Lead-Acid Batteries Richard Perez In 1970, we realized that our dreams depended on cheap land. The only desirable property we could afford was in the outback. Everything was many miles down a rough dirt road and far from civilized conveniencies such as electricity. The 40 acres we finally bought is 12 miles from the nearest paved road, telephone, or commercial electrical power. We were ready to do without. This is not, however, an account of doing without-- it is a story of having one's cake and eating it too. We solved the problem of the rough road with a 4WD truck and countless hours of mechanical maintenance. The electrical power problem was not so easy to solve. We had to content ourselves with kerosene lighting and doing all our construction work with hand tools. The best solution the marketplace could offer was a motordriven generator. This required constant operation in order to supply power, in other words expensive. It seemed that in America one either had power or one didn't. We needed inexpensive home power. And we needed it to be there 24 hours a day without constantly running a motor. We decided on a 12 volt battery system. A lawnmower motor driving a car alternator recharges the batteries. To this we added a homemade control system. Later, we installed an inverter. We now have all the power we need, both 12 volts DC and 120 volts AC. This information on batteries is based on my over 17 years of actual experience with battery based alternative energy systems. Battery Terms The battery is the heart of all alternative energy systems. A battery is a collection of cells which store electrical energy in chemical reactions. Not all batteries are the same. They have evolved into different types to meet different needs. We are primarily interested in the true "Deep Cycle" lead-acid battery. This type is the most cost effective for home energy storage. In order to discuss these batteries, we need to agree on certain terms. The more we know about batteries, the better we can use them, and the cheaper our power will be. Voltage Voltage is electronic pressure. A car uses a 12 volt battery for starting. This voltage is the addition of the six lead-acid cells which make up the battery. Each individual lead-acid cell has a voltage (or electronic pressure) of about 2 volts. Commercial household power has a voltage of 120 volts. Batteries for alternative energy are usually assembled into packs of 12, 24, 32, or 48 volts. Current Current is the flow of electrons. The rate of this flow per unit time is the ampere. A car tail light bulb draws about 1 to 2 amperes. The headlights on a car draw about 8 amperes each. The starter draws about 200 to 300 amperes. Current comes in two forms-- direct current (DC) and alternating current (AC). Regular household power is AC. Batteries store power as direct current (DC). Power Power is the amount of energy that is being used or generated. The unit of power is the Watt. A 100 watt lightbulb consumes 10 times as much energy as a 10 watt lightbulb. The amounts of power being used and generated determine the capacity of the battery pack required by the system. The more electricity we consume the larger the battery must be. The power source must also be larger to recharge the larger battery pack. Battery Capacity Battery capacity is the amount of energy a battery contains. This is usually rated in ampere-hours at a given voltage. A battery rated at 100 ampere-hours will deliver 100 amperes of current for 1 hour. It can also deliver 10 amperes for 10 hours, or 1 ampere for 100 hours. The average car battery has a capacity of about 60 ampere-hours. Alternative energy battery packs contain from 350 to 4,900 ampere-hours. The specified capacity of a battery pack is determined by two factors-- how much energy is needed and how long must the battery supply this energy. Alternative energy systems work best with between 4 and 21 days of storage potential. A battery is similar to a bucket. It will only contain so much electrical energy, just as the bucket will only contain so much water. The amount of capacity a battery has is roughly determined by its size and weight, just as a bucket's capacity is determined by its size. It is difficult to water a very large garden with one small bucket, it is also difficult to run a homestead on an undersized battery. If a battery based alternative energy system is to really work, it is essential that the battery have enough capacity to do the job. Undersized batteries are one of the major reasons that some folks are not happy with their alternative energy systems. Battery capacity is a very important factor in sizing alternative energy systems. The size of the battery is determined by the amount of energy you need and how long you wish to go between battery rechargings. The capacity of the battery then determines the size of the charge source. Everthing must be balanced if the system is to be efficient and long-lived. State of Charge A battery's state of charge is a percentage figure giving the amount of energy remaining in the battery. A 300 ampere-hour battery at a 90% state of charge will contain 270 amperes-hours of energy. At a 50% state of charge the same battery will contain 150 ampere-hours. A battery which is dicharged to a 20% or less state of charge is said to be "deep cycled". Shallow cycle service withdraws less than 10% of the battery's energy per cycle. State of Discharge State of discharge is the inverse of state of charge. A battery at a 90% state of charge is also at a 10% state of