Scheduling the Charging of Batteries

To provide innovative electrical energy storage solutions to power the world.  Exide Technologies is the world's foremost supplier of lead-acid batteries for network power, motive power and automotive applications.  By offering a comprehensive portfolio of superior technologies, products and people, Exide is a leader in every market segment in which it does business.

Background:

The Elizabeth plant produces lead acid batteries for use in automotive and commercial applications. The plant has a throughput of approx 1.85 million units per annum, and operates 3 shifts, 5 days per week. The production process consists of manufacturing and assembling component parts into a “dry” finished battery, which are then filled with acid, connected in series and placed into racks to receive their “charge” The charging process is achieved by the application of electrical current through rectifiers, with the batteries in “banks” of approx 92. Dependent on a number of factors, (some of these physical, such as the current rating and size of the product, and some related to quality), the charging time can vary from 18 hours to 63 hours, dependent on the product. Approximately 114 spaces are available (@92 per bay) and a mixture of batteries are accommodated according to sales demand. The finishing process (labelling, washing, testing etc) occurs after the charging process is complete.

The Problem:

Due to the chemical reaction which begins when acid is initially added to the battery, heat is generated. Heat is also generated during the charging process, and excessive heat can cause degradation of the structure inside the battery, thereby ultimately shortening the life of the product. If excessive heat is detected, the charging process is shut down, causing the process time to extend. This results in a lack of product available for finishing, causing factory inefficiencies. Because batteries have a “shelf life” and deteriorate slowly after initial charging, it is necessary to reduce the warehouse storage time to a minimum. Hence, it is impractical to produce a large numbers of “stock” batteries. Due to the need to service customer demand, the mix of battery types varies greatly, and hence the charging profiles. Since the charging area is the “bottleneck” in the factory, it must be utitlized at maximum capacity at all times. The combination of charging profiles means that some batteries are ready to be removed at different times, and this needs to be balanced with the requirement of the finishing area to fully utililize it’s labour.

The Question:

Given that there are approximately 50 different charging profiles, which vary from summer to winter, and given the product mix required for sales, what is the optimum mix of battery types to be on charge at any one time, in order to fully occupy the charging area, and to ensure that the finishing area labour  is fully utilized? Other considerations are the quality constraints required to ensure that the battery does not begin to deteriorate during charging, and subsequently fail in service, and the need to produce and warehouse product in accordance with historical and forecast sales volume to ensure that stock is rotated adequately. Further consideration can be given to the need to reduce charging time in all cases, where possible.