GREENCELL-laptop battery,Camcorder battery,cameras battery

Proper charging of nickel-metal hydride cells is the key to satisfaction with their performance in any product. A successful charging scheme balances the need for quick, thorough charging with the need to minimize overcharging, a key factor in prolonging life. In addition,a selected charging scheme should be economical and reliable in use.In general, the nickel-metal hydride cell appears to be more sensitive to charging conditions than the nickel-cadmium cell. It also has yet to develop the volume of operational data that guides design of nickel-cadmium chargers. For these reasons, charging strategies should be selected and charging parameters established in consultation with the cell manufacturer. One advantage today¡¯s application designers do have in developing chargers for nickel-metal hydride cells is the increasing availability of packaged charger circuits.
1. Charging Summary
The keys to successful charging of nickel-metal hydride cells are:
>Use a three-step charging strategy to speed return to service while minimizing excessive overcharge.
>Design for more subtle indications of entry into overcharge.
>Use redundant fast-charge termination techniques.
>Provide fail-safe charge-termination backup(thermal fuse, etc.).When these guidelines are followed, nickel-metal hydride cells can be quickly and reliably charged while maximizing cycle life.

Unlike discharge performance where the behavior of nickel-metal hydride cells andtraditional nickel-cadmium cells is very similar, there are significant differences in behavior on charge between the two cell types that relate to basic electrochemical differences. Specifically nickel-cadmium cells are endothermic on charge while nickel-metal hydride cells are exothermic. This difference is manifested in the interrelationships among voltage, pressure, and temperature as discussed below.

Figure 17 sketches typical behavior of a nickel-metal hydride cell being charged at the C rate. These curves both indicate why charge control is important and illustrate some of the cell characteristics used to determine when charge control should be applied.The voltage spikes up on initial charging then continues to rise gradually through charging until full charge is achieved. Then as the cell reaches overcharge, the voltage peaks and then gradually trends down.Since the charge process is exothermic, heat is being released throughout charging giving a positive slop to the temperature curve. When the cell reaches overcharge where the bulk of the electrical energy input to the cell is converted to heat, the cell temperature increases dramatically.Cell pressure, which increases somewhat during the charge process, also rises dramatically in overcharge as greater quantities of gas are generated at the C rate than the cell can recombine. Without a safety vent, uncontrolled charging at this rate could result in physical damage to the cell.

Figure 17.Nickel-Metal Hydride Cell Charging Characteristics

The effect of temperature on charging efficiency(the increase in cell capacity per unit of charge input)is one area of difference between nickel-metal hydride and nickel-cadmiumcells. Specifically charge acceptance in the nickel-metal hydride cell(as shown in Figure 18)decreases monotonically with rising temperature beginning below 20 Celsius and continuing through the upper limits of normal cell operation. This contrasts with the nickel-cadmium cell which has a peak in charge acceptance in the vicinity of room temperature. With either cell type, the drop in charge acceptance at higher temperatures remains a significant concern to product designers who are mounting the cells in close proximity to heat sources or in compartments withlimited cooling or ventilation.

Figure 18.Effect of Charge Temperature on Discharge Capacity

Figure 19 indicates that the charge acceptance efficiency for the nickel-metal hydride cell is improved as the charging rate is increased.

Figure 19.Effect of Charge Rate on Charge Acceptance

Determining when overcharge has occurred is critical to charging schemes that minimize the amount of time spent at high charge rates in overcharge. In turn, these efficient charging techniques are a key to maximizing cell life, as will be discussed later. Primary charge control schemes typically depend on sensing either the dramatic rise in cell temperature illustrated in Figure 20 or the peak in voltage show in Figure 21. Charge control based on temperature sensing is the most reliable approach to determining appropriate amounts of charge for the nickel-metal hydride cell. Temperature-based techniques are thus recommended over voltage-sensing control techniques for the primary charge control mechanism.

Figure 20.Temperature Profiles During Charge

Figure 21.Voltage Profiles During Charge

Today's trend to faster charge times requires higher charge rates than the 0.1 to 0.3C rates often ecommended for many nickel-cadmium charging systems. Both Figures 20 and 21 indicate that fast-charge rates serve to accentuate the slope changes used to trigger both the temperature and voltage-related charge terminations. A charge rate of 1C is recommended for restoring a discharge cell to full capacity. For charging schemes that then rely on a timed "topping"charge to ensure complete charge, a rate of 0.1C appears to balance adequate charge input with minimum adverse effects in overcharge. Finally a maintenance(or trickle)charge rate of 0.025C(C/40) is adequate to counter self-discharge and maintain cell capacity.

Products using nickel-metal hydride cells often make use of the sophistication of today's chip-level packaged charging systems to tailor the charging profile to fast capacity recovery while minimizing overcharge stress. Two general classes of strategies have evolved:

This approach uses a timer to switch from the initial charge rate to the maintenance charge rate. Because there is no sensing of the cell's transition into overcharge, the charge rate must be kept low(0.1C)to minimize overcharge-related impact on cell performance and life. Charge durations are typically set at 16 to 24 hours to ensure full recharge in cases of complete discharge. Although economical, since this scheme makes no allowance for the degree of discharge or for environmental conditions, its use is rarely recommended for typical nickel-metal hydride applications.

