Recent breakthroughs in nanotechnology have improved the power density and charge/discharge rates for lithium-ion cells, thus opening up applications requiring high power density and high charge/discharge rates. This is, of course, in addition to the traditional battery capacities used in portable entertainment and portable computing applications. The high-power cells are ideal for use in power tools and other motor-driving applications, and deliver the lithium-ion advantages and the low environmental impact (most types qualify for disposal in the regular refuse stream).

As technology makes available concentrated energy devices (high energy density), capable of fast delivery (high discharge rate), safety issues appear. As a reference point, a typical lithium-ion four-cell 2 Ampere-hour (A-h) battery pack stores an energy level of about 100 kJ, while a hand grenade (150g of TNT) has 600 kJ. The potential for personal injury and/or property damage is clear if such amounts of energy are released in a manner that is not controlled.

Enlarge this picture Table 1.

Battery safety has several dimensions. Foremost is to prevent personal injury or property damage. There is also the need to protect the equipment that the battery powers, and to protect the battery itself, since it is an expensive part of the system and may be troublesome or hard to replace if it fails. The mechanical, electrical and chemical design and manufacture of lithium-ion batteries is a mature technology with its safety issues understood. In recent cases in which lithium-ion battery failures made world media headlines, contamination of the chemical components seem to have been the root cause of the failures.

Due to some of the safety concerns, battery vendors do not sell single cell lithium-ion batteries to consumers and even to most OEMs (except for battery pack manufacturers), but rather offer multi-cell battery packs as OEM or consumer products. Packs integrate lithium-ion cells with complex protection electronics. Charging single cells in series or parallel combinations without pack style protections is not advisable.

Battery characteristics

Enlarge this picture Table 2.

Tables 1 and Table 2 summarize the important characteristics of the range of lithium-ion batteries available for mass consumer applications. The first type in the tables is the 18650 interstitial carbon cathode, lithium-cobalt mixed oxide anode, gel electrolyte cell (18 mm diameter, 65 mm long), one of the most popular types for portable electronics applications. The unit, made by Sony, is used here as the representative reference to compare with the newer types, seen in the tables. The cell offers an expected cycle life of 500 charge/discharge cycles, minimum, while retaining a capacity of 80 percent of the nominal value when new (20 percent capacity fade). Cell capacity is between 0.5 A-h and 2 A-h.

The maximum charge and discharge rates are defined by the maximum charge and discharge currents relative to the capacity of the battery, expressed in C units. The term “rate” is used since current is the derivative of electric charge to time. The use of rates in place of straight current values allows the specs to be scaled by the capacity when comparing different battery technologies or different battery sizes for the same technology. For example, a battery subjected to its 1C discharge rate will nominally go from full charge to empty in one hour.

Exceeding the temperature limits of a lithium-ion battery causes severe loss (fade) of the battery’s charge holding ability (capacity). Fade will grow with the time the battery is kept out of the specified temperature range, and the battery will be even more adversely affected if it is discharged or charged under those conditions.

Enlarge this picture Fig. 2a.

Grossly exceeding the charge/discharge rates will cause the opening of the electronic series protection circuit in battery packs, triggered by over-current (short-term, high-current) or over-temperature sensors (long-term). Some of these safety measures can irreversibly disable the battery. Less severe abuses will degrade the battery performance and expected life.

The maximum charge voltage is the tight-tolerance voltage point at which the charge process of a lithium-ion battery must switch from constant current (CC) to constant voltage (CV). The specification is temperature independent. Final charge is very sensitive to small changes of this voltage, as Fig. 1 implies. Exceeding the charging voltage limit has a non-reversible detrimental effect in the battery’s charge-retention ability. The combination of both factors imposes a tight tolerance on the charge voltage limit.

For a present day lithium-ion battery of the Sony type, (see Table 1 and Table 2 for other types) if the voltage between terminals is allowed to reach 4.45 V, the battery life is compromised.

The charging voltage limit (4.2 V for the portable electronics battery type) is the value at which the battery is charged at 100 percent of its nominal capacity when new. If in the charging process, the CC to CV switchover happens at a lower voltage than the nominal charge voltage limit, the charge current will take an early downturn rather than follow the curve in Fig. 1, and the final stored charge will be considerably less than 100 percent of nominal capacity.

Violation of the cut-off discharge minimum voltage specification has different consequences. If a battery is allowed to discharge below this value, a chemical imbalance is created inside the vessel. If at this point the electrochemical state of the battery is disturbed (i.e. max rate charging is attempted) a sudden, abnormal chemical reaction could be triggered that will kill the battery, with a complete loss of the charge-retention ability. All systems developed around a lithium battery as a power source must include a UVLO (under-voltage, lock-out) switch to prevent over-discharge, as well circuitry to prevent high-current charging of deeply discharged cells.

Charging methods

Enlarge this picture Fig. 2b.

With the exception of the voltage source/resistor method used exclusively for very small capacity (a few mA-h) lithium batteries, CCCV (constant current, constant voltage) is the only one universally accepted lithium-ion charging method. A constant current equal to or lower than the maximum charge rate is applied to the battery under charge until the maximum charge voltage is reached. At that point, the charger operating mode turns to constant voltage output, which is maintained across the battery terminals until the charge termination criterion is satisfied.

