EE Chapter 2 Aircraft Storage Batteries

Transcription

1 EE Chapter 2 Aircraft Storage Batteries Two types of batteries used on nearly all aircraft are nickel cadmium and lead acid batteries. All batteries produce dc voltage. 2.1 Dry Cells and Batteries Voltaic Cells If two plates of dissimilar metals are placed in a chemical solution called an electrolyte, opposite electric charges will be established on the two plates. An electrolyte is technically defined as a compound which, when molten or in solution, conducts electric current and is decomposed by it. Chemical action in a voltaic cell. The carbon and zinc elements are called electrodes. The carbon, which is the positively charged electrode, is called the anode, and the zinc plate is called the cathode. When the two electrodes are connected by an external conductor, the free electrons from the zinc plate flow to the carbon rod. The standard dry cell used in flashlights and for other purposes for which a low voltage d c supply is desired employs a compound called manganese dioxide (MnO 2 ) to prevent the accumulation of hydrogen at the positive electrode in the cell. For a dry cell, the electrolyte is in the form of a paste. And the voltage developed by a zinc carbon cell is approximately 1.5 V. 1

2 A cell that cannot be recharged satisfactorily is called a primary cell. Secondary cells may be charged and discharged many times before they deteriorate to the point at which they must be discarded. (Non rechargeable primary cell and rechargeable secondary cell) Alkaline and Mercury Cells Voltaic cells utilizing an alkaline electrolyte are usually termed alkaline cells. The electrolyte consists primarily of a potassium hydroxide solution. Various electrode materials will determine if the cell is primary or secondary cells. Most common alkaline cells produce approximately 1.5 V without a load applied to the cell. Mercury cells are commonly used for many applications and are made in sizes to fit the device in which they are to operate. A mercury cell consists of a positive electrode of mercuric oxide mixed with a conductive material and a negative electrode of finely divided zinc. Mercury cells are often used for small button batteries found in miniature equipment such as watches and calculators. Nickel Cadmium Cells Nickel cadmium electric cells and batteries have been developed to a high degree of efficiency and dependability. In a charged state, the negative electrode consists of metallic cadmium and the positive electrode is nickel oxyhydroxide. During discharge electrodes alter chemical composition. An advantage of the nickel cadmium secondary cell is that it can stand in a discharged condition indefinitely at normal temperatures without deterioration. If a lead acid battery is left in the discharged condition for a substantial period of time, sulfation of the plates occurs, and the cells lose much of their capacity. Nickel cadmium cells generate gas during the latter part of a charge cycle and during overcharge. Hydrogen is formed at the negative electrode and oxygen is formed at the positive electrode. In ventedtype batteries, the hydrogen and oxygen generated during overcharge is released to the atmosphere together with some electrolyte fumes. 2

3 Open and Closed Circuit Voltages The voltage measured when there is no load applied to the battery is called the open circuit voltage (OCV) The voltage measured while a load is applied to the battery is called the closed circuit voltage (CCV). OCV is always higher than the CCV. The OCV of a fully charged aircraft battery may reach 13.2 V, however when even a small load is applied, the CCV will measure near 12 V 12 V battery. Internal Resistance The resistance present inside a battery while connected to a load is called internal resistance (IR). A battery s IR is equal to the difference between the OCV and the CCV, divided by the applied load. See an example for a calculation. A battery s IR always becomes greater as the battery becomes discharged. A battery s IR will not affect an aircraft electrical system until that the battery becomes over 75% discharged. When the battery reaches this low state, its IR becomes too high and the CCV lowers. 2.2 Lead Acid Storage Batteries Two types of lead acid batteries currently being used in aviation are 1) the vented cell and 2) the sealed (recombinant gas) battery. Lead acid batteries are being used to replace the more expensive nickel cadmium battery in some turbine powered aircraft. But nickel cadmium battery will remain a practical power source for aircraft due to the ability to generated higher current than the lead acid batteries. 3

