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A Guide To Lead-Acid Battries

Posted on Jun 4, 2011

General information about lead-acid batteries, charging practice, determining the state of charge and safety tips.

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Part 1: How Lead-Acid Batteries Work

Posted on Jun 4, 2011

Structure and Operation Most lead-acid batteries are constructed with the positive electrode (the anode) made from a lead-antimony alloy with lead (IV) oxide pressed into it, although batteries designed for maximum life use a lead-calcium alloy. The negative electrode (the cathode) is made from pure lead and both electrodes are immersed in sulphuric acid. When the battery is discharged water is produced, diluting the acid and reducing its specific gravity. On charging sulphuric acid is produced and the specific gravity of the electrolyte increases. The specific gravity can be measured using a hydrometer and will have a value of about 1.250 for a charged cell and 1.17 for a discharged cell, although these values will vary depending on the make of battery. The specific gravity also depends on the battery temperature and the above values or for a battery at 15°C. Specific gravity is defined as: The chemical reactions that occur during charging and discharging are summarised in figures 1 and 2. If lead-acid batteries are over discharged or left standing in the discharged state for prolonged periods hardened lead sulphate coats the electrodes and will not be removed during recharging. Such build-ups reduce the efficiency and life of batteries. Over charging can cause electrolyte to escape as gases. Types of Lead-Acid Battery Starting Batteries – Used to start and run engines they can deliver a very large current so a very short time, discharging by about 2-5%. If deep cycled these batteries quickly degenerate and will fail after 30-150 cycles but should last for a very long time when used correctly. Deep Cycle Batteries – Used to store electricity in autonomous power systems (e.g. solar, mini-hydro), for emergency back-up and electric vehicles. These batteries are designed to discharge by as much as 80% of their capacity over thousands of charging and discharging cycles. True deep cycle batteries have solid lead plates however many batteries that do not have solid plates are called semi-deep cycle. Marine Batteries – Usually a hybrid battery that falls between deep cycle and starting batteries although some are true deep cycle batteries. hybrid batteries should not be discharged by over 50%. Types of Deep Cycle Battery Flooded – These batteries have a conventional liquid electrolyte. Standard types have removable caps so that the electrolyte can be diluted and the specific gravity measured, such batteries are supplied dry and you add distilled water. Standard flooded batteries are cheap and if they are kept topped up they are not overly sensitive to high charging voltages. Sealed batteries are supplied pre-flooded and have fixed valves to allow gases to vent during use however, they will still leak if inverted and the electrolyte can not be replenished so that over charging will cause damage. Gelled Electrolyte – The electrolyte is a jelly and so will not leak. The electrolyte can not be diluted so that over charging must be avoided and these batteries may only last for 2 or 3 years in hot climates although with good care...

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Part 2: Discharging

Posted on Jun 4, 2011

Internal Resistance Batteries transfer energy to electrons so that they ‘flow’ around a circuit, the Electro Motive Force (EMF) is the total amount of energy per coulomb of charge that a battery can supply and is measured in volts. The EMF of a lead-acid cell is provided by that chemical reactions described above (figures 1 and 2) and can be seen as the maximum possible voltage across the cell’s terminals (the open circuit voltage). The path taken when current passes through the lead-acid cell will have resistance. This internal resistance depends on the cell’s design, construction, age and condition. On discharge this internal resistance (Rc) causes the voltage measured across the cell’s terminals to be less than the EMF (E) of the cell (the voltage drop = I x Rc, figure 3a). Thus when a current (I) flows the terminal voltage (U) is given by: Example A cell has an internal resistance of 0.02&omega and an EMF of 2.2V. what is its terminal potential difference if it delivers (a) 1A, (b) 10A and (c) 50A? Note that if a high resistance voltage meter is used to measure the voltage across a battery’s terminals it will register the batteries EMF; as long as there is no current flowing through a load from the battery (figure 3b). If the terminal voltage is measure when a current is flowing through a load from the battery, the meter will register the EMF minus the voltage drop across the internal resistance (figure 3a). When charging a cell the voltage applied across the terminals must be great enough to push the desired current against the cell’s EMF. Therefore the effective voltage across the internal resistance is the difference between the terminal voltage (in this case applied to the cell) and the the cells EMF. Therefore the current that flows is given by: and: Example A cell with an EMF of 2V and an internal resistance of 0.08Ω is to be charged at 5A. What terminal voltage must be applied? Cell and Battery Voltage A well maintained cell should have a cell EMF of about 2.2V falling to about 2V when fully discharged. Once the internal resistance has been taken into account the terminal voltage (the potential difference across the cell terminals) of each cell will be about 2.1V, but this value will drop depending on how much current is being drawn. Six cells in series make up a twelve volt battery which when fully charged will have a terminal voltage of 12.6 to 12.8V. The EMF of lead-acid cells is dependent on chemistry although the actual terminal voltage differs depending on the battery design, this must be taken into account when using a voltmeter to determining the batteries state of charge. Battery Capacity The capacity of a battery is usually expressed as a number of ampere-hours (Ah). One ampere-hour is the amount charge delivered when a current of one ampere is delivered for one hour. Since the capacity of lead-acid batteries depend on the rate...

