|
|
|
|
Lead-acid battery
|
| |
|
| |
Lead-acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest type of rechargeable battery. Despite having the second lowest energy-to-weight ratio (next to the nickel-iron battery) and a correspondingly low energy-to-volume ratio, their ability to supply high surge currents means that the cells maintain a relatively large power-to-weight ratio. These features, along with their low cost, make them attractive for use in cars to provide the high current required by automobile starter motors.
cell contains (in the charged state) electrodes of lead metal (Pb) and lead (IV) dioxide (PbO2) in an electrolyte of about 33.5% v/v (6 Molar) sulphuric acid (H2SO4).
Discussion
Ask a question about 'Lead-acid battery' Start a new discussion about 'Lead-acid battery' Answer questions from other users |
Recent Posts
Encyclopedia
Lead-acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest type of rechargeable battery. Despite having the second lowest energy-to-weight ratio (next to the nickel-iron battery) and a correspondingly low energy-to-volume ratio, their ability to supply high surge currents means that the cells maintain a relatively large power-to-weight ratio. These features, along with their low cost, make them attractive for use in cars to provide the high current required by automobile starter motors.
Electrochemistry
Each cell contains (in the charged state) electrodes of lead metal (Pb) and lead (IV) dioxide (PbO2) in an electrolyte of about 33.5% v/v (6 Molar) sulphuric acid (H2SO4). In the discharged state both electrodes turn into lead(II) sulfate (PbSO4) and the electrolyte loses its dissolved sulphuric acid and becomes primarily water. Due to the freezing-point depression of water, as the battery discharges and the concentration of sulphuric acid decreases, the electrolyte is more likely to freeze.
The chemical reactions are (charged to discharged):
Anode (oxidation):
Cathode (reduction):
Because of the open cells with liquid electrolyte in most lead-acid batteries, overcharging with excessive charging voltages will generate oxygen and hydrogen gas by electrolysis of water, forming an explosive mix. The acid electrolyte is also corrosive.
Practical cells are usually not made with pure lead but have small amounts of antimony, tin, calcium or selenium alloyed in the plate material.
Voltages for common usages
These are general voltage ranges for six-cell lead-acid batteries:
- Open-circuit (quiescent) at full charge: 12.6 V to 12.8 V (2.10-2.13V per cell)
- Open-circuit at full discharge: 11.8 V to 12.0 V
- Loaded at full discharge: 10.5 V.
- Continuous-preservation (float) charging: 13.4 V for gelled electrolyte; 13.5 V for AGM (absorbed glass mat) and 13.8 V for flooded cells
- All voltages are at 20 °C, and must be adjusted -0.022V/°C for temperature changes.
- Float voltage recommendations vary, according to the manufacturer's recommendation.
- Precise (±0.05 V) float voltage is critical to longevity; too low (sulfation) is almost as bad as too high (corrosion and electrolyte loss)
- Typical (daily) charging: 14.2 V to 14.5 V (depending on manufacturer's recommendation)
- Equalization charging (for flooded lead acids): 15 V for no more than 2 hours. Battery temperature must be monitored.
- Gassing threshold: 14.4 V
- After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V.
Measuring the charge level
Because the electrolyte takes part in the charge-discharge reaction, this battery has one major advantage over other chemistries. It is relatively simple to determine the state of charge by merely measuring the specific gravity (S.G.) of the electrolyte, the S.G. falling as the battery discharges. Some battery designs have a simple hydrometer built in using coloured floating balls of differing density. When used in diesel-electric submarines, the S.G. was regularly measured and written on a blackboard in the control room to apprise the commander as to how much underwater endurance the boat had remaining.
Construction of battery
Plates
The principle of the lead acid cell can be demonstrated with simple sheet lead plates for the two electrodes. However such a construction would only produce around an amp for roughly postcard sized plates, and it would not produce such a current for more than a few minutes.
Gaston Planté realised that a plate construction was required that gave a much larger effective surface area. Planté's method of producing the plates has been largely unchanged and is still used in stationary applications.
