5V, exceeds 4.3V, or if the battery current exceeds a predefinedthreshold level when charging or discharging.

Charging methods
The recommended way to charge a Li-ion battery is to provide a±1 percent voltage-limited constant current to the battery untilit becomes fully charged, and then stop.

Methods used to determine when the battery is fully charged includetiming the total charge time, monitoring the charge current, or acombination of the two.

The first method applies a voltage-limited constant current, rangingfrom C/2 to 1C for 2.5 to 3hrs, thus bringing the battery up to 100percent charge. A lower charge current can also be used, but willrequire more time.

The second method is similar but it requires monitoring the chargecurrent. As the battery charges, the voltage rises, exactly as in thefirst method. When it reaches the programmed voltage limit, which isalso called the float voltage, the charge current will begin to drop.

When it first begins to drop, the battery is about 50 percent to 60percent charged. The float voltage continues to be applied until thecharge current drops to a sufficiently low level (C/10 to C/20) atwhich time the battery is approximately 92 percent to 99 percentcharged and the charge cycle ends.

Presently, there is no safe method for fast charging (less than 1hr)a standard Li-ion battery to 100 percent capacity. Applying acontinuous voltage to a battery after it is fully charged is notrecommended, as it will accelerate permanent capacity loss and maycause internal lithium metal plating. This plating can develop into aninternal short circuit, resulting in overheating and making the batterythermally unstable. The length of time required is months.

Figure1: A typical Li-ion battery charge profile is shown, with chargecurrent, battery voltage and battery capacity vs. time.

Some Li-ion battery chargers allow a thermistor to be used tomonitor battery temperature. The main purpose is to prevent charging ifthe battery temperature is outside the recommended window of 0°C to40°C.

Unlike NiCd or NiMH batteries, Li-ion cell temperature rises verylittle when charging. Figure 1 above showscharge current, battery voltage, and battery capacity vs. time for atypical Li-ion charge profile.

The letter “C” is a battery term used to indicate the batterymanufacturers stated battery discharge capacity, which is measured inmilliampere-hours.

For example, a 2,000mAhr rated battery can supply a 2000mA load for1hr before the cell voltage drops to its zero capacity voltage. In thesame example, charging the battery at a C/2 rate would mean charging at1,000mA (1A).

The letter “C” becomes important in battery chargers because itdetermines the correct charge current required and the length of timeneeded to fully charge a battery. When discussing minimum chargecurrent termination methods, a 2,000 mAhr battery using C/10termination will end the charge cycle when the charge current dropsbelow 200mA.

Figure2: The relationship between cell capacity and cycle life is shown.

Float voltage factors
The main determining factor is the electrochemical potential of theactive materials used in the battery's cathode, which for lithium isapproximately 4V.

The addition of other compounds will raise or lower this voltage.The second factor is a trade-off between cell capacity, cycle life,battery life and safety. The curve shown in Figure 2 above shows therelationship between cell capacity and cycle life.

Most Li-ion manufacturers have set a 4.2V float voltage as the bestbalance between capacity and cycle life.

Using 4.2V as the constant voltage limit (float voltage), a batterycan typically deliver about 500 charge/discharge cycles before thebattery capacity drops to 80 percent. One charge cycle consists of afull charge to a full discharge. Multiple shallow discharges add up toone full charge cycle.

Although charging to a capacity less than 100 percent using either areduced float voltage or minimum charge current termination will resultin initial reduced battery capacity, as the number of cycles increasesbeyond 500, the battery capacity of the lower float voltage can exceedthe higher float voltage.

Figure 3 below illustrateshow the recommended float voltage compares with a reduced float voltagewith regard to capacity and the number of charge cycles.

Figure3: The recommended float voltage is compared with a reduced floatvoltage with regard to capacity and the number of charge cycles.

Because of the different Li-ion battery chemistries and otherconditions that can affect battery life, the curves shown here are onlyestimates of the number of charge cycles and battery capacity levels.

Even a similar battery chemistry from different manufacturers canhave dramatically different results due to minor differences in batterymaterials and construction methods.

Battery manufacturers specify a charge method and a float voltagethe end user must use to meet the battery specifications for capacity,cycle life and safety. Charging above the recommended float voltage isnot recommended.

Many batteries include a battery pack protection circuit, whichtemporarily opens the battery connection if the maximum battery voltageis exceeded.

Once opened, connecting the battery pack to the charger willnormally reset the pack protection. Battery packs often have a voltageprinted on the battery, such as 3.6V for a single cell battery. Thisvoltage is not the float voltage, but rather the average batteryvoltage when the battery is discharging.

