ProfileNi-MH BatteryDischarge Characteristics
  DISCHARGE CHARACTERISTICS
 Ni-MH BATTERY
Specification tables
The discharge behavior of the GREENCELL nickel-metal hydride cell is generally well-suited to the needs of today's electronic products - especially those requiring a stable voltage for extended periods of operations.
Overview
Features
Comparison of Ni-MH
   and Ni-Cd Cells
1. Definitions of Capacity
Major applications
The principal battery parameter of interest to a product designer is usually the run time available under a specified equipment use profile. While establishing actual run times in the product is vital prior to final adoption of a design, battery screening and initial design are often performed using rated capacities. Designers should thoroughly understand the conditions under which a cell rating is established and the impact of differences in rating conditions on projected performance.The standard cell rating, often abbreviated as C, is the capacity obtained from a new, but thoroughly conditioned cell subjected to a constant-current discharge at room temperature after being optimally charged. Since cell capacity varies inversely with the discharge rate, capacity ratings depend on the discharge rate used. For nickel-metal hydride cells, the rated capacity is normally determined at a discharge rate that fully depletes the cell in five hours.The published C value may reflect either an average or minimum value for all cells. Typically nickel-cadmium cells are rated based on minimum values while nickel-metal hydride cells are rated on average values. The difference between the two values may be significant (Approximate to 10 percent)depending on the variability in the manufacturing process.Many charge and discharge parameters are normalized by the C rate since cellperformance within a family of varying cell sizes and capacities is often identical when compared on the C basis.
Structural designs
Electrochemical processes
Discharge characteristics
Charge characteristics
Charging methods
Cycle life characteristics
Storage characteristics
Safety characteristics
Designing for Ni-MH cells
Battey pack designs
Battery pack configurations
  designation system
Precautions for using
  Ni-MH batteries
Battery selection 
 
 
  2. Equivalent Circuit
 
For purposes of electrical analysis of the battery cell, the Thevenin equivalent discharge circuit shown in Figure 9 is often used. This models the circuit as a series combination of a voltage source(a series resistance)Rh=the effective instantaneous resistance(and the parallel combination of a capacitor )Cp=the effective parallel capacitance(and a resistor)Rd=the effective delayed resistance.
 
 
 
 
 
 
Figure 9.Equivalent Discharge Circuit for a Nickel-Metal Hydride Cell
  Eo = effective cell no-load voltage
  Re =(Rh + Rd)= total effective internal resistance
  Rh = effective instantaneous resistance
  Rd = effective delayed resistance
  Cp = effective parallel capacitance
  E = cell termination voltage
 
For steady state purposes, the cell voltage at a given current is Eo - iRe, where Re, the state effective internal resistance, is the sum of Rh and Rd. The transient response is shown in Figure 10 where the initial voltage drops immediately to Eo - iReh and then transfers exponentially(with a time constant =Cp*Rd)to the steady-state voltage. Obviously the process reverses when the load is reduced or removed. For many application the steady-voltage is adequate for describing cell performance since the time constant for most cells is small: usually less than 3 percent of the discharge time. When compared to a nickel-cadmium cell, the steady-state voltage for the nickel-metal hydride cell will be reduced since, although the instantaneous resistance is comparable, the delayed resistance will be on the order of 10 percent higher.
 
 
 
 
 
 
 
 
 
 Figure 10.Example of Transient Voltage Profile for a Nickel-Metal Hydride Cell
 

3. Voltage During Discharge
 
The discharge voltage profile, in addition to the transient effects discussed above, isaffected by environmental conditions, notably discharge temperature and discharge rate. However, under most conditions the voltage curve retains the flat plateau desirable for electronics applications.
 
 
  Shape of Discharge Curve
  A typical discharge profile for a cell discharged at the 5-hour rate(the 0.2C rate)is shown in Figure 11. The initial drop from an open-circuit voltage of approximately 1.4 volts to the 1.2 volt plateau occurs rapidly.
 
 
 
 
Figure 11.Typical Discharge Voltage Profile for a Nickel-Metal Hydride Cell
 
Then, as with nickel-cadmium cells, the nickel-metal hydride cell exhibits a sharp "knee" at the end of the discharge where the voltage drops quickly.As can be seen by the flatness of the plateau and the symmetry of the curve, the mid-point voltage(MPV - the voltage when 50 percent of the available capacity is discharged)provides a useful approximation to average voltage throughout the discharge.
 
 
 
  Environmental Effects
 
The principal environmental influences on the location and shape of the voltage profile are the discharge temperature and discharge rate.As indicated in Figure 12, small variations from room temperature(¡À10 Celsius)so not appreciably affect the nickel-metal hydride cell voltage profile. However major excursions,especially lower temperatures, will reduce the mid-point voltage while maintaining the general shape of the voltage profile.
 
 
 
 
 
 
Figure 12. Mid-Point Voltage Variation with Temperature
 
Discharge Rate
 
The effect of discharge rate on voltage profile is shown in Figure 13. There is nosignificant effect on the shape of the discharge curves for rates under 1C; for rates over 1C, both the beginning and ending transients consume a larger portion of the discharge duration.
 
 
 
 
 
Figure 13.Discharge Chracteristics
  4. Discharge Capacity Behavior
 
As with the voltage profile, the capacity available during a discharge is dramatically affected by the cell temperature during discharge and the rate of discharge. The capacity is also heavily influenced by the operating history of the cell, i.e. the recent charge/discharge/storage history of the cell. Obviously a cell can only discharge the capacity which has been returned to it from the previous charge cycle less whatever is lost to self discharge. Charging/charge return issues are discussed in the next section while storage and self-discharge is addressed in a later section.
 
