Lead-Acid Batteries |
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All lead-acid cells have the same basic parts and chemistry. Initially you start with 2 electrodes made of lead sulfate (PbSO4) and electrolyte that is almost all pure distilled water, with just a little sulfuric acid added so it can conduct some current. For the lead-acid cell to deliver energy the electrodes must be 2 dissimilar metals and the electrolyte must be active. When a charging source is connected to the 2 electrodes, the electrode connected to the - pole becomes spongy lead (losing its SO4) and the electrode connected to the + pole becomes lead peroxide (PbO2), (losing its SO4 and getting an O2 from the water). This frees 4 each H ions from the water left over, which combine with the 2 each SO4's from the lead to make 2 molecules of H2SO4 (Sulfuric Acid). This reaction can only take place on the surface of the electrodes, so once all the available surface area is used up, the cell will not accept any more charge. When the cell reaches about 70% charge, oxygen starts being produced by the positive electrode. At about 90% charge hydrogen starts being produced by the negative electrode. This hydrogen and oxygen comes from the disassociation of the water in the electrolyte. In flooded cells this water loss can be made up through the addition of water through the inspection caps, but in sealed cells the hydrogen and oxygen must be recombined. Because lead sulfate is mechanically weak, it is usually made into a paste and fitted into a lead alloy grid which gives it more strength. Antimony is used as the strengthening alloy in open cells, but antimony contributes to the evolution of hydrogen gas at the negative plate, so the plates in sealed, gelled-electrolyte cells are normally alloyed with calcium (PbCa) or lead calcium tin (PbCaSn). Because sealed cells are sensitive to traces of metal contaminates, the lead oxide used in the positive plate is made from pure virgin lead.
Lead-acid batteries can be roughly grouped into three categories by construction and intended use:
Automotive starting batteries are constructed with thin pasted plates inside a lead-antimony alloy grid and are designed to supply high currents for brief periods of time (starting an engine). They are expected to be recharged immediately after discharge. The are essentially designed for float service, being discharged less then 10% and immediately recharged. Starting a car uses only 2 or 3 % of the original capacity of the battery. If used in deep- discharge service they will only last for 100 to 200 discharges. (i.e., the capacity of the battery will diminish fairly quickly. While it will still act as a battery, it will not be able to supply its rated capacity soon after being placed in the wrong kind of service.) The Diesel starting battery delivers a little more starting current, but still has much the same characteristics. The weight of this kind of battery is 35-45 lbs.
The commonly available Deep Cycle Marine/RV batteries have a life of 200 to 1000 cycles at a 50% discharge. Not all deep discharge batteries are constructed the same. The three types are the flooded cell, the gel cell and the absorbed electrolyte. The flooded cell is the older design with caps for testing and electrolyte replacement, or a maintenance-free type with special alloys (calcium or tin) to minimize water consumption and sufficient electrolyte to last the life of the battery. The gel cell, which has been around for 60 years, has about 92% of the electrolyte of a flooded cell. The absorbed electrolyte cell has about 95% of the electrolyte of a flooded cell, captured in a mat of randomly oriented borosilicate glass fibers, between the plates. Both the gel cell and the absorbed electrolyte cell are constructed so that the gases given off in charging and discharging are recombined, thus eliminating the need to add water. Marine/RV batteries weight 55-65 lbs.
Traction batteries are made with thick pasted plates with more antimony added for strength, and have thicker separators between the plates. This makes them more rugged and better able to withstand physical abuse, and also reduces the chance of failure due to dendritic growth during recharging (those are "whiskers" of metal can grow between plates, shorting out the battery). These batteries are sold for use in electric forklifts and golf carts. They are designed to be deep discharged (80% of capacity) 400 to 2600 times in their life, and be recharged each night. Because there is some tradeoff in battery life by using the pasted plate construction to keep the size and weight of the battery down, they are not as long-lived as stationary batteries. A 6 Volt 200 Amp Hour (Ah) golf cart battery weighs 66 lbs.
Stationary batteries are made with thick solid plates (lead-calcium). They are designed to be used as standby power; supplying minimal power most of the time and kept in a state of nearly full charge until needed. They can take deep discharge. Because of the solid plate structure, they are bigger and heavier, and their lifetime is much longer, 20 years is the rated lifetime. (Life to 80% of their original capacity). Exide Corp., for their Stationary Lead-Calcium cells, specifies no more then 2 total discharges per year, for the 20 year life of their battery, to maintain the battery in warranty condition. Some photovoltaic storage batteries (for solar-powered homes and such) are the stationary type, as are back-up power systems in generating stations and telephone companies. A 2 volt cell can be 100 lbs and up and have gallons of sulfuric acid in it.
