The Lead-Acid Cell
The basic lead-acid cell consists of two sets of plates, one of which is negative and the other positive. They are interleaved and prevented from coming into contact with each other by porous separators. The separators have high insulation qualities but permit the unobstructed circulation of the electrolyte at the plate surfaces.
The basic lead-acid cell components are shown in Figure 5.14.
11
Figure 5.14 - Lead-Acid Cell Components
VENT PLUG FILLER OPENING
IN CELL COVER TERMINAL
POST
TERMINAL CONNECTOR PLATE
ST RAP LINK
CONNECTOR
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l E
CONTAINER rd EGAT IVE
PLATE
SEPARATOR
CASE RIB
POSITIVE PLATE
SEDIMENT SPACE
Figure 5.15 - Lead-acid battery construction
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CELL ELEMENT PARTLY ASSEMBLED
Figure 5.16 - Lead-acid battery plate arrangement.
The positive plates are made up of grids of lead and antimony filled with lead peroxide. The negative plates are made up of similar grids, but filled with spongy lead.
The electrolyte is a solution of sulphuric acid and water in contact with both sets of plates.
The type of cell construction permits the electrolyte to circulate freely and also provides a path for sediment to settle at the bottom of the cell.
When an external circuit is connected to a fully charged cell, electrons flow from the negative lead plates, via the circuit, to the positive lead peroxide plates.
As the electrons leave the negative plates, positive ions form. These attract negative sulphate ions from the sulphuric acid of the electrolyte. This causes lead sulphate to form on the negative plates.
The electrons arriving at the positive plates, from the external circuit, drive negative oxygen ions from the lead peroxide into the electrolyte. These combine with hydrogen, which has lost
sulphate ions, to form water.
The positive lead ions that are left on the positive plates also attract and combine with sulphate
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ions from the electrolyte to form lead sulphate on the positive plates.
Once lead sulphate collects on both the positive and negative plates and the electrolyte becomes diluted by the water, which has formed in it, the cell is considered discharged.
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Module 3.5 DC Sources of Electricity 5-23
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A discharged cell is recharged using a direct current of the correct voltage. When the positive plates of the cell are connected to the positive of the charging source and the negative plates to the negative of the source, electrons are drawn from the positive plates and forced onto the negative plates.
Electrons arriving at the negative plates drive negative sulphate ions out of the lead sulphate back into the electrolyte. The sulphate ions join with hydrogen to form sulphuric acid.
When electrons flow from the positive plates they leave positively charged lead ions. These attract oxygen from the water in the electrolyte to form lead peroxide on the plates.
When the cell is fully charged the positive plates again become lead peroxide and the negative plates lead. The electrolyte becomes a high concentration of sulphuric acid.
The specific gravity of the electrolyte of a fully charged cell is approximately 1.260. This falls to about 1.150 when the cell is completely discharged. These values will depend upon the manufacturer's instructions.
The specific, gravity, therefore, is a good indication of the state of charge of the cell and is measured using a hydrometer. Using the rubber bulb, enough electrolyte is drawn up into the hydrometer, to float the float. The specific gravity is then indicated by the calibration mark on the float at the surface of the electrolyte. This is shown in Figure 5-17.
1.100---1.150- ►
1.100 1.250 1.300
1.350 s
7
FLOAT
1.400 01
Figure 5.17 - The Hydrometer
During the charging of the cell hydrogen gas is released from the electrolyte and bubbles to the surface. As the cell nears full charge more hydrogen is released and the bubbling increases. A vent is, therefore, incorporated in the cell cap.
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The voltage of a fully charged cell is approximately 2.2 volts (2 volts nominal) and in the discharged state 1.8 volts.
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The electrolyte level should be just above the top of the plates and the level will generally drop over a period of use due to evaporation and gassing. The level can be adjusted by topping up with distilled water after removal of the vent cap.
Generally lead-acid batteries are made up of cells in a common case so that cells cannot be removed individually as shown in Figure 5.18.
Figure 5.18 - A Typical Lead-Acid Battery
Module 3.5 DC Sources of Electricity 5-25
The Nickel-Cadmium Cell
Aircraft engines, particularly turbines, require extremely high current for starting. High rate
discharges of lead-acid batteries causes their output voltage to fall, due to the increased internal resistance caused by the build-up of sulphate deposits. This drawback led to the development of the alkaline cell for aircraft use.
