VRLA battery float operation management: specific control and operating limits

In a paper presented by J. McAndrews and R. Jones at Intelec `96 a unique circuit was described which provided the flexibility necessary to manage VRLA float operation and to prevent thermal runaway. This paper discusses the logic employed in selecting a float-charge mode to manage the VRLA battery and prevent thermal excursions. Included are the control limits for the battery float, boost and recharge modes for each of the following charge regimens: constant current with current limit compensation; constant voltage to a current limit; and constant current to a voltage limit. The accepted but at times impractical approach employed to prevent thermal excursions is to limit the charge current. This has been accomplished by either decreasing the float voltage as the temperature increases or operating the battery at a constant current. The parameters and controls described in this paper are derived from known VRLA characteristics including Tafel behavior and the temperature dependency of the relative reactions. Although the battery characteristics employed in this study may differ from others, the approach employed to arrive at a satisfactory charge regime should be common to that employed for most VRLA products. The battery management schemes discussed can be accomplished with relative ease when the battery is isolated from the plant rectifier. Although there is this isolation, the battery must be instantly available to service the load when commercial AC power is not available