The key components within a Ni/Cd cell are the electrodes (positive nickel electrodes and negative cadmium electrodes), a separator between electrodes, electrolyte, a container, and connection points or terminals. Actual design will vary depending on application and requirements. Some of the variations and considerations follow.
All electrodes consist of a support or grid structure
which acts to hold active materials, carry current, and withstand
environments. Grids are usually made of nickel metal or nickel plated steel in
the forms listed in Table 6-1. On this grid, a porous structure of nickel
foam, fibers, or sintered metal may be bonded, sintered, or welded to form a
framework. The active materials are pasted, chemically or electrochemically
deposited, or vacuum deposited onto or into this framework, forming a
TABLE 6-1. GRID
- Pierced foil
- Perforated foil
- Woven screen
- Expanded metal
Electrodes are produced flat. In the cylindrical cell electrodes are
designed to be rolled (often called "jelly-roll") as a single sandwich of
positive electrode, separator, and negative electrode.
Cell capacity or rating is determined by the amount of active material contained within the positive and negative electrodes. In sealed cells, the amount of negative active material exceeds the amount of positive active material by a factor of 1.3 to 1.8. Because of this, at the end of charge the positive electrode reaches full charge and generates oxygen which is recombined by the excess negative material, maintaining low pressure. In vented cells, negative material exceeds the amount of positive material by a factor of 1.1 to 1.25 since gases do not have to be recombined and may vent freely.
The separator material keeps the electrodes apart, holds electrolyte, and allows passage of ions and gases in sealed cells. In vented cells, the separator prevents the passage of gases. The separator of choice in sealed cells is non-woven (felt) nylon. Other materials such as non-woven polypropylene and inorganic fibers have also been used. In vented cells, woven and non-woven nylon or polypropylene is used as the main separator and microporous plastic (such as CELGARD) is often used as the gas barrier.
Separator materials must be resistant to potassium hydroxide electrolyte, have low resistivity when wet, resist the action of gases and metals present, and not introduce any contaminants (such as wetting or bonding agents) into the cell.
Commonly used nylon separator materials deteriorate with extended operation at high temperatures. Batteries for continuous high temperature exposure use special polypropylene or inorganic separators.
The electrolyte is usually a solution of 30 - 35% potassium hydroxide (KOH). A 31% solution has the highest conductivity and is the most common concentration. The concentrated KOH is typically high purity (Mercury Cell grade or better). KOH is diluted to the desired concentration with distilled water (specification O-B-41), analyzed, and kept in storage to minimize absorption of carbon dioxide from the atmosphere. Carbon dioxide forms potassium carbonate which increases cell resistance and lowers discharge voltage.
Sealed cells contain sufficient electrolyte to wet all internal components with little or no "free" electrolyte. This condition, know as starved electrolyte design, allows oxygen gas generated at the positive electrode (as it approaches overcharge) to migrate to the negative electrode where some of the oxygen is recombined into water. Excess oxygen (mainly during high-rate charge) builds up internal pressure in a hermetically sealed cell in the range of 10 to 100 psig.
Vented cells contain an excess of electrolyte and are called flooded cells. As gases are generated on charge, the resealable vent allows their escape along with a small amount of water. Periodically water must be added to maintain electrolyte level above the electrodes. The frequency and amount of water addition will vary with charging method, temperature of operation, and application. A maintenance interval should be established for each application which is based on water addition records. The internal operating pressure of vented cells is in the range 0 - 10 psig.
Features are compared for vented and sealed cells and
summarized in Table 6-2.
TABLE 6-2. SALIENT DESIGN FEATURES OF SEALED AND VENTED
Single or double layer porous
Close electrode spacing
Enhances performance maintenance
Provides short O2 diffusion path to
Shortens ultimate life
negative-to-positive material ratio (1.3/1 to 1.8/1)
cells are capable of unsupported operation at
internal pressures to 100 psig.
1 or 2 layers of woven or non-woven nylon
polypropylene and 1 layer acting as gas barrier
Pocket plate cells, no separators used, but some
Larger electrode spacings provide for longer
Catalytic recombination can reduce water
material ratio (1.1/1 to 1.25/1)
Rectangular plastic cells require
support (usually from
battery container) during charge, pressures range
Sealed cells are contained within a stainless steel or nickel plated steel container. In button and cylindrical cells, the cover is separated from the can by a nylon or polypropylene ring which acts as an insulator. The cover serves as the positive connection area (terminal). The can or body is the negative connection area (terminal). A resealable safety pressure release is integral with the cover.
The prismatic hermetically sealed aerospace cell utilizes 304L stainless steel for the case and cover. It has no vent and utilizes ceramic-to-metal solid nickel feedthroughs for electrical connections.
The cell electrode stack is insulated from the case and cover by polypropylene sheets. Electrical connection is made by a welded internal connection to the case and cover. However, in some low rate button and cylindrical cells the connections may be achieved by pressure contact. This can result in poor high discharge rate performance.
Intercell connections are usually made by spot welded nickel or nickel plated tabs between cells.
Vented cells typically have metal or nylon cases and covers. Terminal connections are nylon or polypropylene insulated studs which are either solid nickel, stainless steel, or nickel plated steel. The pressure relief valve is screwed into the cover. The valve may be plastic or metal. The case to cover bond is welded in both metal and plastic (solvent welded) cased cells.
Button and cylindrical cells are typically spot welded together with nickel or nickel plated steel metal strips. Individual cells may be purchased with the metal strips as an integral component.
Prismatic cells (vented and sealed) may be interconnected with solid metal bus bars or wires held to the threaded terminals with nuts and lockwashers depending on ampere rating and application.
The battery container holds the cells in place during use and handling. Containers may vary from a simple shrink wrap sleeve on button and cylindrical cells to a machined and fabricated complex structure for aerospace cells. Mounting provisions are an integral part of any container and must be configured to the coordination drawing provided by the system integrator. Depending on the application, the container may have a removable cover or be a sealed unit. For vented cell batteries, the cover must be removable so water can be added periodically.
If the container is metal there will be insulating material between the cells and container walls. If aluminium (although not recommended) is used an insulating paint should also be used. The electrolyte (KOH), if leaking, attacks aluminium. The insulation may be potting or encapsulation material, depending on requirements for individual cell replacement.
Some form of connection is needed between the battery and the system. Often, this will be a pigtail lead from each of the positive and negative terminals of the battery. The leads are soldered to terminal posts. Often a Military Specification (MS) style connector is used and will be specified on the system integrator drawing. The battery power connector should preferably be a receptacle type unit and should be properly sized to minimize voltage drop. Provision should be made for color coding, keying, or in some way assuring correct connection where battery removal and replacement are contemplated.
In engine starting and space applications where
operation at low temperatures is required, a heater is often supplied as an
integral part of the battery structure. The heater is thermostatically
controlled and is powered from the system bus. Heater sizing is based on
anticipated environment and is usually covered in a design standard or
Status information on the battery is often desired. Battery voltage and temperature are typically monitored at the system level. Temperature sensors and conditioning circuitry are usually mounted directly in the battery. Voltage is monitored through wires separate from the current lines to avoid line impedance drops.
In some applications, the charger may be provided by the cell/battery manufacturer as an integral part of the battery package. This has the advantage of matching the charger to the battery, but may result in size and weight penalties. Before the decision is made to incorporate an integral charger, a thorough analysis of potential thermal problems and life-cycle costs should be performed.