discharge. These terms are important. It is critical for users to know when the battery is nearly empty and should be charged. We also need to know when the battery is full and when it is time to stop charging. We must know the battery's state of charge (or discharge) in order to properly cycle the battery. Lead-acid batteries Lead-acid batteries are really the only type to consider for home energy storage at the present time. Other types of batteries, such as nickel-cadmium, are being made and sold, but they are simply too expensive to fit into low budget electrical schemes. We started out using car batteries. Automotive Starting Batteries The main thing we learned from using car batteries in deep cycle service is DON'T. Automotive starting batteries are not designed for deep cycle service; they don't last. Although they are cheap to buy, they are much more expensive to use over a period of several years. They wear out very quickly. Physical Construction The plates of a car battery are made from lead sponge. The idea is to expose the maximum plate surface area for chemical reaction. Using lead sponge makes the battery able to deliver high currents and still be as light and cheap as possible. The sponge type plates do not have the mechanical ruggedness necessary for repeated deep cycling over a period of many years. They simply crumble with age. Types of Service Car batteries are designed to provide up to 300 amperes of current for very short periods of time (less than 10 seconds). After the car has started, the battery is then constantly trickle charged by the car's alternator. In car starting service, the battery is usually discharged less than 1% of its rated capacity. The car battery is designed for this very shallow cycle service. Life Expectancy and Cost Our experience has shown us that automobile starting batteries last about 200 cycles in deep cycle service. This is a very short period of time, usually less than 2 years. Due to their short lifespan in home energy systems, they are more than 3 times as expensive to use as a true deep cycle battery. Car batteries cost around $60. for 100 ampere-hours at 12 volts. Beware of Ersatz "Deep Cycle" Batteries After the failure of the car batteries we tried the so called "deep cycle" type offered to us by our local battery shop. These turned out to be warmed over car batteries and lasted about 400 cycles. They were slightly more expensive, $100. for 105 ampere-hours at 12 volts. You can spot these imitation deep cycle batteries by their small size and light weight. They are cased with automotive type cases. Their plates are indeed more rugged than the car battery, but still not tough enough for the long haul. True "Deep Cycle" Batteries After many battery failures and much time in the dark, we finally tried a real deep cycle battery. These batteries were hard to find; we had to have them shipped in as they were not available locally. In fact, the local battery shops didn't seem to know they existed. Although deep cycle types use the same chemical reactions to store energy as the car battery, they are very differently made. Physical Construction The plates of a real deep cycle battery are made of scored sheet lead. These plates are many times thicker than the plates in car batteries, and they are solid lead, not sponge lead. This lead is alloyed with up to 16% antimony to make the plates harder and more durable. The cell cases are large; a typical deep cycle battery is over 3 times the size of a car battery. Deep cycle batteries weigh between 120 and 400 pounds. We tried the Trojan L-16W. This is a 6 volt 350 ampere-hour battery, made by Trojan Batteries Inc., 1395 Evans Ave., San Francisco, CA (415) 826-2600. The L-16W weighs 125 pounds and contains over 9 quarts of sulphuric acid. We wired 2 L-16Ws in series to give us 12 volts at 350 ampere-hours. Types of Service The deep cycle battery is designed to have 80% of its capacity withdrawn repeatedly over a long period of time. They are optimized for longevity. If you are considering using battery stored energy for your homestead, this is the only type to use. Deep cycle batteries are also used for motive power. In fact more are used in forklifts than in alternative energy systems. Life Expectancy and Cost A deep cycle battery will last at least 5 years. In many cases, batteries last over 10 years and give over 1,500 deep cycles. In order to get maximum longevity from the deep cycle battery, it must be cycled properly. All chemical batteries can be ruined very quickly if they are improperly used. A 12 volt 350 ampere-hour battery costs around $400. Shipping can be expensive on these batteries. They are corrosive and heavy, and must be shipped motor freight. "Deep Cycle" Lead-acid Battery Performance The more we understood our batteries, the better use we made of them. This information applies to high antimony, lead-acid deep cycle batteries used in homestead alternative energy service. In order to relate to your system you will need a voltmeter. A Radio-Shack #22-191 Digital Multimeter (DMM) is a good deal. An accurate voltmeter meter is the best source of information about our battery's performance. It is essential for answering the two basic questions of battery operation-- when to charge and when to stop charging. Voltage vs. Current The battery's voltage depends on many factors. One is the rate, in relation to the battery's capacity, that energy is either being withdrawn or added to the battery. The faster we discharge the battery, the lower its voltage