4 Lead acid secondary cells consist of lead compound plates immersed in a solution of sulfuric acid and water which is the electrolyte. When connected to a substantial load, the voltage is approximately 2 V. Aircraft storage batteries of the lead acid type are generally rated at 12 or 24 V. To increase both the voltage and the amperage by combining single cells, the cells are connected in a series parallel circuit. It should be noted that the aircraft storage battery is used only when other sources of electric power are not available. The storage battery would supply for about 30minutes of emergency power. 4

5 Cold Weather Operation A battery will give much better performance in temperature or tropical climates than in cold climates. However, a battery will deteriorate faster in a warm climate. 2.3 Lead Acid Battery Maintenance Procedures Precautions: 1) Always wear safety glasses. 2) Remove the negative lead first and install it last. 3) Do not cause a short circuit between the batter terminals. Be cautious of jewelry and watches. 4) Never service the batteries near an open flame or sparks. 5) Never jump start an aircraft from another power source if the plane s battery is discharged. Lead acid battery inspection and service General guide 1) Inspect the mounting of the battery 2) Remove the cover from the battery case, if it is the covered type, and inspect the interior. The top of any storage battery should be kept clean and dry. 3) Check the electrolyte level in the battery. If the liquid is below the plates of the battery cell, add clean distilled water until it is approximately 3/8 in (0.95 cm) above the plate. Always add distilled water only, never electrolyte. The proper level should be above the plate and about 1 in. below the top of the battery. 4) If the battery is suspected of being defective, perform a batter load test or a hydrometer test. 5) Inspect the terminal connections. 6) Inspect the battery cables 7) Replace the cover on the battery case, making sure that the hold down nuts are tightened sufficiently. 8) Inspect the ventilation system of the aircraft and the battery box. Hydrometer Test and Battery Load Testers A hydrometer is a tool used to measure the specific gravity or density of a liquid (for determining the batteries state of charge). 5

6 The specific gravity of a substance is defined as the ratio of the weight of a given volume of that substance to the weight of an equal volume of pure water at +4 C. The specific gravity of the electrolyte in a lead acid cell decreases as the charge in the cell decreases. The specific gravity will vary from the true specific gravity as the temperature goes above 90 F or below 70 F. The correction points represent thousandths. For example, if the temperature of the electrolyte is 10 F and the hydrometer reading is 1.25, the correct reading will be = While an automatic battery load test is being performed, the load is applied for 15 s, and OCV and CCV are automatically compared. The high rate discharge battery test is probably the most common and practical test used. This test is designed to simulate the load placed on the battery during an engine start. Any CCV below 9 V indicates a very weak or dead battery. Battery charging: Secondary cells are charged by passing a direct current through the battery in a direction opposite to that of the discharge current. 6

7 Two types of charging equipment are constant current chargers (left) and constant voltage chargers (right). Constant current charge 1) Constant current charge supplies a consistent current to a battery for the entire charge cycle. 2) The constant current charger will increase its voltage supplied to the battery during charge in order to maintain the current flow set by the operator. 3) Constant current chargers are often used on new batteries where the initial charge state is known, but they are seldom used on lead acid batteries that have been placed in service. 4) Nickel cadmium batteries often use constant current charging equipment. Constant voltage charge 1) Constant voltage charge supplies a consistent voltage to a battery and allows current to change as the battery becomes charged. 2) Constant voltage charger supplies 14 V for charging 12 V batteries and 28 V for charging 24 V batteries. 3) Since the charger supplies a constant voltage, a relatively high current will flow into a discharged battery, and that current will slowly diminish as the battery becomes charged. 4) Because the current supplied to the battery drops to a very low value as the battery becomes charged, constant voltage charging is usually considered the safest method of battery charging. Charging precautions 1) Always charge batteries in a well ventilated area. 2) Always turn off the battery charger before disconnecting any connections between the battery and the charger. 3) Always removing the battery from the aircraft, always disconnect the negative lead first. When installing the battery, always connect the negative load last. 4) Make sure that the caps of each cell of the battery vented and the vents are clean. 7