The best battery for large back-up applications is the Stationary battery. For most home supplies, much more readily obtainable are the Marine/RV batteries. An Eveready Marine/RV battery (12V,115Ah) is $40 at Price Club (life 150 deep cycles), a Trojan Deep Cycle Golf Cart battery (6V, 217Ah) is $100 (life 700 deep cycles), A West Marine absorbed electrolyte Deep Cycle (12V,86Ah) is $180 (life 400 deep cycles). A 2 volt stationary cell of 2000 Amp Hour (Ah) capacity costs about $940.00. That would be $5640 for 12 Volts, or $282 /yr. over a 20 year life. The 20 Golf Cart Batteries needed to equal this would cost $2000 and have a life of maybe 7 years. This would be $286/yr.
| Note: | All the voltages given are for batteries at working temperature - typically 77 to 80F (25 to 26.6C). |
Battery capacity depends on the type of service and how the battery is constructed. When the flooded cell is compared with the gel cell or an absorbed electrolyte cell in extended discharge service, such as over a 5 hour period, the flooded cell can supply more power. An absorbed electrolyte cell will provide 70-80%, while a gel cell will give 68-75% of the power that can be provided by a flooded cell of the same size.
The Battery Council International (BCI) has a set of standard for battery comparison. Automotive batteries are rated in Cold Cranking Amps (CCA) which is the amount of available current for 30 seconds at a temperature of 0 degrees F, that will cause the output voltage to drop to 7.2 volts. Some deep discharge batteries are also rated in Marine Cranking Amps, this is at 32F. Another measure is Reserve Capacity (RC). This is the amount of time for a battery to reach the cut-off voltage, at a temperature of 77F, when delivering 25 amps. Both charge and discharge rates are specified in terms of the capacity of the battery "C". For traction type batteries, capacity is a 5 hour rate, so a fully-charged 100 Ah traction battery in good condition can supply 20 amps for 5 hours before it reaches its' cut-off voltage. This is a C/5 rate. Stationary batteries are usually rated at a 8 hour rate, C/8; while the capacity of an automobile battery is figured at a 20 hour rate, C/20. You will get the most efficient output, and the most amp hours, if you size your batteries such that you use the C/20 rate for all types of batteries. This means if you use 5 amps, your battery should be 5 X 20, or 100 Ah. The most efficient depth of discharge in terms of capacity and recharge energy is 80%, but this will reduce the number of cycles that the battery can be used compared to a more conservative 50% In the example above, the 100 Ah battery should only be discharged for 10 hours at the 5 Amp rate (10 hrs x 5 Amps = 50 Ah or 50% of C), with a maximum of 16 hours (16 hrs X 5 Amps = 80 Ah, or 80% of C).
The discharge curve is NOT linear; if you double the current drain, you will get less than half the time. Similarly, if you halve the drain, you will get more than twice the time. The chemical reaction between lead and sulfuric acid is only about 80% efficient, and high discharge currents waste energy heating the battery and electrolyte.
Each type of battery has a specified voltage at which it is considered completely discharged. If discharge continues below this voltage, the battery life will be considerably shortened, and repeated abuse of this kind can result in a battery which is difficult to recharge. Each battery manufacturer specifies this voltage; in general, the final voltage for the three types of batteries are:
Thus a typical 12 volt marine battery with 6 cells should not be discharged below about 10.2 volts. Another way of looking at it is that no cell should be discharged more than about .3 v below its full-charge rest voltage.
A typical cell will show the following voltages:One time honored method of testing a charge is to measure the specific gravity of the electrolyte. At 77F a fully charged automotive cell should show a Sp.Gr. of 1.260 to 1.280. A traction battery cell may be between 1.280 and 1.300. A fully discharged cell will read 1.130 or less. In very cold climates batteries can use more acid to promote the chemical reaction (and help prevent freezing), and in the tropics a little lower specific gravity will retain capacity while reducing corrosion and extending life. If in doubt, contact the manufacturer. In large stationary cells, acid is drawn from the top, middle and bottom and the readings are averaged. Smaller cells can be mixed by drawing and returning the acid to the battery several times. Maintenance-Free Batteries sometimes have fill plugs beneath plastic labels, allowing access to the cells for testing and adding distilled water if necessary.
