Performance reserve batteries

The reserve battery design is used to meet extremely long or environmentally severe storage requirements that cannot be met with an active battery designed for the same performance characteristics. These batteries are used, for example, to deliver high power for relatively short periods of time, in missiles, torpedoes, and other weapon systems. 

Ruoboric acid, rather than the more common sulfuric acid electrolyte, is used for these applications because it performs better at the very low temperatures required for these military applications. This low-temperature performance is due in part to the absence of insoluble reaction products as the reserve battery discharges. 

Operating Temperature Limits. Like most other batteries, the performance of liquid-electrolyte reserve batteries is affected by temperature. Military applications frequently demand battery operations at all temperatures between -40 and 60°C, with storage limits of -55 to 70°C. These requirements are routinely met by the lead/fluoboric acid/lead dioxide systems and, with some difficulty at the low-temperature end, by the lithium/thionyl chloride and zinc/potassium hydroxide/silver oxide systems. Provision is occasionally made to warm the electrolyte prior to the activation of the two latter systems. 

In the selection of a lithium anode electrochemical system for packaging into a reserve battery, besides such important considerations as physical properties of the electrolyte solution and performance as a function of the discharge conditions, factors such as the stability of the electrolyte and the compatibility of the electrolyte with the materials of construction of the electrolyte reservoir are of special importance. 

As lithium bromide appeared to initiate the reactions causing electrolyte instabilities, Honeywell investigated other lithium salts for use in reserve battery electrolytes and concluded that lithium hexafluoro-arsenate (LiAsFe) combined with acetonitrile and sulphur dioxide was a suitable electrolyte, which did not exhibit discoloration or deposition of solids during storage. Table 24.1 compares the performance of batteries made up using the lithium bromide- and lithium hexafluoroarsenate-based electrolytes. Clearly, the 0.5 molal lithium hexafluoroarsenate electrolyte is functionally equivalent or superior to the lithium bromide electrolyte. 

Table 24.1 Effect of electrolytesand storage conditions on discharge performance of Honeywell lithium - sulphur dioxide reserve batteries at 560 mAh... 

Various problems related to the construction and performances of these batteries, such as changes in materials of membranes and additives both to the electrode materials and to the electrolyte, were studied in recent years. Some instability of the silver electrode during such storage period and the ways of avoiding these difficulties were studied and discussed [347]. Reserve activated silver oxide-zinc cells were constructed [348] with synthetic Ag20 and Pb-treated zinc electrodes were produced by a nonelec-trolytic process. The cells were tested before and after thermally accelerated aging. 

The stability of BaPbOs and its effect on the performance of a battery over its useful life have also been investigated for automotive applications [12-14]. A conventional automotive cell with 1 wt.% BaPbOs in the positive paste and a control cell were formed by means of a standard high-rate formation procedure. Cell performance was then evaluated by means of a standard Battery Council International (BCI) sequence of reserve capacity and cold-cranking tests. The cell containing BaPbOs formed three times faster with 12% less input capacity. The BCI test results of the two cells were comparable. 

The calculation is heavily dependent on the life curve (Fig. 11.10) and periodic checks should be made against this curve with the selected battery in the region of interest. As a general process, however, the size of reserve capacity can be assessed, provided that the battery meets all other performance specifications. 

Because the service is performed against preset parameters, the batteries are charged and discharged under true field conditions, a feature that provides accurate test results as well as fast service. Batteries with shorted, mismatched or soft cells are identified in minutes, their deficiencies displayed and, if necessary, the service halted. The derived battery capacities are organized into residual nd final capacities. Problems, such as insufficient capacity reserve at the end of field use, are easily identified to allow necessary corrections. 

The key battery performance parameters in this market are cold cranking amps (CCAs) and reserve capacity (RC). For a 12-V SLI battery, CCA is defined as the number of amps a lead-acid battery at 0°F can deliver for 30 sec while maintaining at least 7.2 V. The RC is defined as the amount of time (in minutes) that a battery can deliver 25 A at 80°F while maintaining terminal voltage of at least 10.5 V. 

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