The Faure pasted-plate construction is typical of automotive batteries. Each plate consists of a rectangular lead grid alloyed with antimony or calcium to improve the mechanical characteristics. The holes of the grid are filled with a mixture of red lead and 33% dilute sulphuric acid. (Different manufacturers have modified the mixture). The paste is pressed into the holes in the plates which are slightly tapered on both sides to assist in retention of the paste. This porous paste allows the acid to react with the lead inside the plate, increasing the surface area many fold. At this stage the positive and negative plates are similar, however expanders and additives vary their internal chemistry to assist in operation when in use. Once dry, the plates are then stacked together with suitable separators and inserted in the battery container. An odd number of plates is usually used, with one more negative plate than positive. Each alternate plate is connected together. After the acid has been added to the cell, the cell is given its first forming charge. The positive plates gradually turn the chocolate brown colour of lead dioxide, and the negative turn the slate gray of 'spongy' lead. Such a cell is ready to be used. Modern manufacturing methods invariably produce the positive and negative plates ready formed, so that it is only necessary to add the sulphuric acid and the battery is ready for use.
One of the problems with the plates is that the plates increase in size as the active material absorbs sulfate from the acid during discharge, and decrease as they give up the sulfate during charging. This causes the plates to gradually shed the paste during their life. It is important that there is plenty of room underneath the plates to catch this shed material. If this material reaches the plates a shorted cell will occur.
The paste material used to make battery plates also contains carbon black, blanc fixe (barium sulfate) and lignosulfonate. The blanc fixe acts as a seed crystal for the lead to lead sulfate reaction. The blanc fixe must be fully dispersed in the paste in order for it to be effective. The lignosulfonate prevents the negative plate from forming a solid mass of lead sulfate during the discharge cycle. It enables the formation of long needle like crystals. The long crystals have more surface area and are easily converted back to the original state on charging. Carbon black counteracts the effect of inhibiting formation caused by the lignosulfonates. It has been found that sulfonated naphthalene condensate dispersant is a more effective expander than lignosulfonate and can be used to speed up the formation of the battery plate. This dispersant is believed to function to improve dispersion of barium sulfate in the paste, reduce hydroset time, produce a stronger plate which is resistant to plate breakage, to reduce fine lead particles and thereby improve handling and pasting characteristics. It extends the life of the battery by increasing the end of charge voltage. The sulfonated naphthalene condensate polymer dispersant can be used in about one-half to one-third the amount of lignosulfonate and is stable to higher temperatures than lignosulfonate
About 60% of the weight of an automotive-type lead-acid battery rated around 60 Ah (8.7 kg of a 14.5 kg battery) is lead or internal parts made of lead; the balance is electrolyte, separators, and the case.
Separators
Separators are used between the positive and negative plates of a lead acid battery to prevent short circuit through physical contact, mostly through dendrites (‘treeing’), but also through shedding of the active material.
Separators obstruct the flow of ions between the plates and increase the internal resistance of the cell.
Various materials have been used to make separators:
- wood
- rubber
- glass fiber mat
- cellulose
- sintered PVC
- microporous PVC/polyethylene.
An effective separator must possess a number of mechanical properties; applicable considerations include permeability, porosity, pore size distribution, specific surface area, mechanical design and strength, electrical resistance, ionic conductivity, and chemical compatibility with the electrolyte. In service, the separator must have good resistance to acid and oxidation. The area of the separator must be a little larger than the area of the plates to prevent material shorting between the plates. The separators must remain stable over the operating temperature range of the battery.
Wooden separators were originally used, but deteriorated in the acid electrolyte. Rubber separators were stable in the battery acid.
Applications
- Wet cell stand-by (stationary) batteries designed for deep discharge are commonly used in large backup power supplies for telephone and computer centers, grid energy storage, and off-grid household electric power systems.
- Motor vehicle starting, lighting and ignition (SLI) batteries (car batteries) provides current for starting internal combustion engines.
- Gel batteries are used in back-up power supplies for alarm and smaller computer systems (particularly in uninterruptible power supplies) and for electric scooters, electrified bicycles and marine applications. Unlike wet cells, gel cells are sealed, with pressure relief valves in case of overcharging. In normal use they cannot spill liquid electrolyte.
