The use of portable electronic devices and equipment has become a major part of modern warfare. These portable devices require light weight, cost effective and reliable energy storage devices to power them.

The armed forces have been using a recently redesigned battery pack to power the PRC-112D survival radio. This pack uses eight non-rechargeable commercial AA batteries, and provides the Warfighter with a more cost effective alternative. Now there is a new task to improve upon this first redesign of the battery pack to increase the reliability and manufacturability, further lower the unit cost, while adding the ability to recharge the battery in the field. These new rechargeable units would also have a charge monitoring device built into the battery pack body. The charge monitoring device would ensure that the batteries are in good working order before being shipped out to the field, as well as for the user in the field to be able to monitor the charge status of the batteries.

The design and development process of the rechargeable battery pack began by taking advantage of the lessons learned during the initial battery pack redesign experience. Both Concurrent Engineering Practices and Design for Manufacturing (DFM) principles were used to produce a reliable cost effective re-usable Battery Pack for the customer.

Design
The AA design uses two molded parts to build the body of the battery pack by ultrasonically welding the parts. A lid, along with rubber gaskets, is then screwed on top of the battery case to provide a water tight seal. It is usually difficult to provide a quality water tight seal along an ultrasonic weld joint, especially with welds of a great length. So in this case the parts have very tight tolerance controls on features and dimensions that come together during the welding process to form a water tight seal. The need for tight tolerances makes the mold very expensive to make and maintain. In addition, the molding parameters need to be tightly controlled to get a consistently quality water tight weld in the ultrasonic weld process, otherwise the reject rate would be significant.

One of the goals of the redesign was therefore to eliminate the ultrasonic weld. The two piece part design is eliminated by making the housing as a single piece molded part. This eliminates the ultrasonic welding, and also reduces the cost of the piece part by, first, making the mold tool simple and less expensive since the need for tighter tolerances can now be relaxed, and second, only one piece is now needed for the battery pack body instead of two.

Assembling the lid to the housing in the AA design uses three screws and three rubber gaskets to provide the water tight seal. In the new design the screw assembly has been replaced by a snap fit assembly with only one rubber gasket required to create the seal. The cost savings due to elimination of the three screw assembly process by a one step snap type assembly is significant, in addition to the cost savings due to having four fewer parts than the AA model. Furthermore, the snap fit feature eliminates the need to have a tool to open and close the lid with screws to change the battery, especially, if this is being done in the field where changing the battery in the shortest possible time can be of critical importance.

The internal battery contacts and pressures between various mating surfaces have been modified to reduce the electrical contact resistance between the parts thereby decreasing the voltage drop between the batteries and the radio.

The design of the external contacting surface and other mating parts of the sub-assemblies have been further modified in the new design to reduce the amount of wear and tear on the mating surface as a result of frequent changing of battery packs. Changes made to the design have enhanced the overall reliability of the battery pack by providing a consistently tighter connection to the radio.

These and other changes were incorporated into the new design using Solidworks 2006 as the design tool. Solidworks allows for the creation of 3D models for individual parts and assemblies which reduced the overall design time and facilitated quick checks of the parts and assemblies for assembly tolerances, expected smooth operation, and potential interferences. Additionally, 3D modeling allows the designers to quickly run some of the "what if" scenarios to help make quick decisions between many available design and material options. Figure 4-2 is a rendering of the battery pack using Solidworks 2006 sotware.

Prototyping
To shorten the time from design to high level verification testing, Stereo Lithography was used to build prototype models straight from the Solidworks generated 3D models. The availability of these models helps by allowing for quick evaluation of the fit and form characteristics of the design before giving a go ahead to the mold maker and other tool makers. These prototype samples were also used to assess the manufacturability and the difficulty level for assembling the design. The stereo lithography model also allowed for early discussion of the design with the customer and to get a quick approval decision.

Testing
Preliminary drop and water submersion testing was performed on the stereo lithography models. The drop tests resulted in cracking on some areas of the housing. FEA (Finite Element Analysis) was performed using Algor V19 software (see figure 4-3) to simulate drop testing impact forces, which allows the engineer to see the dynamic interaction of these forces to create high stress concentration zones. The results of the FEA analysis provided good probable cause of failures. Based on the results and analysis of these tests, the design was modified to eliminate, wherever possible, the stress concentration zones or at least minimize their effects.

Manufacturing
One of the key decisions that needed to be made at the early design stage of the product development process was whether to assemble the product manually or using an automated process. This is important since the part design features needed for manual versus fully automated assembly could be vastly different. Automated assembly requires unique handling and orientation features on the piece parts for automated machines to be able to assemble the new battery packs without high reject rates.

Having designed the parts to suit the pre-selected manual assembly approach, the other factors that play a key role in developing a cost effective assembly line/manufacturing process are the material flow, jigs & fixtures, assembly line tools, in process and final inspection tools. The manufacturing team needs to be an integral part of the overall team to help envision the assembly techniques that could be incorporated in the design to minimize the tooling and machines needed to make the assembly cost effective.

By using some simple principles of Concurrent Engineering, Design for Manufacturing, and fast prototyping techniques, and building a partnership of suppliers, toolmakers, prototype shops and manufacturing engineers, the realization of going from "Design to Manufacturing" yielded a high quality product to the customer.