ProfileNi-MH BatteryDesigning for Ni-MH Cells
  DESIGNING FOR NI-MH CELLS
 Ni-MH BATTERY
Specification tables
Incorporation of nickel-metal hydride cells into applications is generally straightforward, particularly for designers accustomed to designing with nickel-cadmium cells. Primary differences between the two cell chemistries are:>Nickel-metal hydride cells offer higher energy densities.>Environmental and occupational health issues relating to cadmium are eliminated with nickel-metal hydride cells.>More care is required in design of nickel-metal hydride charging systems.>Since nickel-metal hydride cells may emit hydrogen in heavy overcharge or overdischarge, both charge-control redundancy and location of the battery package in the product deserve careful scrutiny.>Nickel-metal hydride cells have yet to offer the wealth of sizes and design variations found in the mature nickel-cadmium line.
 1. Capacity Guide
A convenient aid to early analysis of battery systems is the cell selection guide shown in Figure 25. This chart allows estimation of the run times available from specified cell sizes when exposed to a given constant discharge rate.
Overview
Features
Comparison of Ni-MH
   and Ni-Cd Cells
Major applications
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 
 
Figure 25.Nickel-Metal Hydride Cell Selection Guide
manufacturer at Included on the chart are nickel-metal hydride cell sizes available from the the publication date. Other sizes are being added rapidly; consult the manufacturer for an updated capacity guide covering existing offerings. Note that comparison information is also provided for one size of nickel-cadmium cell to allow estimation of the actual performance increment achieved with nickel-metal hydride cells.Typical use for the capacity guide is to enter the guide with a given discharge rate. The intersection of that discharge rate with the performance line for each cell size then indicates the amount of run time nominally available from that cell. The values provided by this guide should be used for planning purposes only; final cell selection should be based on actual discharge times obtained from testing under realistic application scenarios.
  2. Materials of Construction
 
The materials of construction for the nickel-metal hydride cell external surfaces are, like the nickel-cadmium cell, largely comprised of nickel-plated steel, and therefore, are resistant to attack by most environmental agents.
 
 
  3. Orientation
 
Nickel-metal hydride cells will operate satisfactorily in any orientation.
  4. Environmental Suitability
 
The nickel-metal hydride cell is designed to operate effectively in all environments normally experienced by portable electronic equipment. Application designers intending to use nickel-metal hydride cells in especially adverse environments should consult closely with the cell manufacturer to ensure design suitability.
 
 
 
  5. Temperature
 
Like most other battery cells, nickel-metal hydride cells are most comfortably applied in a near-room-temperature environment(-25 celsuis); however, with careful attention to design parameters, they can be successfully utilized when exposed to a much wider range of temperatures.
 
 
 
  Operating
Nickel-metal hydride cells can be successfully applies in temperatures from 0 to 50 celsius with appropriate derating of capacity at both the high and low ends of the range. Design charging systems to return capacity in high or low temperature environments without damaging overcharge requires special attention.
Storage
Cells are best stored in temperatures from 0 to 30 celsius although storage for limited periods of time at higher temperatures is feasible.
6. Shock and Vibration
Expect nickel-metal hydride cells to easily withstand the normal shock and vibration loads experienced by portable electronic equipment in day-to-day handling and shipping. Consult with GREENCELL regarding applications required operation in more intense shock and vibration environments.
7. Ventilation and Isolation
The primary gas emitted from the nickel-metal hydride cell when subjected to excessive overcharge is hydrogen as opposed to oxygen for the nickel-cadmium cell. Although venting of gas to the outside environment should not occur in a properly designed application, isolation of the battery compartment from other electronics(especially mechanical switches that might generate sparks)and provision of adequate ventilation to the compartment are required to eliminate concerns regarding possible hydrogen ignition.Isolation of the battery from heat-generating componetry and ventilation around the battery will also reduce thermal stress on the battery and ease design of appropriate charging systems.
8. Termination
Since the exterior of the nickel-metal hydride cell is nearly identical to that of the nickel-cadmium cell, all termination procedures accepted for the nickel-cadmium cell apply equally well to the nickel-metal hydride cell. The recommendation against use of mechanical(pressure)contacts in favor of welded terminations, especially to nickel- metal hydride cells. The prohibition against soldering directly to the cell to prevent heat damage to plastic seal components also applies.
9. Other Selections Considerations
To date, applications for nickel-metal hydride cells have been focused on electronics that have nominal drain rates of 2C or less. As a result, cell internal current-carrying components such as tabs and current collectors have not been designed for high currents such as found in portable tools and appliances. Although there appear to be no intrinsic constraints on discharge rates imposed by cell chemistry, existing cell designs are for applications with maximum currents of less than 4C.