 
 
 
  Effect of Temperature
 
The primary effects of cell temperature on dischargeable capacity, assuming adequate charging, are at lower temperatures(<0)as shown in Figure 14. Use of nickel metal hydride cells in cold environments may force significant capacity derating from room-temperature values.
 
 
 
 
 Figure 14.Variation of Discharge Capacity with Temperature
  Effect of Discharge Rate
 
Figure 15 illustrates the influence of discharge rate on total capacity available. There is no significant effect on capacity for discharge rates below 1C. At the discharge rates above 1C and below the current maximum discharge rate of 4C, significant reductions in voltage delivery occur. This voltage reduction may also result in capacity reduction depending on the choice of discharge termination voltage.
 
 
 
 
 
 
Figure 15. Effect of Discharge Rate on Capacity
  Discharge Application Considerations
 
In general, the discharge behavior of nickel-metal hydride cells closely follows that of similar nickel-cadmium cells used in the same environment. Thus much of the design expertise gathered for nickel-cadmium cells is directly applicable to nickel-metal hydride cells. Discussed below are some specific issues often raised by designers using nickel-metal hydride cells. As the nickel-metal hydride experience base builds, additional information that will help designers optimize the use of nickel-metal hydride cells is becoming available. For this reason, close consultation with the factory during the design effort is encouraged.
 
 
 
 
 
  5. State-of Charge measurement
 
A major issue for users of portable electronics is the run time left before they need torecharge their batteries. Users of portable computers, in particular, expect some
 form of "fuel gauge" to help them determine when they need to save their work. A variety of schemes for measuring state-of-charge have been suggested. In general, experience with nickel-metal hydride cells indicates that, due to the flatness of the voltage plateau under normal discharge rates, voltage sensing cannot be used to accurately determine state-of-charge. To date, the only form of state-of-charge sensing found to consistently give reasonable results is coulometry-comparing the electrical flows during charge and discharge to indicate the capacity remaining.Many devices already have the electronics available to perform sophisticated tracking of charge flows including estimation of self-discharge losses. Some off-the-shelf charging circuitry includes this form of charge tracking as part of  the package.With careful initial calibration and appropriate compensation for environmental conditions, predictions accurate within 5 to 10 percent of actual capacity can be obtained.
 
 
 
 
 
 
 
 
 
 
 
  6. Memory/Voltage Depression
 
The issue of "memory" or voltage depression has been a concern for many designers of devices, using nickel-cadmium cells. In some applications where nickel-cadmium cells are routinely partially discharged, a depression in the discharge voltage profile of approximately 150 mV per cell has been reported when the discharge extends from the routinely discharged to rarely discharged zones. While the severity of this problem in nickel-cadmium cells is open to differing interpretations, the source of the effect is generally agreed to be in the structure of the cadmium electrode. With the elimination of cadmium in the nickel-metal hydride cell, memory is no longer a concern.
 
 
 
 
 
 
  7. Discharge Termination
 
To prevent the potential for irreversible harm to the cell caused by cell reversal in discharge, removal of the load from the cell(s)prior to total discharge is highly recommended. The typical voltage profile for a cell carried through a total discharge involves a dual plateau voltage profile as indicated in Figure 16. The voltage plateaus are caused by the discharge of first the positive electrode and then the residual capacity in the negative. At the point both electrodes are reversed, substantial hydrogen gas evolution occurs, which may result in cell venting as well as irreversible structural damage to the electrodes. It should be noted that the nickel-metal hydride cell, because it uses a negative electrode that absorbs hydrogen, may actually be somewhat less susceptible to long-term damage from cell reversal than the sealed nickel-cadmium cell.The key to avoiding harm to the cell is to terminate the discharge at the point whereessentially all capacity has been obtained from the cell, but prior to reaching the second plateau where damage may occur. Two issues complicate the selection of the proper voltage for discharge termination: high-rate discharges and multiple-cell effects in batteries.
 
 
 
 
 
 
 
 
 
 
 
 
Figure 16.Nickel-Metal Hydride Cell Polarity Reversal Voltage Profile
Voltage Cutoff at High Rates
Normally discharge cutoff is based on voltage drops with a value of 0.9 volts per cell(75 percent of the 1.2 volt per cell nominal mid-point voltage)often being used. As can be seen in Figure 12, 0.9 volts is an excellent value for most medium to long-term discharge applications(<1C). However, again as seen in Figure 13, with high drain-rate usage(1-4C), the change in shape in the voltage curve with the more rounded "knee" to the curve means that an arbitrary 0.9V/cell cutoff may be premature, leaving a significant fraction of the cellcapacity untapped. For this reason, a better choice for voltage cutoff in high-rateapplications is 75 percent of the mid-point voltage at that discharge rate. Note, however, that this choice of end-of-discharge voltage (EODV)is dictated only by considerations of preventing damage to the cell. There may be end-application justification for selection of a higher voltage cutoff with the resulting sacrifice of some potential additional capacity.
Discharge Termination in Batteries
Normal manufacturing variation produces a range of capacities for battery cells. As these cells are combined in batteries, the effects of cell capacity variations are amplified by the number of cells in the battery. Use of termination voltage based on a simple multiple of 0.9V/cell times the number of cells may result in a weaker cell being driven into reverse significantly before the battery reaches the termination voltage. Both charging techniques that minimize the amount of overcharge applied to the cell and frequent repetitive discharging of the battery may exacerbate the problem. The result may be premature battery failure due to the damage caused by reversal of the weak cell. Experience indicates selection of the EODV by the following formula providesacceptable margin to minimize battery failure from repeated cell polarity reversal:
EODV= [(MPV-150mV)(n-1)]-200mV
where MVP is the single-cell mid-point voltage at the given discharge rate and n is the number of cells in the battery.Selection of the proper discharge termination voltage, especially for large batteries or complicated application profiles, should be done in consultation with the cell manufacturer.