Manufacturing Engineering Magazine November 2017
By Glenn Nausley
If the visionaries have it right, the future will increasingly be powered by electricity, most of which will come from clean, renewable resources like the sun and wind. Before that vision can become a reality, however, there is a major challenge to be overcome.
The hard truth is that neither sunshine nor wind is reliably available anywhere 24/7, and that means the power generated when the resource is available has to be stored until it’s needed at some other time. Batteries, not more efficient solar cells or better windmills, are the key technology that’s needed to make the vision a working reality.
Power density comparable to fossil fuels is the Holy Grail of battery technology. Every step toward that goal requires ever greater precision in preparing the electrochemically active components and assembling them into a safe, durable and efficient package.
Even as chemists continue their quest for better materials, the engineers who design and implement the assembly systems are perfecting the technologies that will be required to bring them to market. In that regard, it’s worthwhile to take a look at current battery assembly best practices for some insight into where the technology may be going in the future.
Button Batteries Require Precision
The ubiquitous “button” or “watch” battery consists of two stainless steel case halves and an internal assembly containing the electrolyte, a collector and an insulator. The insulator keeps the battery from shorting when the two halves are joined in a roll-forming operation.
The case halves have to be mated precisely and held in position while the roll forming operation is performed. Too little pressure and the battery will be too thick, too much pressure and the battery will be too thin, or even crushed.
Contacted by a major battery manufacturer, my company, Promess, recommended that the clamping operation be performed with an Electro-Mechanical Assembly Press (EMAP). The Promess EMAP is essentially a CNC press consisting of a ballscrew driven by a servomotor and equipped with an array of sensors to measure position, force, and any number of other process parameters that may be necessary. It applies force with extreme precision while measuring the functional result and feeding that data back into the control system.
That initial system has been duplicated for various customers several times. Today, rather than use a separate roll-forming tool, the entire operation can be performed with a Promess Rotational Electro-Mechanical Assembly Press (REMAP) which combines an EMAP and a rotary torque device (TorquePRO), to which the roll-forming tooling can be attached.
These intelligent assembly systems ensure uniform quality and also provide feedback that can detect missing or out-of-spec components to virtually eliminate field failures from that cause.
Pressing Issues for Medical Batteries
Pacemakers are probably the most common battery-powered implantable medical devices in use today. They have been available for decades and that makes them a very useful application with which to track the evolution of precision assembly technology.
The battery, usually a rechargeable lithium ion cell today, represents about 90% of the volume of a pacemaker, with the electronics and other operational components making up the other 10%. All of the components are packed in a stainless steel container about the diameter of a half-dollar and 1/8″ (3.18-mm) thick.
Consumer and commercial lithium ion batteries are typically manufactured in large sheets with the lithium compound on either side of a center collector. The sheets are then cut to size and assembled into a container.
Pacemaker batteries, however, typically use highly proprietary mixtures of elements to achieve the exact properties needed in that application. The material is pressed into a precision preform and then assembled around a collector. Pressing two preforms of the proprietary electrolyte powder separated by an extremely thin collector to an extremely uniform thickness is a major challenge.
A little over a decade ago, Promess was asked to evaluate the battery assembly operation of a major pacemaker supplier. What we found was a manual process in which an operator weighed the powder for each preform on a mechanical scale and then poured it into a die and pressed it into a pellet on a hydraulic press. The only control instrumentation for the process was a load cell placed under the press that the operator used to calibrate a hydraulic pressure relief valve.
Each time the die was changed to produce a different battery, the valve was manually recalibrated. From then on the accuracy of the pressing process was determined entirely by the accuracy of the pressure relief valve. To make things even more interesting, the powder had the unusual characteristic of becoming sticky as it was compressed, which complicated the process.
Needless to say, the manufacturer was interested in reducing the scrap rate while improving the consistency of the final product. Remember, these batteries were going inside pacemakers that were going inside people. Even one field failure was one too many.
Our recommended solution was to perform the compaction in two steps using one of our programmable and instrumented EMAPs. The first step is performed with a single EMAP. It produced a semi-solid blank that was close to the final dimension. Two of these blanks were then used in the final assembly process.
In the second step, a pair of blanks separated by the collector are placed in a die and compressed to final thickness and density by a pair of EMAPs. The two blanks must be identical and the thickness tolerance is only 25 µm. To accomplish this, the EMAPs are networked, so they can communicate during the operation. Sensors report the exact thickness of each blank and the controller modulates the stroke of each EMAP to produce identical finished dimensions.
By capturing sensor outputs during the operation, a traceable record is created for each individual battery. This is extremely valuable information for a medical device manufacturer for whom a defective product can represent an extremely high liability exposure.
Testing Automotive Propulsion Batteries
The large amount of energy stored in an automotive propulsion battery, and the highly reactive chemicals used in its construction, make crashworthiness an extremely important design goal. To obtain useful data, the batteries must be tested to destruction under controlled conditions that produce repeatable results. A major automaker is utilizing EMAPs to perform both puncture and crush testing on their batteries.
The fully instrumented presses are used to crush and/or drive penetrators through the batteries to simulate the possible punctures encountered in real-world crashes. The data collected during the simulation is used to verify the original design and develop improved protective packaging for future applications.
As chemists continually increase the power density of tomorrow’s batteries, precision assembly technologies will become increasingly critical to the production of safe, durable, and efficient products. Today’s best assembly and test practices are the foundation of the technologies that will enable the electric future, and the batteries that will power it.
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