Improved estimation methods for lead acid utility arrays for microgrids

In this paper, an enhanced mathematical model is introduced for accurately estimating the injection (charging) and extraction (discharging) of current from a lead acid cell. This is then extended to a much broader scale for use in utility energy storage applications. A comprehensive analysis is conducted on a common lead acid cell model to interpret it in terms of a normalization metric to accurately forecast the energy transfer based on the current state-of-charge (SoC). Discharge and charge models are created where the normalized parameters are used, in conjunction with the published capacity of the battery (in Ah), to obtain the charging and discharging currents. Particular focus is placed on the nonlinear characteristics present in the constant voltage and float charging regions where a 3rd order polynomial model is used. The charging algorithm is further capable of identifying a drift in the published full charging current as a battery ages to prevent overcharge. An experimental verification is conducted on a 12 Ah 3-cell lead acid battery demonstrating the usage of the proposed algorithm can accurately estimate the battery behavior.