Aviation: Battery potx

Aviation: Battery DG Vutetakis, Concorde Battery Corporation, West Covina, CA, USA & 2009 Elsevier B.V. All rights reserved. Introduction Batteries are an essential component of virtually all air- craft electrical systems. Aircraft batteries are used to start engines and auxiliary power units (APUs), provide emergency backup power for essential avionics equip- ment and lighting systems, assure no-break power for navigation units and fly-by-wire computers, and to pro- vide ground power capability for maintenance and pre- flight checkouts. Many of these functions are critical to the safe operation of the aircraft, so the reliability of an aircraft battery is of utmost importance. Other important characteristics include power and energy density, en- vironmental durability, operating temperature range, ease of maintenance, rapid recharge capability, shelf life, cycle life, and tolerance to abuse. Traditionally, two types of batteries have been widely used as aircraft batteries: lead–acid batteries and nickel–cadmium batteries. The only other types that have seen limited use are silver–zinc batteries and, most re- cently, lithium-ion batteries. This article will only deal with those types found in use today: lead–acid, nickel– cadmium, and lithium-ion aircraft batteries. Determining the most suita ble battery type and size for a given aircraft type requires detailed knowledge of the application requirements (load profile, duty cycle, environmental extremes, and physical constraints) and the characteristics of available batteries (performance capabilities, charging requirements, maintenance re- quirements, environmental ruggedness, shelf life, and service life). With the various battery technologies available today, con siderable expertise is required to se- lect the best type and size of battery for a given aircraft application. The information contained in this article will provide general guidance for this purpose. More detailed information can be found in the sources listed at the end of this article. Operational Principles Lead–Acid Aircraft Batteries Theory of operation The chemical reactions that occur in a lead–acid battery are represented by the following equat ions: Positive electrode : PbO 2 þ H 2 SO 4 þ 2H þ þ 2e À $ discharge charge PbSO 4 þ 2H 2 O ½I Negative electrode : Pb þ H 2 SO 4 $ discharge charge PbSO 4 þ 2H þ þ 2e À ½II Overall reaction: PbO 2 þ Pb þ 2H 2 SO 4 $ discharge charge 2PbSO 4 þ 2H 2 O ½III As the cell is charged, the sulfuric acid concentration increases and reaches the highest concentration when the cell is fully charged. Likewise, when the cell is dis- charged, the acid concentr ation decreases and becomes most dilute when the cell is fully discharged. The acid concentration is generally expressed in terms of specific gravity (SG), which is the weight of the electrolyte compared with the weight of an equal volume of pure water. The cell’s SG can be estimated from its open- circuit voltage using the following empirical equation: specific gravity ðgcm À3 Þ¼open-circuit voltage ðVÞÀ0:84 ½1 There are two basic cell types: vented and recombinant. Vented cell s have a flooded electrolyte, and the hydrogen and oxygen gases generated during charging are vented from the cell container. Recombinant cells have a starved electrolyte, and the oxygen generated from the positive electrode during charging diffuses through the separator to the negative electrode where it recombines to for m water according to the following chemical reaction: Pb þ H 2 SO 4 þ 1 2 O 2 -PbSO 4 þ H 2 O ½IV The recombination reaction suppresses hydrogen evo- lution at the negative electrode, thereby allowing the cell to be sealed. In practice, the recombination efficiency is not 100% and a resealable valve is used to regulate the internal pressure at a relatively low value, generally below 5 psig (35 kPa). For this reason, recombinant lead– acid cells are often called valve-regulated lead–acid (VRLA) cells. Charge methods Constant voltage charging at 2.3–2.4 V per cell is the preferred method of charging lead–acid aircraft batteries. For a 12-cell battery, this equates to 27.6–28.8 V, which is generally compatible with the voltage available from the aircraft’s 28-V direct current (DC) bus. Thus, lead–acid aircraft batteries can normally be charged by direct connection to the DC bus, avoiding the need for a dedicated battery charger. If the voltage regulation on the 174 . Aviation: Battery DG Vutetakis, Concorde Battery Corporation, West Covina, CA, USA & 2009 Elsevier B.V. All rights. life, and service life). With the various battery technologies available today, con siderable expertise is required to se- lect the best type and size of battery for a given aircraft application lead–acid, nickel– cadmium, and lithium-ion aircraft batteries. Determining the most suita ble battery type and size for a given aircraft type requires detailed knowledge of the application requirements