Float current

The majority of VRLA batteries produced today are used in standby applications to provide a reliable source of power in the event of failure of the mains supply. Discharges are infrequent and the batteries are maintained by float charging at a preset voltage. For example, European practice is to use parallel strings (usually 48 V) across a 54.5 V supply (2.27 V per cell). Adequately designed new cells, after conditioning and free of impurities, have a float current of < 1 mA per Ah at 20°C. 

Under normal operating conditions, the battery is fully charged and the float current, /float, should maintain this condition by compensating for self-discharge and secondary reactions. In a VRLA cell, the current distribution at each electrode is At negative electrode... 

Under normal float-charging conditions this heat, though excessive, can be easily dissipated. If conditions hinder heat dissipation and/or higher float currents are drawn, however, this process can become a vicious cycle that can result in thermal... 

Fig. 9.13. Comparison of float currents, heat evolution, and heat removal for a vented (flooded, upper graph) and VRLA (lower graph) battery [18].

Changes in float current with float time on polarization of industrial batteries at 2.26 V per cell. Cell... 

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. 

Figure 27.30. Dependences of accumulated charge determined on the basis of float-current experiments from the polarization potentials in 0.05 M (triangles), 0.5 M (circles), and 5 M (squares) H2SO4.

Thus, float-current measurements can provide a direct and simple procedure for the choice of optimum conditions of operation of porous carbon materials with a high surface area used in ECSCs. 

The float current is determined by the sum of the internal oxygen cycle and grid corrosion and has no direct relation to water loss. 

Since the potential of the positive electrode is equally reduced, a corresponding (slight) reduction of the rate of corrosion and the float current is observed. 

Another important parameter is the dependence of the float current on float voltage and the temperature. Both parameters markedly influence the float current and thereby the water loss by electrolysis. Furthermore, both parameters also influence corrosion of the grid and all conducting elements that are connected to the positive plate. 

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