HFXC-5000 High Frequency Transformer Onboard Battery Charger for Electric Vehicles
Summary of the Constant Current and Constant Power
Charging Results for the Hawker Sealed Lead Acid Battery
Prof. A. Burke
M. Miller, Graduate Student
Institute of Transportation Studies
University of California, Davis
July, 1996
Coherent Power, Ltd., Santa Rosa, California has developed a battery charger that utilizes constant power steps rather than constant current in the charging algorithm. Charging with constant power in the initial portion of the battery charging clearly results in higher charging currents for the same maximum power capability of the charger. Using a constant current intitial charge, the charger experiences maximum power only when the voltage reaches the clamp voltage. Both charging approaches utilize current or power taper after the clamp voltage has been reached. The key question to be answered is how much faster a given lead-acid battery can be charged using the constant power approach compared to the customary current approach. Further, it is of interest to determine how much of the charging time reduction occurs in the initial portion of the clamp voltage and how much occurs after the clamp voltage has been reached.
Tests were run at two charging rates - low power (230 W per module) resulting in a charging time of 5.6 hours using the standard constant current algorithm and high power (460 W per module) resulting in a charging time of 2.26 hours using the standard constant current charging algorithm. A 12.7A discharge (approximately C/3) was used for all the discharges.
The test results are summarized in Figures 1-6 and in Table I [Not shown here]. In these tests, the constant power algorithm resulted in shorter charging times in all cases. In the low power case, the constant current charging was 75% longer and in the high power case, the constant current charging was 52% longer. However, it should be noted that nearly all of this difference occurs in the taper portion of the charge and that the differences in the time to the clamp voltage and the Ahs returned to the battery when the clamp voltage was reached are much smaller. Using constant power rather than constant current steps during the taper portion of the charge resulted in a smoother tapering of the current and more rapid approach to the limit voltage (16 V) and to the specified overcharge factor (6%). It appears that the detailed specification of the algorithm in the tapered portion of the charging has a greater effect on charging time than whether one utilizes constant power or constant current in the initial portion of the charging. The constant power approach results in a slightly higher charge/discharge efficiency than the constant current approach, but the difference is only 2-4 percentage points, being about 86% for the constant power charges and 82-84% for the constant current charges.
[Complete Report, with Figures and Table, available upon request.]
[Copyright © 1996 Institute of Transportation
Studies. All Rights Reserved.]
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