Fuzzy logic estimation of SOH of 125Ah VRLA batteries

It is important to be able to accurately estimate the condition of VRLA batteries used in the UPS/telecom industry. A common size used in this application is the 12 V 125Ah module. While impedance-based meters are commonly used to perform this estimation, the only reliable means to determining battery SOH is through off-line discharge testing of the batteries. Significant cost savings in VRLA battery monitoring could be achieved if a reliable meter could be developed to accurately estimate battery SOH. Battery Intelligence, Inc. ("BII") is developing battery monitoring hardware to accurately estimate the state-of-health (SOH) of lead acid batteries of sizes commonly used in the UPS and telecom industries. The patented approach combines a battery interrogation technique, such as impedance measurements, with fuzzy logic analysis of the data. In this paper the combination of AC impedance measurements and fuzzy logic data analysis is used to estimate the SOH of 12 V 125 Ah VRLA batteries. AC impedance measurements were made using a Solartron 1280B electrochemical measurement unit. Measurements were made over a frequency range of 0.0178 Hz to 100 Hz in a galvanostatic mode with AC current amplitude of 300 mA. From this data it was observed that the magnitude of the impedance at 10 Hz, the magnitude of the impedance at 5.6 Hz and the phase angle at 3.16 Hz showed monotonic behavior with the battery SOH. These data were used to develop a fuzzy logic model for estimating the SOH of the batteries to an accuracy of ∼10%. This model has been programmed into a handheld battery tester and preliminary test results are provided.