Batteries limit current

Forced discharge (cell reversal) Voltage cutoff Use low-voltage batteries limit current drain... 

Another important concept for batteries is resistance. Expressed in ohms, resistance is what limits current. For example, if a 12V car battery were connected to a circuit with 4 ohms resistance, the current would be 3 A. If the battery had a capacity of 90 A-h, it would supply 3 A for 30 hours. (Resistance of 4 ohms here refers to external resistance of the battery and the headlight circuit.) The greater the resistance of the circuit and cell itself, the less the current for a given applied voltage. 

The maximum values of electric power and unit output of electrochemical cells vary within wide limits. The total current load admitted by individual electrolyzers for the electrochemical production of various materials in plant or pilot installations (their capacity) is between 10 A and 200 kA, while the current loads that can be sustained by different types of battery (their current ratings) are between 10 A and 20 kA. Corresponding differences exist in the linear dimensions of the electrodes (between 5 mm and 3 m) as well as in the overall mass and size of the reactors. 

In addition to mass transport from the bulk of the electrolyte phase, electroactive material may also be supplied at the electrode surface by homogeneous or heterogeneous chemical reaction. For example, hydrogen ions required in an electrode process may be generated by the dissociation of a weak acid. As this is an uncommon mechanism so far as practical batteries are concerned (but not so for fuel cells), the theory of reaction overvoltage will not be further developed here. However, it may be noted that Tafel-like behaviour and the formation of limiting currents are possible in reaction controlled electrode processes. 

Overhead from D-1 is called the raffinate. It is washed counter-currently with water in D-4 for the recovery of the solvent, and then proceeds beyond the battery limits for further conversion to isoprene. Both wash columns operate at substantially atmospheric pressure and 100°F. The product streams are delivered to the battery limits at 100 psig. 

The system comprising the resistor Re and capacitor C in series provides an example of a class of systems for which, at the zero-frequency or dc limit, current cannot pass. Such systems are considered to have a blocking or ideally polarizable electrode. Depending on the specific conditions, batteries, liquid mercury electrodes, semiconductor devices, passive electrodes, and electroactive polymers provide examples of systems that exhibit such blocking behavior. 

The bottom of this tower, a mixture of C4 and ether produced in the second stage, is recycled to the first column while the overhead stream (C4 and ethanol) is sent to the alcohol extraction tower. In this column, ethanol is removed by counter-current washing with alcohol-free water while C4 raffinate, isobutene-free, is sent to battery limits. The stream water/ethanol is finally sentto an additional fractionation tower where the ethanol-free water, recovered as bottom stream, is recycled to the washing tower, and the overhead ethanol/water azeotropic stream is recycled to the reaction stages. 

Fig. 13.17. Fast-charging characteristics of a 12-V, 65-Ah AGM battery with current limit of 100 A and voltage of 14.40 V.

Based on the described process design, an economic feasibility study has been performed [48]. The total processing costs are calculated from the costs of the process equipment, the chemicals, and the utilities required. In addition, the start-up costs (5% of total fixed capital), maintenance (5% of inside battery limits, a standard term denoting the part of the plant considered), plant overheads (80% of labor costs and 20% maintenance), and capital charges (10 year depreciation) are included in the calculations. As the infrastructure is aheady present at the current polymerization plant, no additional costs are assumed to be incurred outside battery limits. The operators on duty are assumed to handle the extraction process therefore no additional labor costs are made. The purchased costs of the process equipment shown in Table 14.9 were determined using the cost estimation program Chemical Process Equipment Individual Equipment Costing [55]. 

The DET power system designers believe that for optimum system reliability, the minimum solar-array size should consist of 110 strings in parallel and 63 cells in series per string, which will involve a minimum of 6,930 solar cells. These solar cells will limit battery charge current to approximately 5.52 amperes. 

Amperometric sensors as well as most electrocatalytic and battery electrodes operate under diffusion-controlled conditions, and thus their response depends on the flux of electroactive solutes (or their transformation products) to the conductive electrode surface, be it a film-coated electrode or the three-dimensional conductive network of porous electrodes. In order to examine the qualitative factors that influence the flux to such electrodes, it is valuable to examine a simple conceptual model. We examine here the diffusion-limited current to a test case planar electrode of area. A, coated by a thin flhn of thickness, d. Diffusion-limited conditions frequently accompany a high overpotential operation such that the concentration of the key analyte is negligible at the surface of the electrode. Under these conditions, the concentration drop across the membrane determines the faradaic current ... 

In general, the principles and various electrochemical techniques described in this chapter can be used to study all the important electrochemical aspects of a battery or fuel cell. These include the rate of electrode reaction, the existence of intermediate reaction steps, the stability of the electrolyte, the current collector, the electrode materials, the mass-transfer conditions, the value of the limiting current, the formation of resistive films on the electrode surface, the impedance characteristics of the electrode or cell, and the existence of the rate-limiting species. 

The maximum continuous-current capability of a zinc/air battery, as determined by gas diffusion regulation, is typically specified as the limiting current, denoted by 7. The relationship between gas transfer regulation, limiting current, and service life is illustrated in Fig. 13.5. 

FIGURE 13.13 Voltage-time response of 675-size zinc/air batteries subjected to continuous loads above and below limiting current. Courtesy of... 

High-rate discharging or short-circuiting Low-capacity or low-rate batteries may be self-limiting Electrical fusing, thermal protection limit current drain apply battery properly... 

Overheating Limit current drain Fusing, thermal cutoff, PTC devices Design battery properly Do not incinerate... 

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