Battery characteristics and performance criteria

It is therefore seen that eqs (2.42) and (2.43) for the on-load cell voltages during discharge and charge, respectively, can be expanded to give 

A cell may be characterised in terms of (1) its available capacity, (2) its available energy, and (3) the power it can deliver. It is not very useful to 

The theoretical capacity of a cell or half-cell may be calculated as 

The coulombic efficiency of a cell is defined as Qp/Qj. It is often more useful to determine the capacity of each half-cell separately, since for operational reasons, most practical batteries do not have an equal number of equivalents of anodic and cathodic reactants. 

For purposes of comparison, it is convenient to calculate the specific capacity defined as the capacity divided by the mass of the cell or half-cell, and usually given in units of Ah/kg. In some cases a volume-based specific capacity is preferred (e.g. Ah/dm3). 

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Energy and power density (ratio of energy and power deliverable by a battery to its volume) of batteries determine their suitability for most electrical devices. Other important criteria for use in medical devices are safety and reliability in operation during lifetime of the device (up to 10 years or longer), predictability of battery performance characteristics, and provision of device replacement indicators based on battery depletion and mechanical design features such as shape and size to allow efficient packaging in the device and for patient comfort. 

Table 1 presents the characteristics of several lithium-ion batteries in various stages of development. These batteries promise markedly different levels of performance for the various criteria for which batteries are evaluated for the HEV and PHEV applications. 

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