Here a fast charge restores approximately 90 percent of the discharged capacity, an intermediate timed charge completes the charge and restores full capacity, then a maintenance charge provides a continuous trickle current to balance the cells and compensate for self-discharge. The fast charge(with currents in the 1C range)is typically switched to the intermediate charge using a temperature-sensing technique which triggers at the onset of overcharge. The intermediate charge normally consists of a 0.1C charge for a timed duration selected based on battery pack configuration. This intermediate-charge replaces the need to fast-charge deeply into the overcharge regime to ensure that the cell has received a full charge. Three-step charging, such as illustrated in Figure 22, requires greater charger complexity(to incorporate a second switch point and third charge rate)but reduces cell exposure to life-limiting overcharge.

Because of the sensitivity of cell life to overcharge history and the greater subtlety of some of the overcharge transitions, charge termination redundancy in charger design is recommended. This applies to both built-in redundant charge control techniques and fail-safe charge termination techniques such as thermal fusing. Both of these considerations are discussed in more detail in the cell and battery design sections.

Use of charge control based on the temperature rise accompanying the transition of the cell to overcharge is generally recommended because of its reliability(when compared to voltage peak sensing techniques)in sensing overcharge. However, temperature sensing is typically more expensive to implement than voltage sensing since it requires additional sensors. The exothermic nature of the nickel-metal hydride charge process(as illustrated in Figure 20)results in increasing temperature throughout charging. This requires care in selection of setpoints to avoid premature charge termination.

Figure 22.Recommended Charge Regime for Nickel-Metal Hydride Cells

Charge switching based on the change in slope of the temperature profile eliminates much of the influence of the external environment and can be a very effective technique for early detection of overcharge in a three-step charging scheme.

The simple form of temperature-based switching is to use an absolute increment in temperature from the start of charging, e.g. a 20¡æ increase in cell temperature from onset of charge. The chosen DT has to account for both normal temperature gain during charge and the spike at overcharge. Selection of the proper temperature increment can be greatly influenced by the environment surrounding the cell. Thus it should be done based on bench testing of the cell in the application and done after consultation with the cell manufacturer.

Charge switching based on the absolute cell temperatur as opposed to temperature increment¡¿is subject to varying use patterns¡ªAlaska or the Sahara¡ªand is recommended only as a fail-safe strategy to avoid destructive heating in case of failure of the primary switching strategy.

Charge control based on voltage changes is attractive because it can be accomplished using only existing leads to the battery, eliminating the expense and complexity of additional temperature-sensing leads to the cell. However, the voltage peak typically occurs later in the overcharge process, the voltage overcharge is not as distinct as that seen with temperature, and the voltage behavior may change with cycling. For these reasons, most product designers choose to use voltage-sensing techniques only as backups to temperature-based control.

Despite the concerns voiced above, Figure 21 does indicate a significant knee to the voltage early in overcharge when charging at the 1C rate. Sensing this slope change in a dV/dt£¨or delta v/delta t¡¿system can provide an effective economical approach to detecting early entry to overcharge.

Sensing the absolute voltage rise, if carefully performed, can be a useful charge control strategy. It can be most easily utilized if cells are usually fully discharged prior to recharge. This approach is subject to the same caveats mentioned previously regarding consultationand bench-level verification.

Since the voltage does peak during overcharge, switching on the voltage decrease is feasible. This eliminates the concerns faced in both voltage and temperature increment methods about determining the increment that ensures charge return without excessive overcharge.

Charge control through the absolute value of the voltage is relatively imprecise and unsuited for primary charge-control techniques. It can be used as a redundant control technique in, for example, a dV/dt scheme.

Timer-controlled charging systems are the simplest and most economical of all charging strategies. However, to avoid adverse effects on cell life and performance, charging rates must be limited to 0.1C, which constrains time-based charging to those products where overnight return of charge is acceptable. In typical application scenarios where the degree of discharge varies widely, a charging system using time as the primary control variable will either undercharge or overcharge the battery. However, time-based redundant charge termination and/or time-based control of intermediate charging¡¾topping charge¡¿in a three-step system are often key elements of an integrated charge-control strategy.

The discussions above are most pertinent for devices operating in the room-ambient range.Designers of products predominantly operating at either temperature extreme shouldconsult closely with their cell suppliers in designing their charging system.

Although high-temperature performance¡¾in the 40 to 55¡æ range¡¿is equivalent or even slightly better than the standard nickel-cadmium product, charging of nickel-metal hydride cells in high-temperature environments requires careful attention for two reasons: The selection of setpoints, for both temperature and voltage-sensing systems, can
be affected if the cells are already at elevated temperatures prior to starting charge; Charge duration may have to be extended due to the charge acceptance inefficiencies illustrated in Figure 19.

Even though low temperature charge acceptance is better for the nickel-metal hydride cell than for nickel-cadmium cells, designers must ensure that low temperatures do not adversely affect their charge-control scheme. The charge time increases at lower temperatures so charge durations must be carefully considered to provide adequate low-temperature charging while avoiding excessive charge at normal temperatures. Charge rates must also be reduced at low temperatures. An upper limit of 0.1C is recommended below 15 selsius. Charging below 0 celsius is not advisable. Consult the factory for more details on low-temperature charging.

Traditionally, application designers tailored their charging system to their application. With the rapid evolution of chip-based charging circuitry, designers can now use standardized designs providing a sophisticated charging scheme while allowing the designer wide latitude in selecting charge parameters. Such systems are available from a variety of sources including both cell manufacturers and integrated-circuit design houses, in forms ranging from basic chip to complete charger packages.