AeroElectric Connection - Everything you ever wanted to know about RG batteries but were afraid to ask.

Everything you ever wanted to know about RG batteries but were afraid to ask. . .

Shamlessly purloined from the website of our friends at Concord Battery Corporation

Battery Elements

The lead-acid secondary cell used in aircraft batteries consists of positive plates made of lead peroxide (PbO2); negative plates of pure spongy lead (Pb); and a liquid known a electrolyte, consisting of a mixture of sulfuric acid (H2SO4) and water (H2O). The sulfuric acid and water are mixed in such quatities that the solution has a specific gravity of 1.275 to 1.300 for a fully charged battery.

The specific gravity of a substance is defined as the ratio of the weight given volume of the substance to the weight of an equal volume of pure water at +4 degrees celsius.

Simplified lead-acid electrochemical reaction.
Discharge > < Charge
PbO2 + Pb + 2H2SO4 + 2H2O <=> 2PbSO4 + 2H20

Chemical Reaction

A chemical reaction takes place when a battery is delivering current. The sulfuric acid in the electrolyte breaks up into hydrogen ions (H2) carrying a positive charge, and sulfate ions (SO4) carrying a negative charge. An ion is an atom or molecule which is either positively or negatively charged. A positively charged ion has a deficiency of electrons, and a negatively charged ion has an excess of electrons. The SO4 ions combine with the lead in the plates and forms lead sulfate (PbSO4). At the same time, they give up their negative charge, thus creating an excess of electrons on the negative plate.

The H2 ions go to the positive plate and combine with the oxygen of the lead peroxide (PbO2) forming water (H2O). During the process, they take electrons from the positive plate. The lead of the lead peroxide combines with some of the SO4 ions to form lead sulfate on the positive plate. The result of this reaction is that the positive plate has a deficiency of electrons and the negative plate has an excess of electrons.

When the plates are connected together with an external conductor, the electrons from the negative plate flow to the positive plate. This process will continue until both plates are converted to lead sulfate and no further chemical action is possible. The battery is discharged.

During the charging process, current is passed through the storage battery in the reverse direction. A direct current supply is applied to the battery with the positive pole connected to the positive plate and the negative pole onnected to the negative plate. If the electromotive force (emf) of the source is greater the emf of the battery, this causes the current to flow in the reverse direction.

The result is that SO4 ions are driven back into sulution where they combine with the H2 ions of the water, thus forming sulfuric acid. The plates then return to their original composition of lead peroxide and spongy lead. When this process is complete, the battery is charged. In as much as the sulfuric acid in the electrolyte is used up as the battery is discharged, and returned to the electrolyte as the battery is charged, a measurement of the specific gravity of the electrolyte will give a good indication of the state of charge in the battery.

In flooded (vented) batteris the oxygen generated at the positive electrode when on charge escapes from the cell. Concurrently, at the negative electrode, hydrogen is generated from water and escapes from the cell. The overall result is the gassing of the cells and water loss. Therefore, flooded cells require periodic water replenishment.

In valve-regulated (sealed) Recombinant Gas (RG) batteries, oxygen combines chemically with the lead at the negative electrode in the presence of H2SO4 to form lead sulfate and water. This oxygen recombination suppresses the generation of hydrogen at the negative electrode. Overall, there is no water loss during charging. A very small quantity of water may e lost as a result of self-discharg reactions, however, such loss is so small that no provisions are needed for water replenishment. The battery cells have a pressure relief safety valve that may vent if the battery is overcharged.

Concorde's valve-regulated lead acid batteries (VRB) are recombinant gas (RG) batteries. The cells are sealed with pressure relief valves that open to relieve excessive pressure within the battery.

The plates are sandwiched between layers of micro fiber glass mat. Eloctrolyte is absorbed and held in place by the capillary potential of the fluid and the absorptive glass mat (AGM) fibers.

The AGM by design is approximately 92% saturated with eletrolyte. The remainder is filled with gas. This void space provides the channels by which oxygen travels from the positive to the negative plate during charging. The freshly generated gases, which are in their atomic state and very reactive, recombine rapidly and safely.