The nickel-cadmium, or ni-cad, battery has a very distinct advantage in that its internal
resistance is very low. Its output voltage, therefore, remains almost constant until it is nearly totally discharged. The low resistance also allows high charging rates without damage.
The ni-cad cell has positive plates made from powdered nickel which is fused, or sintered, to a porous nickel mesh. The mesh is then impregnated with nickel hydroxide.
The negative plates are of the same construction but are impregnated with cadmium hydroxide.
Separators of nylon and cellophane, in the form of a continuous strip wound between the plates, keeps the plates from touching each other. Cellophane is used because it has low electrical
resistivity and also acts as a gas barrier preventing oxygen, given off at the positive plates t during overcharge, from passing to the negative plates. If the oxygen were allowed to reach the l negative plates it would combine with active cadmium, reduce cell voltage and produce heat as t a result of chemical reaction.
The cell construction is shown in Figure 5.19, where the complete plate group is mounted in a 17, sealed plastic container.
CELLOPHANE
l f
NYLON NYLON l
Figure 5.19 - Nickel-Cadmium Cell Construction i,.
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VENT AND FILLER CAP TERMINAL
SEPARATOR
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LU
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r-1
Figure 5.20 - Nickel-cadmium cell.
The electrolyte is an alkaline solution of potassium hydroxide and distilled or de-ionized water with a specific gravity of 1.24 to 1.30.
The specific gravity of the electrolyte does not change during charge or discharge so it cannot be used to indicate the state of charge.
The electrolyte does not play an active part in the chemical reaction and is used only to provide a path for current flow.
During charging of the cell an exchange of ions takes place. Oxygen is removed from the negative plates and added to the positive plates, the electrolyte acting as an ionized conductor.
The positive plates are, therefore, brought to a higher state of oxidation.
When the cell is fully charged all the oxygen is driven out of the negative plates, leaving only metallic cadmium, and the positive plates are highly oxidized nickel hydroxide.
The electrolyte is forced out of both sets of plates during charging so that the electrolyte level in the cell rises. The electrolyte level is, therefore, only checked and any water added when the cell is fully charged.
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Towards the end of the charging process and during overcharging, gassing occurs as a result of electrolysis. This only reduces the water content of the electrolyte.
During discharge the chemical action is reversed. The positive plates gradually lose oxygen to become less oxidized and the negative plates regain lost oxygen and change to cadmium hydroxide.
Module 3.5 DC Sources of Electricity 5-27
The plates absorb electrolyte so that the level in the cell falls but it should always cover the top of the plates. The charge and discharge levels are shown in Figure 5.21.
Charged level
Discharged level
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Figure 5.21 - Nickel-Cadmium cell electrolyte levels
The discharge and charging cycle of a ni-cad cell produces high temperatures which, if not correctly monitored, can break down the cellophane gas barrier. This creates a short circuit allowing current flow to increase. More heat is produced, causing further break down. The condition is aggravated by the internal resistance of the cell falling as the temperature rises.
These factors all contribute to a process known as "thermal runaway", which ultimately results in the destruction of the cell.
The ni-cad electrolyte would be contaminated and its specific gravity reduced if it were to be exposed to the carbon dioxide in the air. The atmosphere must, therefore, be kept out of a nicad cell. Three basic types of ni-cad cell are, therefore, produced:
a) The sealed type where the cell is completely sealed, as used in small capacity batteries.
b) The semi-sealed type where the cell is almost fully sealed but has a safety pressure valve.
c) The semi-open type which has a non-return valve, allowing the cell to gas yet preventing the electrolyte from being contaminated by the air. This type is used in the main aircraft battery.
The individual ni-cad cell produces an open circuit voltage of between 1.55 and 1.80 volts, depending on the manufacturer.
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1
Although the nickel-cadmium battery has become the preferred type in today's aircraft, there are also the nickel-iron and silver-zinc types of alkaline cell. Silver-zinc rechargeable batteries have been used in the space programme, where size and weight factors greatly outweigh initial cost.The capacity of each cell is added together to obtain the total capacity. In effect the area of the L% plates has been increased. The voltage, on the other hand, does not increase.
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Figure 5.22 -- Examples of NiCad Batteries
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L I
L i
Module 3.5 DC Sources of Electricity 5-29
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