All batteries self-discharge, (lose their charge) when they sit. Because this is a chemical action, higher temperatures cause batteries to self-discharge quicker. A battery that only loses 3 % of its charge a month at 77F, like a good maintenance-free absorbed electrolyte Marine Battery, will lose 10% a month at 95F. A flooded cell battery can lose 5% a week and have a 20% a week self-discharge rate at 95F. When they near the end of their useful life the self-discharge rate can be 50% a month.
When a battery discharges, water is formed and sulphur combines with the lead making lead sulfate. If not recharged soon, the lead sulfate "hardens" and causes a condition called "sulfation". Sulfation is one of the major causes of battery failure. One theory is that the lead sulfate changes from an ionic to a covalent chemical bond and it requires a lot more energy to break this bond apart. This covalent bond doesn't change back to spongy lead or lead peroxide when the battery is recharged, thus lessening the capacity of the battery.
CHARGING:In general, liquid-electrolyte lead-acid batteries can be recharged at any rate that exceeds internal and surface discharge rates, and which does not cause excessive gassing (liberation of oxygen, hydrogen, and steam). Do not charge at higher rates then the manufacturer recommends. Excessive gassing (as from overcharging) causes the electrical resistance of the plates to increase, so the energy of the charger goes into heating the cell, and breaking down the water. This rate of gas production can be so great that it causes sealed cells to explode. Even small flooded cell batteries, such as motorcycle batteries, can be melted by the output of an automobile type charger. NEVER OVERCHARGE A BATTERY. The water loss causes the sulfuric acid to get stronger, causing damage to both the plates and the grid structure through both corrosion and sulfation. The steam will also carry sulfuric acid into the air, causing corrosion to the wiring leading to the battery, as well as the battery mounting hardware.
There are four types of charge. A bulk charge, with a rate from C/3 to C/10; a finishing charge, C/15 to about C/25; a trickle charge, which should replace the energy lost due to self-discharge; and the equalizing charge, which puts 2.75 volts per cell into the battery and is a deliberate overcharge condition. Because the conversion from electrical to chemical energy is only 80% efficient, a battery at a rate of C/14 should be charged about 17.5 hours (14 hrs x 1.25 = 17.5 hrs).
In non-float service, there are several simple chargers. A single-rate (constant-current) charger limits its charge rate to about 7% of the Ah capacity of the battery; for a 100 Ah battery, it would charge at a rate of 7 amperes. Since the battery will start at about 2.1 v/cell, and finish at about 2.7 v/cell, the charger must be able to vary its voltage over this range. For a "12 volt" battery with 6 cells, the charger will need to supply between 12.6 and 16 volts over the duration of the charge. Charging is complete when the battery reaches 2.65 to 2.7 v/cell.
A simple taper charger is a constant-voltage source set to 2.8 v/cell with a series ballast (typically a resistor, but a choke or the internal resistance of the supply can be used) that limits the output current to 7% of the capacity at 2.1 v/cell, as the voltage increases the current will drop. This type of charger may take 24 hours to bring a battery up to full charge. Again, charging is complete when 2.7 v/cell is reached.
Trickle-charging of a fully-charged battery can be done to keep it charged. This is done by supplying .5 to 1 milliAmp per Ah capacity. Trickle charging should be discontinued when it has continued for at least 24 hours and the battery has reached 2.25 v/cell. Typically, trickle chargers are set to run perhaps once a week.
Some interesting research results are that using pulsating rectified AC (positive and negative pulses) or superimposing a small AC current on pure DC charging current increases battery life by up to 30%. Apparently the mechanism is that it reduces gassing (which increases charging time and battery heat) and leads to a more porous lower-resistance plate, and lessens the tendency to form dendrites during charging. Immediately after the pulse the battery can accept a large charging pulse of energy. (on the order of 1000 amps). This is repeated at a fast rate until the battery is charged. This is one of the methods that will be used to fast charge electric vehicles.
