Parallel and Series Connections pptx

Parallel and Series Connections J McDowall, Saft America Inc., North Haven, CT, USA & 2009 Elsevier B.V. All rights reserved. Introduction Series and parallel connections are simple to define in electrical terms, but underlying these basic concepts is a wealth of detail. The battery voltage determines the number of series connections and can have a significant effect on battery safety and reliability. Paralleling is used to increase system capacity and to add redundancy, but there are many pitfalls in designing a paralleled battery system. Adding to the confusion is the fact that a number of myths concerning paralleling have assum ed ‘common wisdom’ status in the battery industry. This article dis- cusses these issues, provides some basic insights into statistical battery reliability, and describes the way in which these concepts are likely to evolve with the emergence of advanced battery technologies such as lithium ion. Basic Concepts Cells, Strings, and Batteries The terms ‘cell’, ‘string’, and ‘battery’ are used throughout this article. In this context, the cell is the basic electrochemical unit containing the electrodes and electrolyte; a string is a grouping of interconnected cells with the same nominal voltage as the direct current (DC) system; and a battery is the overall assembly of one or more strings. Although, in pr actice, both single cells and series and/or parallel arrangements of these cells are called batteries, the text of the article avoids the common misnomer in which a single cell is described as a ‘battery’. Series Connections A typical definition of series connections is an arrange- ment of cells in a battery made by connecting the positive terminal of each cell to the negative terminal of the next cell so that their voltages are additive. Such a definition covers the most common series arrangement, in which the battery capacity is equal to the capacity of a single cell and the nominal battery voltage equals the nominal cell voltage multiplied by the number of cells. Impli cit in this definition is that all cells in the battery are identical, and indeed the battery would not function correctly if this were not so. In many batteries, the series arrange- ment is more complex, because the units to be connected in series may in fact be parallel groupings of cells, as described in the next section. Cells may be supplied as separate units or in multicell modules. Multicell modules have factory-made series connections between the cells within each module, and these connections may be external (on top of the module) or internal (through the vertical partition be tween cells, as shown in Figure 1). In the case of lead–acid batteries these intramodule connections are generally cast in place with lead alloy. Examples of multicel l modules include automobile batteries, which typically have internal series connections, and forklift truck batteries, in which indi- vidual cells are grouped together in a steel tray and ex- ternal lead-alloy series connections are cast in place, as shown in Figure 2. Series connections between separate cells or modules comprise a conductor, which may be a cable, wire, or metal plate, and a means for attachi ng that conductor to the cell terminals. In small cells, typically with a rated capacity of 10 Ah or less, a wire may be directly soldered to th e cell terminals, or there may be a connector as- sembly consisting of a wire with a female connector at each end, attaching to male spade-type terminals per- manently attached to the cells. In larger cells it is more normal to have bolted connections using metal bars, or cables with terminal lugs attached. These connectors are fixed to the cell terminals, which may be horizontally drilled for a nut-and-bolt fastener set, vertically drilled and tapped, or externally threaded. In larger stationary battery installations the cells are typically numbered, and by convention the cell with the Figure 1 Portion of multicell lead–acid module showing through-the-partition intramodule series connections. 499 . Parallel and Series Connections J McDowall, Saft America Inc., North Haven, CT, USA & 2009 Elsevier B.V. All rights reserved. Introduction Series and parallel connections are. voltage determines the number of series connections and can have a significant effect on battery safety and reliability. Paralleling is used to increase system capacity and to add redundancy, but there. as the direct current (DC) system; and a battery is the overall assembly of one or more strings. Although, in pr actice, both single cells and series and/ or parallel arrangements of these cells