8 5) Remove the battery from the aircraft prior to charging whenever possible. The electrolyte will corrode the aircraft if the battery is charged while in the airplane. 6) Always take precautions not to spill electrolyte on skin or clothes. Always wear safety glasses. 2.4 Battery Ratings When cells are connected in series, the total battery voltage is equal to the sum of all the voltage of each cell. Capacity is the measure of a battery s total available current. Small batteries are normally rated in milliampere hours (mah). Large batteries are rated in ampere hours. A battery s capacity is equal to the time required to fully discharge that battery multiplied by the current draw applied to the battery. For example, if a battery can supply 2 A for 2 h, it has a capacity of 4 Ah. All aircraft batteries must be discharged over a consistent time when determining capacity. The standard of a 5 h discharge rate is typically used. All batteries will be discharged for 5 h to determine their capacity rating. Storage batteries of all types are rated according to voltage and ampere hour capacity. The fully charged lead acid cell is approximately 2.1 V when the cell is not connected to a load. A nickelcadmium cell is rated at about 1.22 CCV. Batteries are normally rated as 6 V (3 cells), 12 V (6 cells) and 24 V (12 cells). Another rating applied to storage batteries is known as 5 min discharge rate. This rating is based on the maximum current a battery will deliver for a period of 5 minutes at a starting temperature of 80 F and a final average voltage of 1.2 V per cell. The 5 min rating gives a good indication of the battery s performance for the normal starting of engines. 2.5 Nickel Cadmium Storage Batteries An advantage of the Ni Cd cell is that it contains a greater power to weight ratio than a lead acid battery. The CCV of a Ni Cd battery remains nearly constant during the entire discharge cycle. Ni Cd batteries are much more costly than a typical lead acid battery and therefore are usually found on turbine powered aircraft. The Ni Cd cell is a vented cell similar to that of a lead acid battery The cells are placed in an insulated metal or plastic case in proper order and then connected in series by the cell connectors. Most Ni Cd aircraft batteries contain vented cell caps to allow any expanding gases inside the cell to escape. 8

9 The thermal runaway is a condition where the battery chemicals overheat to such a degree that the battery can be destroyed or even explode. If the battery reaches a thermal runaway condition, the pilot must disconnect the battery from the electrical system and land the aircraft as soon as practical. The electrolyte for a Ni Cd is a solution of 70% distilled water and 30% potassium hydroxide, which gives specific gravity of 1.3. Active material is not dissolved by the electrolyte of potassium hydroxide. The cells are very stable even under a heavy load, and the chemicals last a long time before the battery requires replacement. The specific gravity of the electrolyte remains essentially constant at any state of the discharge. The OCV of 1.28 V is consistent for all Ni Cd vented cells, regardless of cell size. The CCV of a vented cell Ni Cd battery ranges between 1.2 and 1.25 V. The Ni Cd is suitable for staring turbine engines due to the fact that it has tremendous peak power and delivers far more power than a lead acid battery of the same size and weight. 9

10 Most Ni Cd batteries are designed for 24 V system with a capacity between 22 and 80 Ah. The internal resistance of most vented Ni Cd cells is very low. This allows it to recharge very rapidly. 2.6 Nickel Cadmium Battery Maintenance Procedures The Ni Cd requires specific maintenance procedures. Battery inspection 1) Inspect the battery case for cracks, distortion or other damage. 2) Inspect the vent system for proper airflow 3) Inspect the cells and clean as needed. 4) Inspect the cell connectors for corrosion, cracks and overheating. 5) Inspect the cell caps 6) Check the cell electrolyte level for proper amounts. Never add water to a discharged battery or a battery of unknown charge. The electrolyte level increases significantly during charging, therefore, if water is added before charging, an overfill situation is likely. The reconditioning of Ni Cd aircraft batteries is usually performed between 100 and 300 flight hours. Any service area used to recondition Ni Cd batteries should be separated from the service area for leadacid batteries. During reassembly, always observe correct cell polarity and always use the proper torque values on each cell connector bolt or nut. 10

