The individual parts of a battery will be discussed to provide fundamental information concerning its Construction and the chemical reactions taking place inside it. Each part will be discussed separately, beginning with the basic plate grids and progressing to the complete battery assembly.

PLATE GRID

The grids are the supporting framework for the active material of the plates. They also conduct the current to and from the active material plates. They are made of an alloy of lead. Some alloys use a small amount of antimony to strengthen and stiffen the soft lead. This is necessary so the grids can be handled during the battery manufacturing process without bending or other damage. There is a trend toward lower antimony content in automotive battery grids, with manufacturers producing batteries which require less frequent water addition as a result of the reduced gassing experienced with the lower antimony content. Batteries using grids containing other metals such as calcium or strontium for strength in place of antimony also reduce the gassing, water usage and self-discharge rates of batteries. Small amounts of other metals are used in the alloy to obtain various desired effects. Various grid mesh designs are used. The object of these designs is to use the grid metal more efficiently, placing more metal where the current density is greater and less metal where there is less current flowing.

The first step in making a positive or negative plate is to paste a material (which has a consistency of firm mud) onto a grid. This paste is a mixture of lead oxide, sulfuric acid and water. Other additives, as well as fibers to help bind the active material together, are often incorporated in the paste mix. The main difference between the paste used for the positive and negative plates is that "expanders" are added to the negative paste. These "expanders" are required to prevent the negative material from contracting in service and reverting to a dense, inactive state. After the plates have been pasted and dried (hydroset), the positive plate has a light brown color. The negative plates have a slightly grey color due to the expander material in them. The next step in manufacturing the plates is to give them a "forming" charge while immersed in a dilute sulfuric acid electrolyte solution. This charge electrochemically converts the lead oxide of the positive plate to lead dioxide. This is a highly porous material which allows the electrolyte to freely penetrate the plate. The positive plate is now a dark, chocolate brown color. The same "forming" charge converts the lead oxide of the negative plate to a grey, sponge lead. The "spongy" lead allows the electrolyte to penetrate freely allowing the material beneath the plate surface to take part in the chemical reactions. The negative plates are now a darker grey color. Whether a plate becomes a positive or negative plate depends on the direction of the "forming" charge. If it is made in the reverse direction by accident. the polarity of the formed plates would be reversed. The plates with expanders would be formed into lead dioxide positive plates. The sponge lead negative plates would not contain expander material and the performance of the battery would be greatly impaired.

SEPARATORS

Separators are thin sheets of electrically insulating porous material used as spacers between the plates to prevent short circuits within the cells. Fine pores in the separators allow ionic current flow in the electrolyte between the positive and negative plates. The important characteristics of a separator are uniform plate spacing. oxidation resistance, low electrical resistance, porosity, pore size and distribution, wettability and good acid diffusion. Generally, the ribs on the separator face the positive plate to provide greater acid volume next to the positives and to minimize the area of separator contact. The ribs also provide space to improve acid circulation and to permit any gas formed to rise to the surface of the electrolyte. Some separators are provided with a glass fiber mat which is placed against the positive plate as a retainer for the active material. A highly porous mat provides ample space for acid circulation. Separators are put into batteries in two ways. One is called "leaf construction" and the other is referred to as "envelope construction". Leaf separators are single sheets that are inserted between the. The separators are made of latex-impregnated glass fiber, resin-impregnated cellulose fibers, sintered PVC, silica/polyethylene. or silica/rubber. Commercial and cycling batteries often use silica/polyethylene or silica/rubber separators because the greatly reduced pore size is needed to inhibit dendrite growth. Sometimes, a glass fiber mat may be attached to the ribs to retard the loss of active material under conditions of excessive vibration. In batteries with envelope construction, the separator is generally folded around the bottom of the plate and sealed on the sides. The envelopes are open at the top to allow the gases generated at the plates to escape to the surface of the electrolyte. The plate feet and sediment space can be eliminated if these separators are used. Envelope separators are made from microporous plastic materials, usually the silica/polyethylene type because they can be readily folded and sealed. A third method of battery plate separation is that incorporated in gas recombinant automotive batteries, in which the plates are wrapped in an acid absorptive microglass mat. Either the positive plates or negative plates are wrapped but not sealed.

