A battery is a device for converting chemical energy into electrical energy. Batteries can consist of a single voltaic cell or a series of voltaic cells joined to each other. (In a voltaic cell, electrical energy is produced as the result of a chemical reaction between two different metals immersed in a solution, usually a liquid.) Batteries can be found everywhere in the world around us, from the giant batteries that provide electrical energy in spacecraft to the miniature batteries that power radios and penlights.
Batteries are an excellent emergency power source, but require some basic information to use properly. They are electrochemical devices. They have plates, usually metallic, and either a solution or a moist compound between the plates. A chemical reaction takes place in the battery when it is discharged that produces a flow of electrons out one plate on the negative side and into another plate on the positive side.
Actually a single unit of a battery is a cell. A battery is called a "battery", because it is a "battery" of cells together. Each cell will have a characteristic voltage range between charged and discharged that is set by the electrochemical nature of the metals used and the reactions that go on in the solution, gel, wet powder, etc. between the plates.
Some non-rechargeable batteries contain other chemicals to absorb waste by products from the chemical reaction that moves the electrons along. This is what an "alkaline" battery is and why it lasts longer and costs more than a standard carbon/zinc cell. It has an excess of these chemicals to absorb more by products before the cell becomes poisoned. One such chemical is manganese dioxide, which is mostly what the damp black powder inside a typical dry cell battery consists of.
2. TYPES OF BATTERIES:
Batteries can be classified as
1. Primary: A primary battery is one designed to be used just once. When the battery has run down (produced all the energy it can), it is discarded
2. Secondary batteries (or cells): Secondary batteries, on the other hand, can be recharged and reused.
The secondary cell with which you are likely to be familiar is the lead storage battery found in automobiles. The lead storage battery usually consists of six voltaic cells connected to each other. The total amount of energy produced by the battery is equal to the sum of the electrical energy from the six cells. Since each cell produces about two volts, the total energy available from the cell is 12 volts.
Some cells or batteries can be recharged. In this case a power supply is hooked up to run the chemical reaction backwards and restore the chemical makeup of the battery back to its uncharged state. Not all batteries can be recharged and attempting to recharge some non-rechargeable batteries can be quite dangerous, as pressures will develop inside the case and cause an explosion.
An example of a rechargeable battery is a lead/acid cell. Here lead plates and sulphuric acid are used and lead sulphate is generated and destroyed as the battery discharges and then gets recharged. A "gel cell" is usually a lead/acid battery that has something in the sulphuric acid solution to make it less slushy or gelled. Because they have more trouble dissipating heat and out gassing, these gel cells should be charged slower than regular lead/acid batteries.
The lead/acid battery has been in common use in automobiles since 1915 or so. It has plates of lead in sulphuric acid solution in water. One of the sets of lead plates is coated with lead dioxide. As such a battery discharges it creates two chemical reactions, one at the anode that ends up with an excess of electrons, and one at the cathode that ends up short electrons. If a wire is connected between the two, the excess electrons from the anode will travel through the wire as a current to the cathode where they are needed to complete the electron deficient reaction there.
The correct use of the term battery is reserved for groups of two or more voltaic cells. The lead storage battery found in automobiles, for example, contains six voltaic cells. However, in common usage, a single cell is often referred to as a battery. For example, the common dry cell battery found in flashlights is really a single voltaic cell.
3. LEAD STORAGE BATTERY:-
(A) INTRODUCTION
Lead acid batteries were invented in 1859 by Gaston Planté and first demonstrated to the French Academy of Sciences in 1860. They remain the technology of choice for automotive SLI (Starting, Lighting and Ignition) applications because they are robust, tolerant to abuse, tried and tested and because of their low cost. For higher power applications with intermittent loads however, Lead acid batteries are generally too big and heavy and they suffer from a shorter cycle life and typical usable power down to only 50% Depth of Discharge (DOD). Despite these shortcomings Lead acid batteries are still being specified for PowerNet applications (36 Volts 2 kWh capacity) because of the cost, but this is probably the limit of their applicability and NiMH and Li-Ion batteries are making inroads into this market. For higher voltages and cyclic loads other technologies are being explored.
(B) CONSTRUCTION
Inside a lead storage battery is a series of plates. Half of the plates are made of lead dioxide and the other half are made of a spongy form of lead. The plates are bathed in a solution of sulfuric acid which serves as an electrolyte (a chemical solution that conducts electricity). Two posts extend to the outside of the battery through sealed openings in the battery wall. One of the posts---the negative post---is connected to the lead plates, and the other---the positive post---to the lead dioxide plates.
Sulphuric acid, chemically, is composed of two hydrogen atoms, a sulphur atom and four oxygen atoms. Inside the battery, the sulphuric acid molecules are in solution with water and so are dissociated. This means that the sulphur atom with the four oxygen atoms attached to it are in the water, separated from the hydrogen atoms. The sulphur with the four oxygen atoms is called a sulphate ion and has a double negative charge. The free hydrogen atom is called a hydrogen ion and has a positive charge. (An ion is simply an atom or a molecule with a positive or negative charge.)
(C) WORKING
The battery performs its function through a series of chemical reactions involving these ions. In the condition described in the introductory paragraph, the battery is charged and has the capacity to supply an electric current through cables connected to the two posts. As the battery supplies electric current, the sulphuric acid reacts by giving up its sulphate ions to the lead and lead dioxide plates. This forms lead sulphate that deposits on the plates. While this process occurs, the sulphuric acid concentration is decreasing, and the battery is discharging.
