Battery capacity, definition

One issue is how to define the capacity of a fully charged battery. It can be the rated capacity (given by the manufacturer), the measured capacity (which changes with age), or the practical capacity (battery capacity that is available during operation conditions). The differences between these capacities is illustrated in Fig. 8.7. Definitions of SoC, SoH, measured capacity, practical capacity etc. are given in Table 8.1. A detailed discussion of these terms has been presented by Sauer et al. [4]. 

After 15 years of service at Crescent, the BESS continues its peak-shaving duty although the battery capacity is somewhat diminished. There is not definite information on the total number of cycles that the battery has experienced. The automated system fimctions so routinely that Crescent tends to forget it is there . 

The nominal capacity of valve-regulated batteries is defined between I5 and I20 according to the application and the technology of the battery. For definition the rated capacity following discharge rates is valid at T = 20°C ... 

In this paper, we presented new information, which should help in optimising disordered carbon materials for anodes of lithium-ion batteries. We clearly proved that the irreversible capacity is essentially due to the presence of active sites at the surface of carbon, which cause the electrolyte decomposition. A perfect linear relationship was shown between the irreversible capacity and the active surface area, i.e. the area corresponding to the sites located at the edge planes. It definitely proves that the BET specific surface area, which represents the surface area of the basal planes, is not a relevant parameter to explain the irreversible capacity, even if some papers showed some correlation with this parameter for rather low BET surface area carbons. The electrolyte may be decomposed by surface functional groups or by dangling bonds. Coating by a thin layer of pyrolytic carbon allows these sites to be efficiently blocked, without reducing the value of reversible capacity. 

Equation (42) expresses the total current from both the electrodes regardless of the limiting one. The user can follow any of the two definitions for the capacity of the battery (Eqs 38 or 42) however, Eq. (38) gives a closer value to the real capacity of the battery than Eq. (42). Equation (38) is commonly used when building and testing batteries. 

The first step is to draw a schematic diagram of the battery with a clear definition of the desired predicted variables (dependent variables) and the provided variables (independent variables). As mentioned before, a battery is a combination of several electrochemical cells connected in series or in parallel depending on the desired voltage and capacity, which increases the complexity of the model. 

State of health is 1, when the measured capacity equals the rated capacity. A state-of-health greater than 1 means more measured capacity than promised by the rated capadty. As per definition, a battery is at its end of lifetime at a state-of-health of 0.8. 

The SoH is defined as the quotient of the measured capacity and the rated capacity. The measured capacity is the capacity of a fully charged battery at standard discharge conditions. In some applications (e.g., automotive), the high-power capability is of interest. Therefore, other definitions for SoH are also possible. 

Perhaps the most contentious and important value concerning the battery condition in EVs is the SoC calculation. SoC is used to determine maximum source and sink current, battery health estimates, and is one of the direct outputs to the customer information cluster. It should be understood that SoC can be interpreted to mean many things, but most definitions cast it as the ratio between the limiting active material available divided by the total active material available. As described above, the SoC for high-rate batteries (such as hybrids) may have little meaning. At the lower current densities used in EVs, however, it is reasonable to assume that the capacity is fairly stable, so in theory an accurate SoC gauge should be possible. Some of the methods used to calculate SoC (see also Chapter 8) are as follows ... 

Memory Reversible capacity loss found on NiCd and to a lesser extent on NiMH batteries. The modem definition of memory refers to crystalline formation of the cell plates. 

The choice of a secondary battery (or a battery of accrrmulators) requires knowledge of its characteristic data. These characteristics are primarily its nominal voltage, its energy or the charge that it can store (capacity) or, for certain applications, its maximum current or maximum power. The definitions of these characteristics are given below. 

Depending on the conditions of discharge (intensity, temperature, end of discharge criterion, etc.) and the secondary battery s operational history (previous conditions of charge and discharge), this capacity may be variable or have changed in relation to an initial value. There are many definitions of capacity which are given from section 2.3.3.3 onwards. 

End of life definition for small portable batteries is usually based on capacity loss. In contrast, NiMH for EV and HEV applications find end of life is due to power limitations. 

---