February 2004

Electric Applications Inc. staff were contracted to develop an electric push back tractor (EPBT) that demonstrates the advantages of using a high-power density, high voltage battery and the safety features of state-of-the-art electric vehicles. In the first phase of the program, a simulated operating strategy that mimics EPBT operation at airports was developed to allow the testing and evaluation of various candidate batteries for use with the EPBT. Seven valve-regulated lead-acid (VRLA) battery technologies were then selected (based on a combination of voltage, cost, capacity and claimed partial-state-of-charge performance) for evaluation under the EPBT schedule. Each battery type was then subjected to repeated discharges that simulate EPBT duty. Only two of the battery types were found to meet the energy, power and reliability requirements that have been established for successful EPBT operation, namely, a gelled-electrolyte VRLA battery from Sonnenschein (6-V; 110 Ah, 1 h rate) and an absorptive glass-mat (AGM) VRLA battery from Vision (12-V; 124 Ah, 1 h rate).In the second phase of the work program, the Sonnenschein and Vision batteries have been operated under simulated EPBT conditions to evaluate the potential cycle-life of these batteries under field conditions. The simulated conditions are based on what is called partial-state-of-charge (PSOC) duty. This mode of operation involves cycling batteries below a full state-of-charge (SoC) for extended periods, between full recharges. Such operation has been shown to extend the lifetime Ah’s available from VRLA batteries significantly, provided that the frequency and intensity of the full recharge is carefully controlled. Two Sonnenschein units have performed almost 6000 simulated towing operations (i.e., approximately two years of simulated field service), and are still at 90% of their nominal capacity. Based on this data, and the fact that the simulated conditions used in this study are more severe than those expected in the field, it is predicted that under normal operating conditions, the batteries should be capable of providing at least four years of field service. These results have greatly exceeded expectations and provide direct evidence that EPBTs can be less expensive than diesel-powered alternatives. The first two Vision batteries evaluated did not perform as well as the Sonnenschein units and failed before completing 1200 simulated towing operations. This poor performance is attributed mainly to the use of a charging strategy that was not suitably optimized for the specific battery type. Another two Vision modules operating under an improved charging regime delivered 2700 and 3400 towing operations before failure. Given that the Vision batteries are effectively half the price of the Sonnenschein units, the former are also considered to be an economic option for powering EPBTs. Finally, the performance improvement obtained for the Vision batteries by optimizing the operating strategy reinforces the importance of tailoring charging algorithms to both battery type and application.


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