Self-discharge time

Table 3. Self-Discharge Times for a 3-D Nanobattery as a Function of Electrolyte Thickness and Electrical Conductivity...

Retention time (self-discharge time) constant x describing the time needed for a potential drop of 37% after the capacitor was charged to the rated voltage yR x = RiSC, where Ris the isolation resistance. The associated power loss P is P = U2/Ris = 2 W/%, where W is the energy stored in the capacitor. 

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. 

Batteries of this type have rather high specific energies (up to 600 Wh/kg), then-reliability is satisfactory, and they can be stored for a long time without marked self-discharge. A certain disadvantage is the relatively low value of the highest admissible discharge current density (this is not over 2 to 3 mA/cm ), which is dne to the slowness of lithinm ion intercalation into the cathode materials. 

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... 

The self-discharge is an important parameter for applications in which the DLCs are not connected to an electric network and therefore need to maintain their state of charge. In those applications, the device is supposed to be able to deliver power with a performance not deteriorated by the rest time. 

FIGURE 11.7 DLC self-discharge controlled by oxidation-reduction reactions shows a linear voltage fading when plotted in a time logarithmic scale. This is the case for the particular prototype BCAPproto. 

The DLC self-discharge performance is the result of a compromise with its power capability. The manufacturers could use a thicker separator to improve the voltage retention but this operation would increase in the same time the series ionic resistance. 

It has also been shown that a module in a vehicle is able to work for years without any on-board balancing. The principle is to equalize the maximum voltage of the DLCs in the module from time-to-time with an external device [63,64], The cells must have a relatively uniform self-discharge performance for this method to be successful. It is nevertheless necessary to have an on-board surveillance system to avoid problems caused by unexpected failures. 

To work with high power, it is necessary to have a low ESR to limit the ohmic losses. The parallel resistance, Rv, has an effect visible only at ultralow frequency (below the millihertz range). It is responsible for the capacitor self-discharge. Its value must be as high as possible to limit the leakage current. The time constant x of the self-discharge is equal to x = RVC. 

Shelf life — A period of time in which a -> battery can be stored under specified conditions before it fails to deliver minimum specified performance of energy and - power. The shelf fife of batteries, primary and secondary, is dependent on the type of battery, the storage temperature and humidity, and on the state of charge. The main sources that limit batteries shelf fife is self - discharging, - corrosion, and dehydration. 

Many types of batteries loss a certain amount of the initial charge when they are unused, and this phenomenon is called self-discharge. For that reason, some batteries need to be periodically recharged, if they are left unused for long periods of time. Self-discharge is related to kinetics of chemical reactions occurring inside each type of battery, and then it is strongly dependent on temperamre. 

This type of dry cell has been widely adopted in Europe where a specially developed carbon is employed in the manufacture.78 The carbon cell is especially suitable where it is necessary to furnish low continuous outputs of energy for long periods of time. The shelf life of these cells is long because there is practically no self-discharge. Consequently these cells are extremely efficient as sources of current over long periods in remote places such as telephone and telegraph block stations, and electric installations for wire fencing. 

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