Battery State

Battery state is used as an input parameter for the electrical management system and, additionally, is an important parameter for the user. The battery state, whieh is described in terms of SoC and SoH, ean be used to estimate both the bridging time (the period of time for which the battery will be able to provide power during an interruption of the normal power supply) and the expeeted life of the battery. In general, the SoC describes the aetual available charge whereas the SoH describes the available charge of the fully charged battery. Both parameters are relative values, normalized to the full battery eapaeity, i.e.. 

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]. 

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The description of the battery states that one electrode is Pb in contact with PbSOq, and the other electrode is Pb02 in contact with PbSOq. This information identifies the two half-reactions ... 

Tirado, J. L., Inorganic materials for the negative electrode of lithium-ion batteries state-of-the-art and future prospects. 

A generalized controller operational flowchart is included in Figure 2. This flowchart shows relative battery state-of-charge (SOC) to be the starting criteria for the decision tree. SOC is defined as the ratio of ampere-hours (Ah) stored in the battery array at any given time to the maximum Ah capacity of the array. During this process the actual controller subroutine also includes the voltage and current that is available and/or required by each component. However, for the purpose of explanation, the power available or required by various system components can be related to battery SOC. 

Figures 3 and 4 depict the system performance of Case 1 for the first weeks of February and August respectively. In these plots the power produced by the PV array is shown along with the power requirement of the system load and battery state-of-charge (SOC). Figures 5 and 6 show system performance for the first weeks of February and August for Case 2, respectively.

Fig. 6.27 Experimental results obtained on the fuel cell power train in hard hybrid configuration for the R47 driving cycle a battery, input electric drive, and output DC-DC converter powers versus cycle length, b hydrogen, input and output DC-DC converter powers versus cycle length, c battery state of charge versus cycle length...

In general, there are two different architectures for BMSs namely, decentralized systems and centralized systems. These two architectures are illustrated for an electric vehicle (EV) application in Fig. 8.3 (decentralized) and Fig. 8.4 (centralized). In the decentralized system (Fig. 8.3), the individual BMS tasks are located in different devices. The charge control is part of the charger, the discharge control is part of the EV drive system, the battery state determination is carried out within a range meter, and so on. Some BMS tasks must be implemented in more than one device, especially in the case of safety management. Normally, there is little or no communication between the devices, so an optimized operation is not possible. Another disadvantage is that the battery-relevant control functions are located in different devices. Thus, each device must be adapted to the particular battery used. 

The actual battery state is mainly influenced by the historical operation of the battery. Therefore, methods for SoC determination should ideally take into account the history of the battery. Such (long-term) methods include simple charge balance and more complex adaptive methods. 

Statistical analysis is often used to determine differences between battery cells. If no charge-equalizing equipment is installed, the current through each cell is equal. Therefore, the cell/block voltages can be compared to estimate differences between cells. It is known that the voltage is not a direct indication of the battery state. Voltage can be used for relative analyses, however, and can show the changes of one cell in comparison with others. 

Changes in vehicle electric system architecture have immediate consequences for the performance profile expected from the battery. More specialized battery designs will be needed. Multiple batteries will be combined to meet demands and battery state detection will help to keep the battery in its best operational window to assure a sufficient power supply for critical components in all vehicle operational conditions. 

To compensate for the response of the fuel processor, a control architecture was developed to allow a battery system to provide for peak power demands. The power plant has been designed to maintain the batteries state of charge at an optimal level while the batteries provide the capacitance necessary to meet any load topography. In this configuration, the transient-following capabilities of the power plant are greatly enhanced. 

Aylor, J.H., Thieme, A., and Johnson, B.W. 1992, A battery state-of-charge indicator, IEEE Trans. Ind. Electr., 39 398-409. 

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