Battery Construction
Battery Construction
Modern automotive batteries are made of cases (usually plastic) containing alternating plates of Lead and Lead Dioxide (or Lead Oxide) separated by insulators. These alternating plates are connected in series to produce a voltage of 12.6 volts, or about 2.1 volts for each set of Lead and Lead Dioxide plates. The negative terminal is connected to a Lead Dioxide plate and the positive terminal to a Lead plate.
The plates are covered with electrolyte which is a solution of 35% Sulfuric acid and 65% Water.
1 - Plastic container.
2 - Positive and negative internal plates made of lead.
3 - Plate separators made of porous synthetic material.
4 - Electrolyte which is a dilute solution of Sulfuric acid and water better known as Battery Acid.
5 - Lead terminals which are the connection point between the battery and whatever it powers.
Battery Case
Most battery cases and their covers are made of polypropylene. The case is divided into six sections or cells, shaped similar to an ice-cube tray.
The case is designed to:
- Withstand hot and cold temperature extremes.
- Resist damage caused by mechanical shock in automotive applications.
- Resist acid absorption and chemical damage.
The Grids
The grids are the supporting framework for the active material of the plates. They also conduct current to and from the active material plates.
The Plates
Plates are grids covered with a paste mixture of Lead Oxide and Sulfuric Acid and water An expander material made of powdered sulfates is added to the paste to produce negative plates.
A forming charge is applied to the positive plates converting the Lead Oxide to Lead Dioxide, a highly porous material which allows the electrolyte to freely penetrate the plate.
A forming charge is also applied to the negative plates converting the Lead Oxide to Sponge Lead. The Sponge Lead allows the electrolyte to penetrate freely allowing the material beneath the plate surface to take part in the chemical reaction.
The 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.
Elements
In the most common method of construction, a stack of alternate positive and negative plates are formed with separators between each positive and negative plate. The lugs of the negative plates 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.
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 battery cell. More or larger plates per cell will increase plate surface area and increase capacity of the battery but will not affect the voltage output.
Electrolyte
The electrolyte is a mixture of Sulfuric Acid and Water. Electrolyte consists of 35% sulfuric acid and 65% water.
The electrolyte is the carrier for the electric current to move between the positive and negative plates through the separators.
The Lead Terminals
BMW's use a 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.
The positive terminal is slightly larger than the negative to minimize the danger of installing the battery in reverse. The positive terminal is 17.5mm in diameter at the top. The negative terminal is 15.9mm at the top.