11 Recharging the battery during reconditioning can be performed by a constant current or constant voltage charger. However, constant current charging typically requires a longer charging time and creates a greater water loss during overcharge than constant voltage charging. Always follow the manufacturers recommendation when charging batteries. Ni Cd batteries can be stored for long periods of time in a charged or discharged state without damage. 2.7 Installation of Aircraft Batteries The battery compartment should be easily accessible so that the battery can be serviced and inspected regularly. Inside the compartment must be coated with a paint that will prevent corrosion caused by electrolyte. The battery must be held firmly in place with bolts secured to the aircraft structure. Batteries should not be located in engine compartments. 11

12 Always perform a thorough battery inspection prior to installation in any aircraft. This inspection should include an electrolyte level check, a cell connector inspection and a check of general battery condition. 2.8 Latest Issues Grounded Boeing 787 Dreamliners Use Batteries Prone to Overheating [20 Jan 2013, boeing 787 dreamliners use batteries proneto overheating/] Two major safety incidents involving Boeing 787 Dreamliners have caused two Japanese airlines to ground their fleets of the aircraft. The problems may be linked to a battery chemistry that s particularly prone to causing fires. Earlier today, a plane in Japan was forced to make an emergency landing after reports of a battery warning light and burning smell. Last week, a battery caught fire on a plane on the ground in Boston. In both cases, the problems may be related to Boeing s decision to use a kind of lithium ion battery chemistry that overheats and catches fire more readily than others. It s not yet clear whether the problems in the 787s originated with the batteries. Faults in the electronic controls have been implicated in other lithium ion battery fires. According to reports, inspectors found liquid leaking from the 787 s batteries after the forced landing in Japan today. The battery was also discolored, but it wasn t clear if it had caught fire. Lithium ion batteries have been known to cause fires in cell phones, laptops, and electric vehicles. But such problems are extremely rare, and usually result from damage to the battery such as piercing or overcharging or problems with the manufacturing process that introduce flaws in the cells. Boeing s 787 is the first commercial aircraft to use lithium ion batteries, according to GS Yuasa, the Japanese battery manufacturer that supplies the batteries. The company also supplies batteries for the International Space Station and electric railcars, among other applications. The chemistry and safety of lithium ion batteries varies. According to GS Yuasa s website, the batteries it uses for Boeing s 787 use lithium cobalt oxide electrodes. These are known for high energy storage 12

13 capacity, but other battery chemistries, such as lithium iron phosphate, are more resistant to overheating. Because of safety concerns, many electric vehicle makers have shifted to alternative chemistries, sacrificing some energy storage capacity. Because the electrolyte materials used are flammable, no lithium ion batteries are completely safe. Some companies are developing a version that doesn t use these electrolytes Mechanism of failure [ batteries] Over time, all batteries eventually lose their ability to perform and become eligible for scrap and recycling. In Ni Cd batteries the three main mechanisms of failure are all progressive and can therefore be predicted in advance, with high reliability, through proper maintenance. They include oxygen barrier failure, separator failure, and irreversible capacity loss due to degradation of active materials. The principle mechanisms of failure in lead acid include capacity loss due to active material degradation, loss of the active material from the current collecting structure leading to high internal resistance, and corrosion of the current collecting structure leading to sudden death. In contrast to Ni Cd batteries, high internal resistance and corrosion can appear rapidly and without warning especially if the battery has been subject to deep discharge, for example during ground operation. Unfortunately these mechanisms of failure are not detectable during maintenance checks. Future of aircraft batteries The need for more advanced battery technology is being driven by the requirements of weight reduction and more electric aircraft. The electrification of functions that were previously powered hydraulically, like actuation, requires high voltage architectures. As of today, only the higher voltage associated with lithium ion can provide an appropriate solution to this change. In light of this, cutting edge Li ion battery systems which meet these new requirements are being developed. Li ion is a sensitive electrochemistry which needs a detailed knowledge of its characteristics to allow its benefits to be exploited fully while ensuring maximum safety. We are likely to see continuing improvements in Li ion performance as new electrode materials and electrolyte compositions are already under study. Nano materials now being developed will also have a role to play. Nevertheless, Li ion batteries are not currently envisaged as retrofit solutions so Ni Cd and lead acid batteries still have many years of work ahead of them. EE Aircraft Electricity and Electronics Asst. Prof. Thavatchai Tayjasanant, Ph.D. 13