ELEMENTS

In the most common method of construction, a stack of alternate positive and negative plates is formed with separators between each positive and negative plate. The lugs of the negative plates and a plate strap are welded together as are those of the positive plates. The plate strap of each group of plates is used to connect them in series with the plate group of the next cell, or with a battery terminal. In a second method of construction, the plates and separators for each element are assembled into the final configuration and placed in a "cast-on" machine which forms the straps and attaches the plate lugs to them in a single operation. The assembly resulting from placing one positive plate group and one negative plate group together, with separators, is known as an element. There is one element per cell. Any number or size of plates can be used in the assembly, depending on the desired performance. For example, a greater quantity of plates or larger plates will increase the total plate surface area per element. This will increase the voltage during discharge at high rates such as cranking an engine at low temperatures. The higher the voltage, the faster the starter will crank the engine. However, the open circuit voltage (battery not connected to a charge or discharge) of a single cell will be approximately 2.1 volts regardless of the size or quantity of plates. Therefore, a 12-volt battery has six cells and a 6-volt battery has three cells. The elements are placed in the cells of the container and the post straps of one cell are welded to the post straps of the adjacent cells. This connects the cells in series (positive group to the negative group of the next cell) so the voltage of the battery equals the sum of the voltages of the individual cells. The battery is not active until the electrolyte, a mixture of sulfuric acid and water, is added. This is the last ingredient required for the chemical actions which take place in the battery. The other ingredients are the sponge lead of the negative plate and the lead dioxide of the positive plate. The electrolyte is also the carrier for the electric current to move between the positive and negative plates through the separators.

CELL CONNECTORS

As mentioned previously, the cells of a battery are connected in series and the battery voltage will equal the sum of the cell voltages. Construction techniques commonly used today connect these element terminal posts in series over or through the cell partitions, before the cover is placed on the battery. This type of construction gives a much shorter, lower resistance path through the battery and therefore, higher battery discharge voltage than the external connector construction. It also eliminates acid leaks around external element terminal posts and self-discharge paths which can develop between the external connectors. Some batteries - mostly heavy duty or other special types are still constructed with external cell connectors. In this type of construction, openings are provided in the cell cover for the element terminal posts. After the cover has been added to the battery, the cell connectors are placed over the protruding terminal posts, and are welded to them. Regardless of the type of construction used, the connectors must be large enough to carry high cranking currents.

CELL COVERS

The cell covers are usually made of a plastic material although some are made of hard rubber. Most current designs are one-piece cover construction. Lead bushings are molded in the cover for the two battery terminal posts (not on side terminal design). Some covers are grooved to fit over intercell connectors which go over the top of the partitions. However, the intercell connection is made through the partition in most batteries, in which case a very low cover is generally used. The vent wells of the cover are designed to provide the proper air space above the electrolyte to permit gas to vent from the cell without forcing electrolyte from the battery. There is generally a ring or some other mark near the bottom of the vent well to indicate the proper height to fill the cell when adding water or activating the battery. If there is no level indicator, fill the cell until electrolyte level is 1/2". (l3mm) above the tops of the separators.

CONTAINER

The outside case or shell of the battery is a one-piece, rectangular shaped container with the appropriate number of cells molded of polypropylene, hard rubber or other plastic-like materials. It is designed to: 1. Withstand the temperature extremes of cold and heat. 2. Resist damage caused by mechanical shock in rough road service. 3. Resist acid absorption and chemical attack. The inner bottom portion of the container has element rests or "bridges" running the full length of each cell. These rests may vary according to manufacturers specifications. The plates are at right angles to the element rests. The feet permit the separators to extend below the bottom of the plates' active surface, reducing the possibility of treeing shorts between positive and negative plates. The space below the tops of the rests acts as a sediment chamber for collecting active material shed from the plates. This loss of plate material is a part of the wear process caused by repeated discharging and charging of the battery. If the battery stays in service long enough for the sediment space to fill to the bottom of the element, the shed material will gradually form an electrical path or short circuit between the bottoms of the positive and negative plates. This will interfere with the charging of the battery and it: ability to retain a charge. When the short circuits become severe enough, the battery fails. Normally, the battery fails for other reasons before this condition occurs, however. As mentioned previously, sediment space can be eliminated when envelope separators are employed, since the envelopes protect plates of opposite polarity from becoming bridged, or short-circuited, by the shed material.