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Part 3: Charging

Posted on Jun 4, 2011

Charge State There are two main methods for determining the state of charge for lead-acid batteries: Terminal Voltage – The open circuit voltage (no current flowing) of a fully charged cell depends on its type but will be 2.1V to 2.3V (12.6V to 13.8V for a 12V battery). If the voltage is measured with the charging current flowing it will be increased by the voltage drop across the internal resistance. If discharging the measured voltage will drop due to the internal resistance of the cell. Table 1 gives the approximate battery and cell voltages for various states of charge. Specific Gravity – This is the recommended method if the battery is not sealed and a hydrometer can get into the battery. For a flood-type battery in good condition the specific gravity should vary in the region of 1.25 for a fully charged battery to 1.17 for a fully discharged battery. These figures vary slightly depending on the battery type and the temperature: 0.0007 should be added to these values for each degree above 15°C. Table 2 gives the specific gravity values for several lead-acid batteries. Table 1: The approximate battery and cell voltages for various states of charge. State of Charge (approx.) 12 Volt Battery Volts per Cell 100% 12.70 2.12 90% 12.50 2.08 80% 12.42 2.07 70% 12.32 2.05 60% 12.20 2.03 50% 12.06 2.01 40% 11.90 1.98 30% 11.75 1.96 20% 11.58 1.93 10% 11.31 1.89 0% 10.50 1.75 Table 2: The approximate specific gravity values for several lead-acid batteries in various states of charge. * SG = specific gravity at 25°C. ** OCV open circuit voltage per 2V cell. State of Charge (approx) Apex Suncycle PVStar SG* OCV** SG* OCV** SG* OCV** 100% 1.277 2.12 1.240 2.0866 1.225 2.0950 90% 1.258 2.10 1.230 2.077 1.216 2.0775 80% 1.238 2.08 1.220 2.067 1.207 2.0600 70% 1.217 2.06 1.210 2.058 1.198 2.0425 60% 1.195 2.04 1.200 2.048 1.189 2.0250 50% 1.172 2.02 1.190 2.040 1.179 2.0075 40% 1.148 2.00 1.180 2.031 1.171 1.9900 30% 1.124 1.98 1.170 2.022 1.163 1.9725 20% 1.098 1.95 1.160 2.013 1.153 1.9550 10% 1.073 1.93 1.150 2.005 1.145 1.9375 0% 1.048 1.91 1.140 1.996 1.135 1.9200 Charging The charging voltage must be higher than the battery voltage for current to flow into the battery. There are two basic ways to charge a lead-acid battery from an uninterrupted supply (e.g. mains or a generator): Constant-voltage charge – A constant voltage is applied across the battery terminals. As the voltage of the battery increases the charging current tapers off. This method requires simple equipment but it not recommended. Constant-current charge – An adjustable voltage source or a variable resistor maintains a constant current flows into the battery. Thus requires a sophisticated charge controller. From uninterrupted power supplies lead-acid batteries are normally recharged using the constant-current technique; the manufacturer’s data should be checked to find an appropriate charging rate. A common rule of thumb used to calculate a suitable charging current is that it should be...

