Battery technology for portable applications
Battery design considerations such as capacity, chemistry selection, weight and energy density are driven by the requirements of equipment manufacturers whose devices will be powered by the portable power supply.
As technology advances, the power requirements of portable electronic devices also change. Because of improvements in battery technology, battery cells have consistently increased in capacity. Advances in integrated chip (IC) chip technology reduce the amount of power required to operate newer electronic devices. The result is greater battery capacity, in smaller packages, at lower cost.
Challenges in battery pack design When designing a portable radio, pager, cellphone or some other electronic device that requires a portable battery supply, manufacturers are faced with the following challenges regarding battery selection: * Energy density _ A manufacturer looks for the most capacity available for the specified dimensions of the battery pack. As manufacturers continue to design smaller and smaller products, the demand for a smaller battery ensues. The challenge is to provide a battery that delivers maximum power in the smallest possible package. * Weight _ Especially for portable communication devices, light weight is important. Although new lithium-based battery technology reduces weight, it does so at considerable cost. A more affordable alternative is to use new IC technology in the design of the device to reduce the required operating voltage. Accomplishing this goal makes nickel-cadmium (NiCd) and nickel-metal hydride (NiMH) battery chemistries more practical. * Cost _ A manufacturer needs to consider the cost of the battery, always an issue, in relation to the cost of the device the battery is designed to power. Especially with consumer-related products, the manufacturer must weigh the costs involved with new technologies to arrive at the most practical solution regarding the portable power supply.
Improvements in battery technology Fortunately, developments in technology have allowed battery manufacturers to meet the growing demands of the electronics industry. For example, lithium ion (Li-ion) batteries are popular for cellular phones and portable computers because of their light weight and capacity to store energy. Although cells are expensive, lithium technology will continue to increase in popularity because of the demand for lighter weight and higher energy density.
The more-common battery chemistries, NiCd and NiMH, have advanced as well, resulting in greater ability to store energy without sacrificing size. Table 1 below illustrates the increase in available capacity for the typical AA cell during the past 10 years. As the chart indicates, technological advances have resulted in much higher battery capacity than that which was available only 10 years ago. New technologies are consistently being developed that will continue to increase the current capacity of NiCd and NiMH batteries.
As the power requirements of portable electronic devices diminish, and available current capacity continues to increase, the result is longer operating time for the electronic device. Therefore, the changes in technology allow a battery manufacturer to meet the demand of lower cost and smaller size without sacrificing operating time. In fact, operating time will continue to increase despite the use of fewer cells in a smaller pack because of higher available current capacity.
Just as the demand for higher current capacity is important, so is the demand for smaller size. One factor that has enabled battery size to decrease has been the development of new IC chips that are incorporated into the portable electronic device. These IC chips can operate with power requirements as low as 3V in a typical cellular phone, where the requirement was once as high as 5V. By reducing the required power levels, the number of cells required in a specific battery pack decreases. For example, the Nokia 101 cellular phone battery required six cells to deliver the required output voltage of 7.2V, whereas the new Nokia 636 battery requires only four cells to deliver the required voltage of 4.8V.
Because the number of cells required in a specific battery pack has decreased, so have the overall dimensions of the battery pack. In addition, the overall cost of the battery pack has decreased as a result of fewer cells being used in the pack. Therefore, using more-common battery chemistries, the cost of producing a portable power supply continues to decrease along with the power requirements of the electronic device.
Another method used by battery manufacturers to reduce size is to incorporate flex board (flexible printed circuit board) technology into their battery packs. A flex board takes the place of wires, weld tabs or other means of interconnect using surface-mount technology (SMT) components, which also provide a decrease in size. Flex boards provide an interconnect path within the cell pack, with an average thickness of only 0.005″. Although flex board technology does not supply any critical advantage over conventional wiring in terms of connectivity, the technology does offer a size-and-space advantage.
Battery pack design factors The key challenge for the equipment designer and manufacturer is to reduce the size and weight of the electronic device without sacrificing features or productivity. The battery manufacturer must, in turn, design and build a battery to complement the device. Keep two questions in mind:
1. What are the minimum and maximum output voltages required to power the device? 2. What is the required operating time necessary to power the device?
These two questions drive battery designers to consistently improve their products. The result has been some of the technological advances already discussed.
It is safe to say that future applications will continue to demand higher battery performance, lighter weight and longer run time. Prices of battery packs will continue to drop as fewer cells are required to deliver the voltage necessary to power new electronic devices. Battery capacity will continue to improve with the advent of new chemistries and with improvements to established chemistries. However, the important point is that although cell manufacturers are constantly focusing their efforts on improving their technologies, they typically react to the demands of manufacturers of electronic devices. Improvements in technology will always be driven by the manufacturer of the cellular phone, the land mobile radio, the pager or the portable computer.
When designing an electronic device that requires a portable power supply, or when evaluating a potential vendor for batteries, consider the following:
* The experience and reputation of a battery manufacturer is extremely important. Any company seeking a partner in a battery venture should feel comfortable with the capabilities of their battery manufacturer. * The technical abilities of the battery manufacturer can be a tremendous asset when designing and developing a new battery pack. A battery manufacturer with a strong technical ability can offer valuable input regarding design and component selection. * A battery manufacturer should offer impeccable engineering support to its customers. * Consistency of product quality is a critical issue for a manufacturer of batteries. Look for a low product return rate, and evaluate sample products for quality of construction. * Many battery manufacturers maintain a documented quality plan. This can be an excellent tool to use when evaluating potential battery vendors to determine the steps taken by the vendor to ensure the quality of their products. * The cost of the product is an important issue, but it is critical not to sacrifice quality over price. Bear in mind that the battery can be the most critical component of the product. A battery that delivers below-par performance can result in customer dissatisfaction and a decrease in use of the product for which the battery is designed.
Summary Although requirements such as size, chemistry and cost affect battery applications, the most important factor to consider is the ability to manufacture a product of the necessary quality. A battery manufacturer should possess a thorough working knowledge of battery technology, the ability to provide valuable input regarding design and construction and the facilities and workers necessary to build the specified product.
Amato is senior engineer, and Crisafulli is product manager, for Multiplier Industries, Mount Kisco, NY.