VRLA batteries 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... 

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

Figure 1.27 The float charging situation of a VRLA battery. Current voltage curves as in Fig. 1.23, but at 100% of recombination efficiency of the internal oxygen cycle.

The authors have found that thermal runaway is a consequence of inappropriate combinations of these parameters number of cells packed together, case conductivity, heat-transfer coefficient, water-loss fraction, float current, and ambient temperature. The model could be used to help design more thermal-runaway-resistant VRLA batteries. 

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