Supplying an electric current to the battery rather than drawing it out---such as what happens in an automobile when the engine is running---reverses the chemical reaction and the battery recharges. When recharging, the lead and lead dioxide plates give up sulphate ions to the electrolyte solution. This restores the amount of sulphuric acid in the electrolyte solution and restores the lead and lead dioxide plates to their charged condition.
(D) ELECTROCHEMISTRY:
The electrode reactions that occur during discharge (i.e. when the battery is in use) are as follows:-
At anode
Pb(s) + SO42- (aq) ? PbSOÂ4 (s) + 2e-
At cathode
PbO2(s) + SO42- (aq) + 4H+(aq) + 2e- ? PbSO4(s) + 2H2O
Overall reaction
PbO2(s) + PbO2(s) + 4H+(aq) + 2SO42-(aq) ? 2PbSO4(s) + 2H2O
From the above equations, it is obvious that H2SO4 is used yup during the discharge. As a result, the density of H2SO4 falls. When it falls below 1.2 g/cm3, the battery needs recharging.
During Recharging, the cells is operated like an electrolytic cell, i.e. now electrical energy is supplied to it from an external source. The electrode reaction are reverse of those that occur during discharge:
PbSO4(s) + 2e- ? Pb(s) + SO42-(aq)
PbSO4(s) + 2H2O ? PbO2(s) + SO42-(aq) + 4H+(aq) + 2e-
2PbSO4(s) + 2H2ÂO ? Pb(s) + PbO2(s) + 4H+(aq) + 2SO42-(aq)
Such an operation is possible because the PbSO4 formed during discharge is solid and sticks to the electrodes. It is therefore, in position to either receive or give up electrons during electrolysis.
(E) CHARACTERISTICS:
The charge algorithm for lead-acid batteries is similar to lithium-ion but differs from nickel-based chemistries in that voltage rather than current limiting is used. The charge time of a sealed lead-acid battery is 12-16 hours (up to 36 hours for larger capacity batteries). With higher charge currents and multi-stage charge methods, the charge time can be reduced to 10 hours or less. Lead-acid cannot be fully charged as quickly as nickel or lithium-based systems.
It takes about 5 times as long to recharge a lead-acid battery to the same level as it does to discharge. On nickel-based batteries, this ratio is 1:1, and roughly 1:2 on lithium-ion.
A multi-stage charger first applies a constant current charge, raising the cell voltage to a preset voltage (Stage 1 in Figure 1). Stage 1 takes about 5 hours and the battery is charged to 70%. During the topping charge in Stage 2 that follows, the charge current is gradually reduced as the cell is being saturated. The topping charge takes another 5 hours and is essential for the well being of the battery. If omitted, the battery would eventually lose the ability to accept a full charge. Full charge is attained after the voltage has reached the threshold and the current has dropped to 3% of the rated current or has leveled off. The final Stage 3 is the float charge, which compensates for the self-discharge.
4. LEAD-ACID BATTERY TYPES:
1. Flooded Lead-Acid Batteries: Flooded cells are those where the electrodes/plates are immersed in electrolyte. Since gases created during charging are vented to the atmosphere, distilled water must be added occasionally to bring the electrolyte back to its required level. The most familiar example of a flooded lead-acid cell is the 12-V automobile battery.
2. Sealed Lead-Acid Batteries: These types of batteries confine the electrolyte, but have a vent or valve to allow gases to escape if internal pressure exceeds a certain threshold. During charging, a lead-acid battery generates oxygen gas at the positive electrode. Sealed lead-acid batteries are designed so that the oxygen generated during charging is captured and recombined in the battery. This is called an oxygen recombination cycle and works well as long as the charge rate is not too high. Too high of a rate of charge may result in case rupture, thermal runaway, or internal mechanical damage.
5. APPLICATIONS:-
There are numerous applications for the use of lead-acid storage batteries. They range from the extremely large battery systems used in load leveling by electrical utility companies to the relatively small batteries used in hand tools. The major categories of lead-acid battery applications are starting, lighting, and ignition (SLI); industrial, including traction and stationary applications; and small portable equipment. A brief description of each type is included below along with example uses of each type.
1. Starting, Lighting, and Ignition: SLI batteries are used by most people every day and are produced in greater numbers than any other type of lead-acid storage battery. These are used to start automobiles and most other kinds of internal combustion engines. They are not suitable for deep discharge applications, but excel for uses needing a high current for a brief time. They are usually charged in a "partial float" manner, meaning that the battery only receives a float charge while the vehicle is running. SLI batteries are usually of the flat pasted plate design.
2. Industrial: Industrial batteries generally have the largest capacity of the three major categories of lead acid batteries. Industrial batteries are used for vehicle traction and stationary applications.
3. Portable: Their operation cannot usually be described as cyclic or float, but is somewhere in-between. Batteries in this category may be frequently deep cycled or remain unused for a relatively long time. Typical applications are portable tools, toys, lighting and emergency lighting, radio equipment, and alarm systems. Most portable batteries may be recharged to 80â€"90% of their original capacity in less than an hour using a constant-voltage charger.
6. HEALTH SIDE EFFECTS:
1. EYES: Direct contact of internal electrolyte liquid with eyes may cause severe burns or blindness.
2. SKIN: Direct contact of internal electrolyte liquid with the skin may cause skin irritation or damaging burns.
3. INGESTION: Swallowing this product may cause severe burns to the esophagus and digestive tract and harmful or fatal lead poisoning. Lead ingestion may cause nausea, vomiting, weight loss, abdominal spasms, fatigue, and pain in the arms, legs and joints.
4. INHALATION: Respiratory tract irritation and possible long term effects.
