Basics notes of batteries

 

                                                                BATTERIES

What is a battery?

Cell is the basic unit of battery. Cells connected in series form a battery.

 

What is a cell?

A cell produces electrical current by chemical reaction. it consists of two electrodes, anode and cathode, of different materials, which are connected by an electrolyte, a chemical which reacts suitably with both electrodes.

 

Types of cell:

1.       Primary cell -  this cannot be recharged and is discarded after its energy is exhausted, e.g. a torch battery.

2.       Secondary cell - this can be recharged, e.g. a car battery.

 

Types of batteries used on board:

1.       Lead Acid cells.

2.       Alkaline cells. (eg. Nickel cadmium).

 

Lead acid cells.

The cell shown above is being discharged.


The lead acid cell has a negative electrode of porous (spongy) lead and a positive electrode of lead dioxide (PbO2). Electrodes are immersed in a solution of Sulphuric acid.

 

 

 

As the cell is discharged the positive electrode of lead dioxide (PbO2) combines with both the H2 and the SO4 of the electrolyte to produce lead sulphate ( PbSO4) and water. The negative electrode of lead combines with the SO4 of the electrolyte to produce lead sulphate (PbSO4).

As the cell is discharged sulphuric acid is consumed and water is formed, this means that the specific gravity of the electrolyte will change as the state of charge changes. The change in specific gravity gives a good indication of the state of charge of the cell.

Item

Discharged

Charged

Positive plate

 

Turns to lead sulphate

Lead dioxide.

Negative plate

 

Turns to lead sulphate

Spongy lead

E.M.F.

Voltage falls as the battery discharges.

Voltage should be measured with the battery on load.

Voltage should not be allowed to fall below 1.8 V per cell.

approx 2.2 V per cell.

Specific gravity

Falls to approx 1.1 at the end of the useful discharge period.

 

Rises to approx 1.25 when fully charged

 

What happens during the charging of cells?

Hydrogen and oxygen gases are evolved during charging due to the chemical breakdown of the water in the electrolyte. The cells are vented to allow gases to escape and prevent internal pressure build up.

 

What happens when we overcharge the cell?

Overcharging causes overheating, distortion of cell plates and consequent dislodging of plate active materials. Expansion distorts the plates and may dislodge the lead dioxide paste which falls to the bottom of the cell and if allowed to accumulate will ultimately short circuit the cell.

 

Why charging of cells should not be done in tropical regions?

The temperature of the electrolyte must not be allowed to exceed 43oC, since charging causes a rise in temperature it may be necessary to reduce the charge current or stop charging in the tropics.

 

What happens when cells are kept at discharged state?

if they remain discharged for a long period the plates become covered with a hard white deposit of lead sulphate which will permanently damage the cell, this is called sulphation.

 

Why do we do trickle charging?

Lead acid cells suffer self discharge. If left idle an internal discharge would slowly dissipate the charged energy. A fully charged cell must be maintained on a trickle charge, a low rate charge, to make up this loss.

 

Alkaline cells

positive plates -nickel hydroxide, negative plates - cadmium oxide. The electrolyte -potassium hydroxide.

On discharge the nickel hydroxide losses oxygen and is reduced to a lower form, while the cadmium in the negative plates is oxidized to cadmium oxide.

On charge the reverse occurs, the oxide being reduced to its original condition.

No chemical change occurs in the electrolyte and the specific gravity remains constant whether it is charging or discharging. The relative density of an alkaline cell does not change with the state of charge but does reduce with the age of the cell.

The state of charge is found by connecting a voltmeter across the battery terminal when it is working the pd per cell should be 1.2 V.

Item

Discharging

Charging

Positive plates

Loses oxygen

Restored to Nickel hydroxide

Negative plates

Gains oxygen, oxidises

Restored to Cadmium

E.M.F.

Usually falls to 1V at end of useful discharge.

Rises to approximately 1.4 V.

Specific gravity

Remains constant at 1.17

Remains constant at 1.17

 

 

Charging of alkaline batteries:

·         Alkaline cells will normally gas continually whilst charging, giving off hydrogen, the cells should be charged until gassing freely and the voltage per cell is 1.7 V.

·         Alkaline cells are able to retain their full charge for a considerable period and do not suffer self discharge except at high temperatures. Periodically they should be given a short refresher charge of 2-3 hours.

·         Alkaline cells are well suited to the float charge mode of operation of standby power batteries. Here the battery is connected across the load supply and is charged up to the supply voltage after which the battery merely floats on the supply, neither taking or supplying electrical energy.

 

Indications of a fully-charged cell:

1.       Gassing 2. Voltage 3. Density of electrolyte.

 

Advantages of lead acid batteries:

  1. Lead acid cells are cheaper.
  2. Lead acid cells are more efficient.
  3. Lead acid cells are more efficient at lower temperatures.

 

Advantages of alkaline cells:

  1. Alkaline cells have a longer life span, typically 20 years compared to 5 to 10 years for the lead acid.
  2. Alkaline cells are more suitable to long periods of idle operation on standby.
  3. Alkaline cells are more reliable.
  4. Alkaline cells are more robust.
  5. Alkaline cells are unaffected by overcharging and no permanent damage is done by over discharging.

 

Maintenance carried out on Lead acid batteries.

·         Connection to be checked for tightness.

·         Casing surface to be kept clean.

·         Terminals are protected with petroleum jelly or Vaseline.

·         Specific gravity to be check with hydrometer.

§  1.21 Charged Condition

§  1.18 Discharged Condition

·         Batteries to be topped up with distilled water.

·         Check & clear vents to ensure H2 gas produced is vented.

 

Safeties while working with batteries:

  1. Wear suitable protective clothing, rubber apron, rudder gloves, goggles.
  2. Use insulated spanners and none metallic jugs to prevent sparks and short circuits.
  3. No source of naked light should be taken into the battery compartment.
  4. Sulphuric acid splashes on the skin should be washed off with fresh water and treated with saline solution.
  5. Potassium hydroxide splashes should be washed off with fresh water but treated with boracic acid solution or boracic powder.
  6. Because of the heat that is evolved when adding acid to the electrolyte, the acid should only be added a little at a time and allowed to mix thoroughly with the electrolyte and cool before any more is added.
  7. Because of the considerable heat evolved it is dangerous to add water to acid, when electrolyte is being prepared the acid must be added to the distilled water.
  8. Use a respirator if using acid in powder form.

 

 

Battery compartment safeties:

  1. Well ventilated to remove explosive hydrogen gas.
  2. Illuminated by explosion protected lights.
  3. Steel work painted with acid resisting bitumastic paint as the fumes given off by lead acid batteries are corrosive.
  4. No naked flames.
  5. No sparks should be made in the compartments.
  6. Lead acid and alkaline cells should not be in the same compartment because of possible damage to the alkaline cells by acid.
  7. Always add acid to water.
  8. Mix electrolyte in a non corrosive container capable of withstanding heat.
  9. Batteries should be arranged over wooden crates.

 

 

 

 

 

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