becomes. The faster we recharge it, the higher its voltage gets. Try an experiment- hook the voltmeter to a battery and measure its voltage. Turn on some lights or add other loads to the battery. You'll see the voltage of the battery is lowered by powering the loads. This is perfectly normal and is caused by the nature of the lead-sulphuric acid electrochemical reaction. In homestead service this factor means high powered loads need large batteries. Trying to run large loads on a small capacity battery will result in very low voltage. The low voltage can ruin motors and dim lights. Voltage vs. State of Charge The voltage of a lead-acid battery gives a readout of how much energy is available from the battery. Figure 1 illustrates the relationship between the battery's state of charge and its voltage. This graph is based on a 12 volt battery at room temperature. Simply multiply the voltage figures by 2 for a 24 volt system, and by 4 for a 48 volt system. This graph assumes that the battery is at room temperature, and is at rest; it is not being either charged or discharged. After recharging, the battery must rest for 6 to 12 hours before the voltage measurement will accurately indicate the state of charge. While discharging it is sufficient to let the battery rest for 10 to 60 minutes before taking the voltage reading. Voltage vs. Temperature The lead-acid battery's chemical reaction is sensitive to temperature. The chemical reaction is very sluggish at cold temperatures. Battery efficiency and usable capacity drop radically at temperatures below 40¡ F. We keep our batteries inside, where we can keep them warm in the winter. Batteries banished to the woodshed or unheated garage will not perform well in the winter. They will be more expensive to use and will not last as long. The best operating temperature is around 78¡ F.. Lead-acid batteries self-discharge rapidly at temperatures above 120¡ F. Consider running your batteries within a temperature range of 55¡ F. to 100¡ F. Determining State of Charge with a Hydrometer A hydrometer is a device that measures the density of a liquid in comparison with the density of water. The density of the sulphuric acid electrolyte in the battery is an accurate indicator of the battery's state of charge. The electrolyte has greater density at greater states of charge. We prefer to use the battery's voltage as an indicator rather than opening the cells and measuring the electrolyte's specific gravity. Every time a cell is opened there is a chance for contamination of the cell's inards. Lead- acid batteries are chemical machines. If their cells are contaminated with dirt, dust, or other foreign material, then the cell's life and efficiency is greatly reduced. If you insist on using a hydrometer, make sure it is spotlessly clean and temperature compensated. Wash it in distilled water before and after measurements. Rates of Charge/Discharge and Equalization Rates of charge and discharge are figures that tell us how fast we are either adding or removing energy from the battery. In actual use, this rate is a current measured in amperes. Say we wish to use 50 amperes of current to run a motor. This is quite a large load for a small 100 ampere-hour battery. If the battery had a capacity of 2,000 ampere-hours, then the load of 50 amperes is a small load. It is difficult to talk about currents through batteries in terms of absolute amperes of current. Battery people talk about these currents in relation to the battery's capacity. Rates of charge and discharge are expressed as ratios of the battery's capacity in relation to time. Rate (of charge or discharge) is equal to the battery's capacity in ampere-hours divided by the time in hours it takes to cycle the battery. If a completely discharged battery is totally filled in a 10 hour period, this is called a C/10 rate. C is the capacity of the battery in ampere-hours and 10 is the number of hours it took for the complete cycle. This capacity figure is left unspecified so that we can use the information with any size battery pack. For example, consider a 350 ampere-hour battery. A C/10 rate of charge or discharge is 35 amperes. A C/20 rate of charge or discharge is 17.5 amperes. And so on... Now consider a 1,400 ampere-hour battery. A C/10 rate here is 140 amperes, while a C/20 rate is 70 amperes. Note that the C/10 rate is different for the two different batteries; this is due to their different capacities. Battery people do this not to be confusing, but so we can all talk in the same terms, regardless of the capacity (size) of the battery under discussion. Let's look at the charge rate first. For a number of technical reasons, it is most efficient to charge deep cycle lead-acid batteries at rates between C/10 and C/20. This means that the fully discharged battery pack is totally recharged in a 10 to 20 hour period. If the battery is recharged faster, say in 5 hours (C/5), then much more electrical energy will be lost as heat. The heating of the batteries plates during charging causes them to undergo mechanical stress. This stress breaks down the plates. Deep cycle lead-acid batteries which are continually recharged at rates faster than C/10 will have shortened lifetimes. The best overall charging rate for deep cycle lead-acid batteries is the C/20 rate. The C/20 charge rate assures good efficiency and longevity by reducing plate stress. A battery should be completely filled each time it is cycled. This produces maximum battery life. We often wish to determine a battery's state of charge while