Self discharge

Self-discharge measurement is a special case of galvanostatic measurement, recording the evolution of potential versus time at zero current, or measuring the residual capacity after some delay 

Carbons for Electrochemical Energy Storage and Conversion Systems 

For similar reasons, aging and robustness versus anomalous conditions of cycling are usually conducted on actual sealed cells. 

This method is well suited for slightly soluble electroactive materials. It has been widely used for the study of oxygen reduction reaction and hydrogen oxidation reaction, the two main reactions occurring in fuel cells. 

FIGURE 1.12 Polarization curves for the reduction of oxygen on a bare platinum disk in oxygen-saturated 0.5 M H2S04 at different rotation speeds. (From Ayad, A., et al., J. Power Sources, 149, 66, 2005.) 

Another problem still to be solved in polymer batteries is the self-discharge of the polymer electrode in common electrolyte media. Effectively, the majority of the polymer electrodes show a poor charge retention in organic electrolytes. In situ spectroscopic measurements (Scrosati et al., 1987) have clearly demonstrated the occurrence of spontaneous undoping processes. A typical example is illustrated in Fig. 9.17 which is related to the change of the absorbance of doped polypyrrole upon contact with the electrolyte. 

The self-discharge phenomena are revealed by the continuous decrease of the low energy bands characteristic of the doped states and by the corresponding increase of those characteristic of the undoped state. These phenomena are not easy to explain. One important fact is that the self-undoping processes do not induce irreversible degradation of the 

The supply of a BMS consumes an electrical current, which discharges the secondary battery. An outside observer will also perceive this consumption as a self-discharge. 

53 The potential of an electrode can only be defined in relation to that of another electrode. In chemistry, it is often measured in relation to the NHE (the Normal Hydrogen Electrode, defined in footnote 58). For lithium secondary batteries, we rather take a reference electrode of metal lithium. Therefore we use the notation XV versus Lf/Li, which denotes a difference of potential of X volts versus (versus) a metal lithium electrode. Alternatively, we use YV versus NHE, which means a difference of potential of Y volts versus a normal hydrogen electrode. We also find the notation Y V/NHE. 

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Trace quantities of arsenic are added to lead-antimony grid alloys used ia lead—acid batteries (18) (see Batteries, lead acid). The addition of arsenic permits the use of a lower antimony content, thus minimising the self-discharging characteristics of the batteries that result from higher antimony concentrations. No significant loss ia hardness and casting characteristics of the grid alloy is observed (19,20). 

Fig. 19. Self-discharge at ambient temperatures for a 35 A-h cell, NTS-2 prototype Sanyo Electric Co. cell.

From these data, the hydride cells contain approximately 30—50% more capacity than the Ni—Cd cells. The hydride cells exliibit somewhat lower high rate capabiUty and higher rates of self-discharge than nickel—cadmium cells. Life is reported to be 200—500 cycles. Though not yet in full production it has been estimated that these cells should be at a cost parity to nickel—cadmium cells on an energy basis. 

Self-Discharge Processes. The shelf life of the lead—acid battery is limited by self-discharge reactions, first reported in 1882 (46), which proceed slowly at room temperature. High temperatures reduce shelf life significantly. The reactions which can occur are well defined (47) and self-discharge rates in lead—acid batteries having immobilized electrolyte (48) and limited acid volumes (49) have been measured. 

Figure 35. Self-discharge characteristics of the CR17335SE lithium-manganese dioxide battery.

Compared with nickel-cadmium and nickel-metal hydride systems RAM cells exhibit very low self-discharge, making them ideal for intermittent or periodic use without the need to recharge before using, even in hot climates. Figure 6 shows a comparison of the temperature characteristics, for various battery systems in the form of Arrhenius diagrams. 

However, even at room temperature, the shelf-life of batteries with nickel oxide cathodes (Ni-Cd, Ni-MeHy, and Ni-Zn batteries) is a source of difficulties for the consumer who relies on the state of charge of his power source when he needs it-without charging time available. Figure 7 compares the self-discharge of RAM cells with Ni-Cd and Ni-MeHy cells at 20 °C. 

Figure 7. Comparison of the self-discharge of RAM cells vs. Ni-Cd and Ni-MeHy cells at 20 °C.

The self-discharge process has made experimental determination of the reversible potential of the Ni(OH)2/NiOOH couple very difficult. A major advance was the realization by Bourgault and Conway... 

Oxygen evolution occurs on nickel oxide electrodes throughout charge, on overcharge, and on standby. It is the anodic process in the self-discharge reaction of the positive electrode in nickel-cadmium cells. Early work in the field has been reviewed [9], No significant new work has been reported in recent years. 

Corrosion of lead starts at the equilibrium potential of the negative electrode. It induces self-discharge of the positive electrode on account of the following couple of reactions Discharge of the positive electrode... 

Fortunately, the kinetic parameters reduce the rates of these reactions so far that the gradual self-discharge of the Pb02 is such a slow reaction that it usually does not affect the performance of the battery. 

The two basic requirements for efficient bromine storage in zinc-bromine batteries, which need to be met in order to ensure low self-discharge and more over a substantial reduction of equilibrium vapor pressure of Br2 of the polybromide phase in association with low solubillity of active bromine in the aqueous phase. As mentioned by Schnittke [4] the use of aromatic /V-substitucnts for battery applications is highly problematic due to their tendency to undergo bromination. Based on Bajpai s... 

Both share more or less the same merits but also the same disadvantages. The beneficial properties are high OCV (2.12 and 1.85 V respectively) flexibility in design (because the active chemicals are mainly stored in tanks outside the (usually bipolar) cell stack) no problems with zinc deposition in the charging cycle because it works under nearly ideal conditions (perfect mass transport by electrolyte convection, carbon substrates [52]) self-discharge by chemical attack of the acid on the deposited zinc may be ignored because the stack runs dry in the standby mode and use of relatively cheap construction materials (polymers) and reactants. 

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