- Absorbed glass mat (AGM) cells are also sealed and used in battery electric vehicles, as well as applications where there is a fairly high risk of the battery being laid on its side or over-turned, such as motorcycles.
- Lead-acid batteries were used to supply the filament (heater) voltage (usually between 2 and 12 volts with 2 V being most common) in early vacuum tube (valve) radio receivers.
- Lead-acid batteries are used in emergency lighting in case of power failure.
- Large lead-acid batteries are also used to power the electric motors in diesel-electric (conventional) submarines and are used on nuclear submarines as well.
Cycles
Starting batteries
Lead acid batteries designed for starting automotive engines are not designed for deep discharge. They have a large number of thin plates designed for maximum surface area, and therefore maximum current output, but which can easily be damaged by deep discharge. Repeated deep discharges will result in capacity loss and ultimately in premature failure, as the electrodes disintegrate due to mechanical stresses that arise from cycling. A common misconception is that starting batteries should always be kept on float charge. In reality, this practice will encourage corrosion in the electrodes and result in premature failure. Starting batteries should be kept open-circuit but charged regularly (at least once every two weeks) to prevent sulfation.
Deep cycle batteries
Specially designed deep-cycle cells are much less susceptible to degradation due to cycling, and are required for applications where the batteries are regularly discharged, such as photovoltaic systems, electric vehicles (forklift, golf cart, electric cars and other) and uninterruptible power supplies. These batteries have thicker plates that can deliver less peak current, but can withstand frequent discharging.
Marine/Motorhome batteries, sometimes called "leisure batteries", are something of a compromise between the two, able to be discharged to a greater degree than automotive batteries, but less so than deep cycle batteries.
Fast and slow charge and discharge
The capacity of a lead-acid battery is not a fixed quantity but varies according to how quickly it is discharged. An empirical relationship exists between discharge rate and capacity, known as Peukert's law.
When a battery is charged or discharged, this initially affects only the reacting chemicals, which are at the interface between the electrodes and the electrolyte. With time, these chemicals at the interface, which we will call an "interface charge", spread by diffusion throughout the volume of the active material.
If a battery has been completely discharged (e.g. the car lights were left on overnight) and next is given a fast charge for only a few minutes, then during the short charging time it develops only a charge near the interface. After a few hours this interface charge will spread to the volume of the electrode and electrolyte, leading to an interface charge so low that it may be insufficient to start the car.
On the other hand, if the battery is given a slow charge, which takes longer, then the battery will become more fully charged, since then the interface charge has time to redistribute to the volume of the electrodes and electrolyte, and yet is replenished by the charger.
Similarly, if a battery is subject to a fast discharge (such as starting a car, which is a draw of some 200 amps) for a few minutes, it will appear to go dead. Most likely it has only lost its interface charge; after a wait of a few minutes it should appear to be operative. On the other hand, if a battery is subject to a slow discharge (such as leaving the car lights on, which is a draw of only 6 amps), then when the battery appears to be dead it likely has been completely discharged.
Valve regulated lead acid batteries
The Valve Regulated Lead Acid (VRLA) battery is one of many types of lead-acid batteries. In a VRLA battery the hydrogen and oxygen produced in the cells largely recombine back into water. In this way there is minimal leakage, though some electrolyte still escapes if the recombination cannot keep up with gas evolution. Since VRLA batteries do not require (and make impossible) regular checking of the electrolyte level, they have been called Maintenance Free (MF) batteries. However, this is somewhat of a misnomer. VRLA cells do require maintenance. As electrolyte is lost, VRLA cells may experience "dry-out" and lose capacity. This can be detected by taking regular internal resistance, conductance or impedance measurements of cells. This type of testing should be conducted on a regular basis, as an indicator that more involved testing and maintenance may be required. Recent maintenance procedures have been developed allowing "rehydration" of cells that have experienced dry-out, often restoring significant amounts of the lost capacity.