The recombination passivates the negative slightly, reducing electrolysis and ultimately eliminating the need to add water.

Because of the compressed construction, the RG batteries have a much lower internal resisitance and thus provide greater starting power and faster recharging, particularly at cold temperatures, than comparable flooded batteries. Additionally, the AGM provides a much higher degree of support against shock and vibration than in the older flooded (vented) batteries. The RG batteries provide electrical performance comparable to nickel-cadium aircraft batteries without the requirement of a temperature or current monitoring system.

Battery Construction

An aircraft storage battery consists of 6 or 12 lead-acid cells connected in series. The open circuit voltage of the 6-cell battery is approximately 12, and the open circuit voltage of teh 12-cell battery is approximately 24. Open circuit voltage is the voltage of the battery when it is not connected to a load.

Grids and Plates

Each cell of a storage battery has positive and negative plates arranged in alternatively, and insulated from each other by separators. Each plate cosist of a framework, called the grid, and the active material held in the grid.

The grid is cast from a lead alloy. The heavy, outside border adds strength to the plate, and the small horizontal and vertical bars support the active material. These bars also act as conductors for the current which is distributed evenly throughout the plate.

The plates ara made by applying a lead paste compound to the grid. The paste is mixed to the proper consistency, and is applied to the grid in much the same manner as plaster is applied to a lath wall.

In compounding the negative plate paste, a material is added known as an expander. This material is relatively inert and makes up less than one percent of the mixture. Its purpose is to prevent the loss of porosity of the negative material during the life of teh battery. Without the use of an expander, the negative material contracts until it becomes quite dense, thus limiting the chemical action to only the immediate surface.

After the paste is applied to the plates, they are dried by a special process until the paste has hardened. The plates are then given a forming treatment in which a large number of positive plates are connecteed to the positive terminal of a charging apparatus; and a like number of negative plates are connecteed to the negative terminal. They are placed in a solution of sulfuric acid and water and chargd slowly over a long period of time. The plates are then washed and dried in an oxygen free atmosphere. The positive plates thus formed are chocolate brown in color and of a hard texture. The negative plate material has been converted to a spongy lead of a pearl gray color. They are then ready to be assembled into plate groups.

Close quality control in processing of all materials used in a storage battery is very important. Close temperature and humidity controls are used in various phases of manufacture. All these factors are essential to produce a reliable storage battery.

Consistent with the various processes by which plates are made, it is extremely important that the user follow Concorde Battery Corporation's recommendations on placing the battery into service and its subsequent maintenance. The information is included with each new battery, Instructions for Continued Airworthiness (ICA) manual.

Plate Groups

Plate groups are made by joining a number of similar plates to a common terminal post by means of a plate strap. The capacity is determined by the number and size of plates in a group. Each plate is made with a lug at the top to which is fused to the strap. A positive group consist of a number of positive plates connected to a plate strap, and a negative group consist of negative plates connected in the same manner. The two groups meshed together with separators between the positive and negative plates constitute a cell element.


The separators used in aircraft batteries are made of high temperature polypropylene material. Their purpose is to keep the plates separated and thus prevent an internal short circuit.

The material of the separators must be extremely porous so that the separators will offer a minimum of resistance to the ions passing through them. The material must also resist the corrosive action of the electrolyte.

Cell Containers

When the cell elements are assembled, they are placed in the cell container which is made of plastic. Usually cell containers are made up in a monobloc with as many compartments as there are cells in the battery. The plastic used is selected for its resistance to sulfuric acid, low permeability and impac strength.

Cell Covers, Vent Valves and Vent Caps

The assembled cell has a cover made of material similar to that of the cell container. The cell or monobloc cover is provided with holes through which the terminal posts extend, and a threaded hole for each cell into which is screwed the vent cap or valve. When the cover is placed on the cell, it is sealed with a special sealing compound to prevent leakage and loss of electrolyte.