it is actually under charge. Figure 2 illustrates the battery's state of charge in relation to its voltage for several charge rates. This graph is based on a 12 volt battery pack at room temperature. For instance, if we are charging at the C/20 rate, then the battery is full when it reaches 14.0 volts. Once again the digital voltmeter is used to determine state of charge without opening the cells and risking contamination. After several months, the individual cells that make up the battery may differ in their states of charge. Voltage differences greater than 0.05 volts between the cells indicate it is time to equalize the state of charge of the individual cells. In order to do this, the battery is given an equalizing charge. An equalizing charge is a controlled overcharge of an already full battery. Simply continue the charging process at the C/20 rate for 7 hours after the battery is full. Batteries should be equalized every 5 cycles or every 3 months, whichever comes first. Equalization is the best way to increase deep cycle lead-acid battery life. Battery voltage during the equalizing charge may go as high as 16.5 volts. This is too high for many 12 volt electronic appliances. Be sure to turn off all voltage sensitive gear while running an equalizing charge. The users of wind machines and solar cells are not able to recharge their batteries at will. They are dependendent on Mama Nature for energy input. We have found that all alternative energy systems need some form of backup motorized power. The motorized source can provide energy when the alternative energy source is not operating. The motorized source can also supply the steady energy necessary for complete battery charging and equalizing charges. The addition of a motorized source also reduces the amount of battery capacity needed. Wind and solar sources need larger battery capacity to offset their intermittent nature. Later in Home Power we will discuss making a very efficient and supercheap motorized 12 volt DC source from a lawnmower motor and a car alternator. Since most homestead battery packs are sized to last several days or weeks, the rate of discharge is not a concern. The same factors which limit the rate of charge also limit the rate of discharge. Deep cycle lead-acid batteries should not be repeatedly discharged at rates exceeding C/10. Self-Discharge Rate vs. Temperature All lead-acid batteries, regardless of type, will discharge themselves over a period of time. This energy is lost; it is not available for our use. The rate of self-discharge depends primarily on the battery's temperature. If the battery is stored at temperatures above 120¡ F., it will totally discharge itself in 4 weeks. At room temperatures, the battery will lose about 6% of its capacity weekly and be discharged in about 16 weeks. The rate of self-discharge increases with the battery's age. Due to self-discharge, it is not efficient to store energy in lead-acid batteries for periods longer than 3 weeks. Yes, it is possible to have too many batteries. If you're not cycling your batteries at least every 3 weeks, then you're wasting energy. If an active battery is to be stored, make sure it is first fully charged and then place it in a cool place. Temperatures around 35¡ F. to 40¡ F. are ideal for inactive battery storage. The low temperature slows the rate of self-discharge. Be sure to warm the battery up and recharge it before using it. Battery Capacity vs. Age All batteries gradually lose some of their capacity as they age. When a battery manufacturer says his batteries are good for 5 years, he means that the battery will hold 80% of its original capacity after 5 years of proper service. Too rapid charging or discharging, cell contamination, and undercharging are examples of improper service which will greatly shorten any battery's life. Due to the delicate nature of chemical batteries most manufacturers do not guarantee them for long periods of time. On a brighter note, we have discovered that batteries which are treated with tender love and care can last twice as long as the manufacturer's claims. If you're using batteries, it really pays to know how to treat them. Battery Maintenance There is more to battery care than keeping their tops clean. Maintanence begins with proper cycling. The two basic decisions are when to charge and when to stop charging. Begin to recharge the battery when it reaches a 20% state of charge or before. Recharge it until it is full. Both these decisions can be made on voltage measurement and the information on Figures 1 and 2. These rules apply to deep cycle lead-acid batteries used in deep cycle service. 1. Don't discharge a deep cycle battery greater than 80% of its capacity. 2. When you recharge it, use a rate between C/10 and C/20. 3. When you recharge it, fill it all the way up. 4. Keep the battery at room temperature. 5. Use only distilled water to replenish lost electrolyte. 6. Size the battery pack with enough capacity to last between 4 to 21 days. This assures proper rates of discharge. 7. Run an equalizing charge every 5 charges or every 3 months, whichever comes first. 8. Keep all batteries and their connections clean and corrision free. More detailed information on all types of batteries and their usage in alternative energy systems is available in The Complete Battery Book (TAB Book #1757) by Richard A. Perez, its ISBN number is 0-8306-0757-9. This book is available from your local library, your local bookseller, or from TAB Books Inc., P.O. Box 40, Blue Ridge Summit, PA 17214.