VRLA types became popular on motorcycles because the acid electrolyte is absorbed into the medium which separates the plates, so it cannot spill. This medium also lends support to the plates which helps them better to withstand vibration. They are also popular in stationary applications such as telecommunications sites, due to their small footprint and flexibility of installation.
The electrical characteristics of VRLA batteries differ somewhat from wet-cell lead-acid batteries, and caution should be exercised in charging and discharging them.
Exploding batteries
Excessive charging of a lead-acid battery will cause emission of hydrogen and oxygen from each cell, as some of the water of the electrolyte is broken down by electrolysis. Wet cells have open vents to release any gas produced, and VRLA batteries rely on valves fitted to each cell. Wet cells may be equipped with catalytic caps to recombine any emitted hydrogen. A VRLA cell will normally recombine any hydrogen and oxygen produced into water inside the cell, but malfunction or overheating may cause gas to build up. If this happens (e.g. by overcharging the cell) the valve is designed to vent the gas and thereby normalise the pressure, resulting in a characteristic acid smell around the battery. Valves can sometimes fail however, if dirt and debris accumulate in the device, so pressure can build up inside the affected cell.
If the accumulated hydrogen and oxygen within either a VRLA or wet cell is ignited, an explosion is produced. The force is sufficient to burst the plastic casing or blow the top off the battery, and can injure anyone in the vicinity and spray acid and casing shrapnel to the immediate environment; an explosion in one cell may also set off the combustible gas mixture in remaining cells of the battery.
VRLA batteries usually show swelling in the cell walls when the internal pressure rises. The deformation of the walls varies from cell to cell, and is greater at the ends where the walls are unsupported by other cells. Such over-pressurized batteries should be isolated and discarded, taking great care using protective personal equipment (goggles, overalls, gloves, etc.) during the handling.
Environmental concerns
Currently attempts are being made to develop alternatives to the lead-acid battery (particularly for automotive use) because of concerns about the environmental consequences of improper disposal of old batteries and of lead smelting operations. Ni-Mn is already widely used in hybrid vehicles. Newer technologies are unlikely to displace lead-acid batteries owing to the much greater cost of potential alternatives. Nickel and Manganese are considerably more expensive than lead or antimony. As an example, at current (May 2008) prices quoted on the London Metal Exchange lead is about one tenth the price of nickel.
Lead-acid battery recycling is one of the most successful recycling programs in the world. In the United States 97% of all battery lead was recycled between 1997 and 2001. An effective pollution control system is a necessity to prevent lead emission. Continuous improvement in battery recycling plants and furnace designs is required to keep pace with emission standards for lead smelters.
Additives
Many vendors sell chemical additives (solid compounds as well as liquid solutions) that supposedly reduce sulfate build up and improve battery condition when added to the electrolyte of a vented lead-acid battery. Such treatments are rarely, if ever, effective.
Two compounds used for such purposes are Epsom salts and EDTA. Epsom salts reduces the internal resistance in a weak or damaged battery and may allow a small amount of extended life. EDTA can be used to dissolve the sulfate deposits of heavily discharged plates. However, the dissolved material is then no longer available to participate in the normal charge/discharge cycle, so a battery temporarily revived with EDTA should not be expected to have normal life expectancy. Residual EDTA in the lead-acid cell forms organic acids which will accelerate corrosion of the lead plates and internal connectors.
Active material (the positive plate lead dioxide and negative plate spongy lead) changes physical form during discharge, resulting in plate growth, distortion of the active material, and shedding of active material. Once the active material has fallen out of the plates, it cannot be restored into position by any chemical treatment. Similarly, internal physical problems such as cracked plates, corroded connectors, or damaged separators cannot be restored chemically.
Maintenance precautions
One precaution in workshops that handle large lead-acid batteries is a supply of ammonia solution to squirt on any spilled battery acid, to neutralize it. Surplus ammonia, and water, evaporate off, leaving a deposit of ammonium sulfate. Sodium bicarbonate (baking soda) is also commonly used for this purpose.
See also
External links
- , trade organization of lead-acid battery manufacturers.
-
-
-
-
-
-
-
- By Chris Yano
|
| |
|
|