None of the sophisticated pulse-charge schemes sometimes used with other battery technologies (such as NiCd) reduces full charge time, increases battery life or coulometric efficiency of lithium-ion. In most cases, those charge schemes have detrimental effects. Trickle charge, the practice of forcing a post-charge long term small current through the battery to keep it at the full charge state, is not necessary and is also ill-advised with lithium-ion batteries.

The graph in Fig. 1 also shows the behavior of current, voltage, and stored capacity as the charge process progresses in a Sony cell of the type described in Table 1 and Table 2. Notice the switchover from CC to CV, and the proportions of the charge acquired by the battery in each phase.

Normal charging termination happens when the battery is fully charged. A popular criterion used to consider a lithium-ion battery fully charged — when the maximum charging voltage has been reached and that the current is below a certain fraction (usually 1/30 to 1/10) of the max rate for the battery. Another termination approach uses a time-out, stopping the charging after the charger has been in constant voltage mode for 2 hours. No termination action is also sometimes used. The battery is left connected to the charger in constant voltage mode indefinitely, and it is said to be “floating” because there is no current circulating. There is some capacity fade associated with long-term floating, but it is reversible in the first charge cycle.

When any of the battery parameters go beyond the acceptable range while charging is ongoing, the charger or the pack protection schemes can perform an “abnormal termination” of the charging process. Also, high-end chargers will terminate charging by time-out if any of the expected state transitions (CC‡ CV, or CV‡termination) does not happen within a pre-programmed time period.

Charger implementations

Enlarge this picture Fig. 2C

Chargers for lithium-ion batteries come in a large variety of types and technologies, depending on the characteristics of the particular battery type, number of cells in the pack, type of energy source available and the nature of the application. In all cases the charger is a CCCV power supply, operating in only one quadrant (the charger must not sink current from the battery under any circumstances, powered or non-powered, etc.), with a highly accurate (better than +/- 1 percent) CV output.

The efficiency of the charger power supply defines its size (volume and board surface), while the energy source influences the power supply topology. Decisions among the different power supply technologies (switching, linear) can be made considering the need for heat dissipation, size, EMI constraints, etc. The key component is the charge controller, an integrated circuit in all cases, which includes the CCCV power supply, the precision reference and a state machine at a minimum.

More sophisticated chargers will include functions such as “battery qualification” detecting short-circuit, open, under cut-off or OK state of the battery, sensing the battery temperature, a “prequalification” function in which a small current is applied to the battery until the no-charge voltage level is exceeded, and timeouts for all states of the charger.

Here are some examples of practical designs of lithium-ion chargers, from a very simple low-power unit to more complete types with more bells and whistles. The examples chosen are of the “stand-alone” variety, no separate intelligence or host control is needed to manage the charging.

The common characteristic among the examples is the CCCV charge algorithm previously described, with the same accurate CV part, defining the max charge voltage, and a medium precision control of the charge CC for all. Since the CV accuracy is guaranteed by the chip supplier, external high-cost precision components are not needed. In all cases, both the CC and the CV functions are regulated through the whole range of input voltage.

Differences between the examples consist of the electrical efficiency (the technology and topology used in the IC choice define it), the range (number) of cells that the charger can be set to handle, max current that the part can be set to in the CC function, number and type of protection and automation functions, and the input voltage range. The number of cells and the max current for these chargers are design variables, but hard-wired in production.

Linear chargers are physically larger since their lower efficiency generates more heat and thus the need for heat sinking to transfer the heat away from the regulator chip. However, the electrical circuitry is simpler than chargers based on switching technology. Switch-mode chargers, though, do offer higher conversion efficiency and are smaller since they must dissipate less heat.

A simple linear charging system, based around the MAX1508, requires only a few extra components. (See Fig. 2a.) It can supply up to 0.5 A to charge a single cell. A self-adaptive power/temperature circuit allows it to operate through a wide range of input voltage and power sources. For multi-cell charging, a single-switch “switcher” type regulator, the MAX1873, can handle from two to four cells, delivering from 4 A to 10 A. (See Fig. 2b.) The circuit also provides an analog voltage readout of the current that it is running through the battery.

Offering still more system features, a charger based around the MAX1737 can charge from one to four lithium-ion cells. (See Fig. 2c.) The power core is a high-frequency power supply, which keeps the charger size very small. The MAX1737 uses two external power switches (high-side power switch and synchronous rectifier) to achieve higher efficiency and thus further reduce any generated heat. It can monitor battery temperature, has adjustable time-outs for all functions, and can drive LEDs that provide cycle-point indication (full charge, fast charge, fault). The output current capability of the MAX1737 is defined mostly by the type of external switches used and the external energy source, and could support high C rates, even for the higher battery capacities.

For more information, enter email: alfredo_saab@maximhq.com

References

Handbook of Batteries - 3rd Edition -Linden & Reddy - McGraw-Hill.
Battery Reference Book - 3rd Edition -T.R.Crompton – NEWNES.
Advances in lithium-ion batteries -van Schalkwijk, & Scrosati.-KLUWER.
MAX1508, MAX1873, MAX1737 datasheets @ http://maxim-ic.com.
Batteries in a Portable World – I. Buchanan – Caltex Electronics.
Li-Ion Cells Build Better Batteries for Power Tools–D. Morrison-Power Electronics Technology–Feb 06.