Primary batteries discharge load

One of the important advantages of primary batteries eonsists of very simple maintenance procedures. Most such batteries require no servicing. Before a battery is switched on, its appearance and the remaining service life are checked sometimes actual parameters are measured (open-circuit voltage (OCV) and the initial discharge voltage). Correct polarity and reliable contacts must be ensured a violation of polarity correspondence may result in serious disorders and even in the breakdown of load circuitry, especially circuits involving transistors and electrolytic capacitors. 

The electrical conditions of cell operation are determined by the load schedule. Typically, primary batteries are used with complicated, and often arbitrary, loading schedules. The current drain of a transistor radio battery, for example, is a function of the volume setting the times of turning the set on and off are arbitrary. Batteries in electronic watches and pacemakers are loaded continuously but the discharge current is pulsed. The cases of continuous discharge to a constant load are rather infrequent. 

For constant resistance discharge as normally carried out for primary batteries, with R(Q) = constant as resistive load, the following relation applies... 

After prolonged storage of primary cells, their increased internal resistance is often misinterpreted as self-discharge. Then the delivered capacity is reduced by an increased voltage drop, although the electrodes are still fully charged, i.e. at a reduced load the battery s full capacity may still be obtained. 

FIGURE 7.5 Typical discharge curves for primary battery systems. AA-size cells, approx. 20-mA discharge rate. f A-size battery. (h) ANSI 1604 battery 9-V, 250-tl discharge load. 

FIGURE 7.7 Comparison of primary battery systems under various continuous discharge loads at 20 U. 

The Jet Propulsion Laboratory (Pasadena, CA) has evaluated several types of lithium primary batteries to determine their ability to operate planetary probes at temperatures of -80°C and below. Individual cells were evaluated by discharge tests and Electrochemical Impedance Spectroscopy. Of the five types considered (Li/SOCl2, Li/S02, Li/Mn02, Li-BCX and Li-CFn), lithium-thionyl chloride and lithium-sulfur dioxide were found to provide the best performance at -SOT. Lowering the electrolyte salt to ca. 0.5 molar was found to improve performance with these systems at very low temperatures. In the case of D-size Li/ SOCI2 batteries, lowering the LiAlCl4 concentration from 1.5 to 0.5 molar led to a 60% increase in capacity on a baseline load of 118 ohms with periodic one-minute pulses at 5.1 ohms at -85 C. 

The desirable characteristics of the Li/S02 battery and its ability to deliver a high energy output and operate over a wide range of temperatures, discharge loads, and storage conditions have opened up applications for this primary battery that, heretofore, were beyond the capability of primary battery systems (see Sec. 6.4). 

Figure 11.10 shows discharge curve of various lithium-liquid electrolyte primary batteries Li/(CFx)u, Li/Mn02, and Li/CuO. These discharge curves have been obtained under load of 13-75 kn. 

In order to make the used battery safe for disposal, for some Uthium batteries the remaining lithium within the battery must be depleted. This is accomplished by placing a resistive load across the cell pack to completely discharge the battery after use. The resistive load should be chosen to ensure a low current discharge, typically at a five (5) day rate of the original capacity of the battery. This feature has been used mainly in military primary lithium batteries. 

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