COVER TO CONTAINER SEAL

Acid cannot be permitted to leak between the cover and the container to the outside surface of the battery. Neither can it leak between the cover and partitions or shorts between cells will increase the self-discharge rate of the battery. Plastic one-piece covers and plastic containers are usually bonded together by a high temperature and pressure process, known as "heat sealing". Special, acid resistant, epoxy resin can be used to bond one-piece covers to containers whether one or both of the two components are plastic or hard rubber. Both of these processes form a permanent seal that cannot be broken by heat or other factors encountered during normal battery life. In the batteries using individual cell covers, "asphalt" sealing compound is often used. This seal is a blend of specially processed bituminous compositions that have resistance to flow at warm temperatures and resistance to cracking at cold temperatures.

VENT PLUGS

Vent plugs of various designs are used in the industry. They are baffled so gas can escape from the cell, but electrolyte splashed into the vent will drain into the cell and not be "pumped" from the battery by the escaping gas. If an individual vent plug is used for each cell, it may be threaded and screw into the vent well, while others are pushed into the vent well and held in place by an interference fit. The "push-in" type of vent plug may be a single plug, three plugs mounted in a manifold (gang vent plug), six plugs mounted in a manifold or in a flexible plastic strip. These designs were developed to reduce the time required to remove and replace the vent plugs when adding water to the battery. There are vent plugs with lead valve systems, which prevent acid draining from the battery when it is turned upside down. These are used in aircraft and marine service and batteries for special vehicles. An important development in the design of vent plugs for the automotive type battery was the introduction of the "flame arrester" vent plug, which is currently used on most batteries produced in North America. If a hydrogen/oxygen mixture outside the battery is accidentally ignited by a spark or flame, the resulting flame front passing through the arrester is cooled below its ability to sustain a flame, preventing the ignition of the gases inside the battery. However, this safety feature will not sustain against repeated ignitions. Flame arresters can be made of several materials including sintered plastics and ceramics. They must be sufficiently porous to pass large quantities of gases with a low back pressure, but not so porous as to allow the flame front to pass through them. Another type of flame arrester has several tiny vent holes in the gang vent. The holes are so small and the vent so designed that the flame is snuffed out. The heat of the flame front could melt the plastic around the opening and seal the hole, but several other openings would still be available to vent the battery cells. Some low water loss batteries do not use vent plugs. Instead, the gas is vented through one or two baffled labyrinths in the cover. The flame arrester is placed at the exits of these intricate passage-ways.

TERMINAL DESIGNS

Tapered Top Terminal This design uses tapered terminal posts built to Industry standards so that all cable clamps will fit any battery with these posts (one size for the positive posts and one size for the negative posts). The positive terminal is slightly larger than the negative to minimize the danger of installing a battery in reverse. The positive terminal is 11/16" (17.5mm) in diameter at the top. The negative terminal is 5/8" (15.9mm) at the top. The minimum height of the taper is 5/ 8" (15.9mm). Various other terminal dimensions are sometimes used outside of North America.

Side Terminals

These terminals are molded into the side wall of the container near the top edge. Each battery cable is attached to the terminal by a bolt which threads into the terminal. If the bolt is missing, install a new cable which is furnished with the proper bolt. When tightening the bolt, use the manufacturer's recommended torque values to prevent terminal damage, while assuring a good connection. Do not over-torque.

The "L" Terminal

The "L" terminal is used on many European car batteries for special applications, such as marine and light duty vehicles. The battery cable is attached to the terminal by a bolt and a wing nut.

Stud Terminal

Another type of "top" terminal is a threaded or "stud" terminal typically used on heavy duty batteries.

TNI Ltd. Industriveien I 1 - 4800 Arendal - Norway - Phone +47 370 54 100 - Fax +47 370 54 101