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Part 4: Battery Banks

Posted on Jun 4, 2011

Battery banks in small power systems normally have nominal voltages of either 12V or 24V however, lead acid batteries are available from 4V up to 24V. Batteries can be combined in series (figure 7a) so that their voltages are added together: two 12V batteries in series will provide 24V. Although voltages are add the same current will flow though each battery, so that two identical batteries 12V in series supplying 5A to a load each supply 5A: therefore the Ah capacity of two identical batteries in series is the same as one battery on its own. The total internal resistance (RC) of batteries in series will equal the internal resistances of the individual batteries added together. Example (a) A 12V battery with an internal resistance of 0.3Ω is connected to a load with a resistance of 4Ω.What Current will flow? (b) What current will flow in the same load if the current is supplied by two similar 12V batteries connected in series? When batteries are connected in parallel (figure 7b) they all operate at the same voltage and only identical batteries should every be connected in parallel. With this arrangement the total current being provided is split equally between the batteries so that two 12V batteries supplying 5A contribute 2.5A each, therefore the total capacity of these two batteries is twice the capacity of one battery supplying 2.5A (which in turn will be greater than the capacity of one battery supplying 5A). The internal resistances must be summed as if they are resistors in parallel; that is that the reciprocal of the total resistance equals the sum of the reciprocals of each resistor. Example From the previous example: (c) If three of the same 12V batteries are connected in parallel to the 4Ω what current flows? Total RC: Therefore: Battery banks may be constructed from several strings of batteries in series connected in parallel (figure 8); note that all of the batteries must be identical and of course all of the series strings must contain the same number of batteries. The EMF of such a bank is equal to the number of batteries in series multiplied by the battery EMF, the Ah capacity is equal to the capacity of one battery (at the appropriate rate) multiplied by the number of string in parallel and the total internal resistance is given by: Example Continuing the previous example: (d) If a battery bank consists of three strings of two batteries each what current will flow? The EMF of the battery bank is: The total internal resistance is: Therefore: Note that this is about the same current that is supplied by two batteries in series however, since there are three strings in parallel so the bank will be able to supply this current for more than three times as long (more than three times because the discharge rate has reduced). If the batteries have a capacity of 50Ah each at the 20 hour rate the bank will have a capacity of (3...

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Part 5: Maintenance And Safty

Posted on Jun 4, 2011

Care and Maintenance Some Lead-acid batteries are designed to be maintenance free, such batteries are sealed and the electrolyte can not be topped up. Batteries that are sealed and not vented should not receive equalisation charges. Batteries should not be left standing for any length of time, either charged or uncharged. Batteries supplied dry have a shelf life of about 2 to 3 years. Batteries should neither be over charged or discharged by more 60% of their capacity. Batteries in a bank should all be the same make and the same model. Once a battery bank has been operating for more than a few weeks, new batteries should not be added. The electrolyte should be topped up regularly with distilled water. Refer to the manufactures literature to find the recommended specific gravity. Equalisation charges should be used about once a month for batteries that are regularly cycled. Safety Lead-acid batteries can be dangerous because they vent hydrogen and oxygen gas during operation. The following points should be remembered: Keep the electrolyte in flooded cells at the correct level with distilled water, to make good losses due to evaporation and gassing. Use no materials or finishes which will be attacked by acid in the battery room. Spilled acid, and acid vapour given off during gassing, will quickly corrode most exposed metals other than lead. Use an asphalt floor where possible and coat wooden surfaces with anti-acid paint. Ventilate the battery room well, if necessary using corrosion-proofed fans. Do not allow a naked flame in the room; and prevent sparking by switching off a circuit before connecting and disconnecting. The gases present are explosive when in the correct proportions. Mop up spilt acid immediately and wash with soda solution. Acid on clothing will quickly cause holes to appear. Do not allow acid to enter the eyes. If it does so, immediately lie down and run clean water over the eyes for as long as possible. Consult a doctor. Acid on the hands is not itself dangerous, but can easily be transferred to more vulnerable parts of the body. When mixing acid for the initial charge of new cells, always add acid to water, and not the reverse. Watch cell temperature, as excessive heat will damage lead-acid cells. Acid temperature should not exceed 36°C. Keep battery terminals clean and coated with petroleum jelly. Remember that a short-circuit across the battery or cell terminals can result in very high currents which may result in fire or burns. Use only insulated tools and take great care not to allow terminals to connect together...

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