Time-dependent discharge current

The capacity of the battery is determined by the electrode of lowest capacity. The actual capacity depends on the discharge current, commonly measured by the decline in the battery voltage with time at a hxed discharge current (see Figure 26.38). 

The principal characteristics of the triboelectret state in polymers recorded experimentally are i) the efficient surface charge density (ESCD) value and ii) the thermally stimulated depolarization (TSD) current spectrum, i.e. the discharge current dependence of the electret on its heating temperature. The analysis of TSD spectra helped to estimate the parameters of the triboelectret state, including the homo- to heterocharge relation in a dielectric, activation energy of the charge relaxation processes, relaxation time and others. 

At present, two procedures are used to characterize the self-discharge behavior of ECSCs and batteries. One of them consists of the measurement of the time dependence of voltage drop under open circuit to distinguish between the self-discharge mechanisms, as described earlier. Another procedure is float current measurement of the dependence of current on time at a constant potential for an individual electrode or at constant voltage V on the cell. 

The amount of time it t ikes to discharge a battery depends upon the battery capacity C expressed in milliamp hours (mAh) (more is better) and the amount of current drawn by the load. Battery charge and discharge currents are normalized with respect to battery capacity and are expressed in terms of C-rate. C-rate = C/1 hour, e.g., to a 150-mAh battery has a C-Rate of 150 mA. The current corresponding to 1C is 150 mA and O.IC, 15 mA. For a given ceU type, the behavior of cells with varying capacity is similar at the same C-rate. More in depth information about batteries can be found in the Handbook of Batteries (Linden, 1995). 

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