Another method, which reduces sulfation of the plates, puts a short (.3uS) pulse of .05 to.1 amp into the battery at a rate of 2 to 10 kpps. This pulse excites the sulfur atoms locked onto the plates and provides the energy for them to return to solution, much like a microwave oven heats water molecules. A solar powered "Solargizer" made by Motor Products of Irving, TX costs about $60 for a 12 volt system, and it is claimed that it will maintain automotive batteries in a charged state without the degrading effects of normal trickle charging. It can plug into a cigarette lighter and the solar cell can lay on the dashboard of a car. It has been adopted by the military to maintain batteries on vehicles in storage or where there isn't a lot of regular use, like snowplows or aircraft ground support equipment. I'm not sure that it would do much for a car that is driven every day and has a good charging system, but it might be a hot ticket for RV's and boats that only see use once or twice a month. It has been used to "restore" batteries that won't take a charge, being left on the battery for several days, then the battery is charged normally. Another company, Pulse Charge Systems, Inc. of Rockwall, TX., claims their "PulsePak" can restore badly sulfated batteries in a few days. In one example they give, a battery with 200 deep cycles failed. After being charged with the Pulse-Pak the life was extended 105 more cycles. It also enables batteries to be recharged faster, accepting more current at the start of the charge cycle. A 12 volt system for up to 1500 Amp Hour of batteries costs $250. (Both are available from Specialized Products Corp. 3131 Premier Dr., Irving, TX 75063, (800)866-5353)
In float service, where the battery is in parallel with the mains supply, the supply voltage must be set to 2.15 to 2.25 v/cell. This will charge the battery, and avoids excessive gassing, but does not serve to "freshen" the cells - there is not enough gassing activity to move electrolyte around, and the electrolyte in the cell stratifies. (Heavy acid drifts down, while the water lies on the top. This reduces the effective plate area and decreases the current the battery can supply.) An equalizing charge causes gassing to take place and mixes the electrolyte. The bubbling action also helps remove sulfation, lead sulfide that has become hard and brittle and won't react with the electrolyte. A disadvantage of the equalizing charge is that it also causes lead from the plates to go into solution, reducing the plate thickness and allowing the lead to precipitate out on the bottom of the battery, forming a layer that will eventually short out the plates.
It is recommended that batteries in float service occasionally be charged to 2.40 v/cell to freshen and equalize the charge. Some types of stationary cells require 2.70 v/cell for this. This is done every 18 months, or when water is added to a cell, or when one cell has a voltage reading of less then 2.13 volts. In large installations, this is done by switching parts of the battery banks out of service in rotation. In smaller systems that can tolerate the voltage excursion, it can be done by simply boosting the output of the mains supply.
Charging of flooded cell type batteries, those with caps to add water, inevitably leads to some water loss due to gassing; 100 Ah of a gassing charge (2.4 v/cell) will yield about 1.2 oz of water loss. Hydrocap Corp ( 975 NW 95th St, Miami FL, (305)696-2504] makes a replacement filler cap that contains a catalytic material that recondenses emitted steam, and recombines the hydrogen and oxygen gasses into pure water that then dibbles back into the cell, greatly reducing the required maintenance. With the available flame arrestor option, they sound ideal for unattended battery systems, and should greatly reduce the danger of fire and explosion from liberated hydrogen. They're about $5-$10 per cell. All batteries should have the electrolyte above the plates. Depending on the construction this can be from 3/8 to an inch or more. Automobile and RV batteries have a ring in the cap well, or markings on the side of a translucent case. Use distilled water to top off a cell that is low. A solution to the problem of outgassing is to use lead alloys that are antimony free. The negative plates of gel-cells are alloyed with calcium or lead-calcium-tin while the positive plates are made of virgin lead.
On The Horizon:The Horizon (trademark name) battery is being developed for electric vehicle (EV) use by Austin, TX based Tracor, Inc and Electrosource, Inc. of San Marcos, Tx. in a joint venture called Horizon Battery Technologies, Inc. It is a lead acid system, but not built like any other battery.
Patented features are:
High tensile strength material
Bipolar grid construction
Horizontal plate orientation
Compression cage assembly
The Horizion battery has a predicted life of 1000 discharges at a moderate depth of discharge (36%). Normal rate recharge should be about 3 hours while a high rate recharge should take about 30 minutes. The battery is light weight, with a specific power of 450 watts per kilogram. Initially the battery will cost about $400/kWh dropping to $150/kWh in a couple of years. Life expectancy is 5 years in EV service.
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