Investigation of Water Consumption Mechanism of FIooded-Type ISS Batteries to Balance with Charge Acceptance

摘要

The market for idling stop system (ISS) vehicles is expected to grow to meet increasing regulation of fuel consumption. Enhanced flooded-type lead-acid batteries (EFB), which have high charge acceptance compared with VRLA, are used for the vehicles and contribute to the reduction of carbon emissions and fuel consumption. On the other hand, it is said that the batteries with higher charge acceptance tend to have higher water consumption, which leads to a significant degradation of high-temperature durability, as it is measured by means of standardized overcharge tests. That seemingly blocks the adoption of beneficial batteries with high charge acceptance in OEMs' markets. It has been reported in recent years, however, that water consumption during simulated high-temperature drive cycles, in which the batteries are operated under partial state of charge (PSOC) conditions, shows little correlation with that during a standardized overcharge test. In order to analyze the discrepancy, a mechanism of EFB water consumption is studied by analyzing flow rates and concentrations of oxygen and hydrogen released during overcharge and PSOC microcycling tests. Two types of Hitachi chemical's batteries are employed for this study. One is an EFB with high charge acceptance and it likely shows high water consumption by an overcharge test. The other is a conventional flooded-type battery. The results show that the molar ratio of hydrogen to oxygen released under POSC microcycling charge conditions largely differ from 2 to 1, and gas flows under the charge conditions are significantly higher than that under a conventional overcharge condition at the same voltage as the PSOC for both batteries. In a comparison of the EFB and the conventional one, water consumption of the EFB by an overcharge test is 2 times higher than that of the conventional one. On the other hand, the increase of gas flows for the EFB under the PSOC conditions compared with those under the conventional overcharge condition is greater than for the conventional one. As a result, the difference of the gas generation between the two batteries under the PSOC conditions is smaller than that during the overcharge test and the gas generation of the EFB under the PSOC conditions is ca. 30% higher than that of the conventional flooded-type. This finding seems to reflect the little correlation between water consumption during the simulated drive cycles and a standardized overcharge test. This paper describes the details of analyses including estimation of oxygen recombination. On the basis of these findings, we will investigate the way to balance both properties of water consumption and charge acceptance as well as discuss the necessity of a new method for the test to evaluate water consumption in the markets.