Choosing a charger
Although a battery charger has no control over a battery'sdepth-of-discharge, discharge current and battery temperature, all ofwhich have an effect on battery life, many chargers have features thatcan increase battery life, sometimes dramatically.

A battery charger's role in extending battery lifetime is mainlydetermined by the charger's float voltage and charge terminationmethod.

Many Linear Technology Li-ion chargers feature a ±1 percent(or lower) fixed float voltage of 4.2V, but there are some offerings in4.1V and 4.0V, as well as adjustable float voltages. Table 2 below lists battery chargersthat feature a reduced float voltage that can increase battery lifewhen used to charge a 4.2V Li-ion battery.

Table2: Battery chargers that feature a reduced float voltage that canincrease battery life when used to charge a 4.2V Li-ion battery arelisted.

Battery chargers that do not offer lower float voltage options arealso capable of increasing battery life. Chargers that provide minimumcharge current termination methods (C/10 or C/x) can provide a longerbattery life by selecting the correct charge current level at which toend the charge cycle.

A C/10 termination level will only bring the battery up to about 92percent capacity, but there will be an increase in cycle life. A C/5termination level can double the cycle life although the battery chargecapacity drops even further to approximately 85 percent.

Table 3 below contains anumber of chargers that provide either C/10 (10 percent currentthreshold) or C/x (adjustable current threshold) charge terminationmode.

Table3: Battery chargers that provide minimum charge current terminationmethods (C/10 or C/x), such as listed above, provide a longer batterylife by selecting the charge current level at which to end the chargecycle.

Longer runtime or longer batterylife?
With present battery technology and without increasing battery size,the answer is no. For maximum runtime, the charger must charge thebattery to 100 percent capacity.

This places the battery voltage near the manufacturer's recommendedfloat voltage, which is typically 4.2V ±1 percent.Unfortunately, charging and maintaining the battery near these levelsshortens battery life.

One solution is to select a lower float voltage, which prohibits thebattery from achieving 100 percent charge, although this would requirea higher capacity battery to provide the same runtime. Of course, inmany portable products, a larger sized battery may not be an option.

Also, using a C/10 or C/x minimum charge current termination methodcan have the same effect on battery life as using a lower floatvoltage. Reducing the float voltage by 100mV will reduce capacity byapproximately 15 percent, but can double the cycle life.

At the same time, terminating the charge cycle when the chargecurrent has dropped to 20 percent (C/5) also reduces the capacity by 15percent and achieves the same doubling of cycle life.

Figure4: Most Li-ion batteries use either a petroleum-based coke material orgraphite. The latter produces a flatter discharge voltage, then dropsquickly.

Discharge voltage drop
As expected, during discharge, the battery voltage will slowly drop.The discharge voltage profile vs. time depends on a number of factors,including discharge current, battery temperature, battery age and thetype of anode material used in the battery.

Presently, most Li-ion batteries use either a petroleum based cokematerial or graphite.The voltage profiles for each are shown in Figure 4 above. The more widely usedgraphite material produces a flatter discharge voltage between 20percent and 80 percent capacity, then drops quickly near the end,whereas the coke anode has a steeper voltage slope and a lower 2.5Vcutoff voltage.

The approximate remaining battery capacity is easier to determinewith a coke material by simply measuring the battery voltage.

Parallel connections
For increased capacity, Li-ion cells are often connected in parallel.No special requirements are needed, other than the batteries should bethe same chemistry, manufacturer and size.

Series connected cells require more care because cell capacitymatching and cell balancing circuitry is often required to assure thateach cell reaches the same float voltage and the same level of charge.

Connecting two cells that have individual pack protection circuitryin series is not recommended because a mismatch in capacity can resultin one battery reaching the over-voltage limit, thus opening thebattery connection. Multicell battery packs should be purchasedassembled with the appropriate circuitry from a battery manufacturer.

The lifetime of a Li-ion battery is determined by many factors ofwhich the most important are battery chemistry, depth of discharge,battery temperature and battery capacity termination level.

<>Charging a battery to the manufacturer's suggested 100 percentcapacity level will provide the stated number of full charge/ dischargecycles. Applications requiring increased battery lifetime will benefitgreatly by selection of a charger that allows charging to less than 100percent capacity.

This is achieved by selecting a battery charger that features alower float voltage or one that terminates earlier in the charge cycle.

Fran Hoffart is an ApplicationsEngineer at LinearTechnology Corp.

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