Parallel and Series Connections pptx

The battery voltage determines the number of series connections and can have a significant effect on battery safety and reliability.. In this context, the cell is the basic electrochemic

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 assumed ‘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 practice, 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 Implicit 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 between cells,

as shown inFigure 1) In the case of lead–acid batteries these intramodule connections are generally cast in place with lead alloy Examples of multicell 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 attaching 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 the 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.

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