Using Li-polymer batteries for military apps

Using Li-polymer batteries for military apps

Significant improvements and unique characteristics of Li-polymer cells make them a good choice for portable military apps

BY JEFFREY VANZWOL
Micro Power Electronics
Beaverton, OR
http://www.micro-power.com

Li-polymer-based batteries have been the “next big thing” in portable power for the last 10 years. They started appearing in small consumer electronics applications, such as wireless headsets, several years ago, but these battery cells are finally becoming mainstream. They are now designed into everything from laptop computers to medical monitors, and have been recently designed into military applications. Many of the initial objectives of the Li-polymer researchers and designers have not been met, while other advantages have been more recently recognized and exploited. This evolution of the Li-polymer products has led to many misconceptions about the advantages and design limits of this new type of cell.

What are Li-polymer batteries?

Li-polymer batteries are most easily thought of as a subset of the more common Li-ion battery. The mainstay of the tradition Li-ion market has been the 18650 configuration (18-mm diameter, 65-mm length), which has a metal can to contain the battery-active material. A majority of the global supply of these cells is used by notebook manufacturers. Li-polymer cells have similar performance characteristics when compared with Li-ion cells, since their materials and manufacture are similar.

Characteristics of Li-polymer cells include a nominal voltage of 3.6 V and 500 duty cycles per lifetime. A less than 1C optimal load current is common, and an average energy density of about 200 Wh/kg is typical. Li-polymer, like Li-ion, exhibits a low self-discharge rate of less than 10% per month when in storage.

The major difference is that the lithium-salt electrolyte is not held in an organic solvent as in the Li-ion design, but in a solid polymer composite such as polyethylene oxide or polyacrylonitrile. This allows a semirigid form factor and very thin cells. Li-polymer cells can be encased in aluminum foil laminate pouches that are just 0.1 mm thick, rather than the 0.25- to 0.4-mm thick aluminum or steel cans traditionally used with Li-ion cells. Li-polymer cells are constructed by stacking electrode and electrolyte materials in a flat sandwich, rather than winding them in a jelly-roll fashion as is done with Li-ion cells.

Li-polymer cells are constructed by stacking electrode and electrolyte materials in a flat sandwich.

Concerns and solutions

One should be aware of misconception about Li-polymer and its flexible packaging. This flexibility is often misleading, as Li-polymer cells should remain flat when installed in a device, not even bending for installation in the battery system. Bending of the cell brings the anode and cathode materials closer together, which can cause preferential plating and shorting. This results in reduced cycle life and presents a potential safety hazard.

While many reports exist on the safety improvements with Li-polymer, it’s fundamental material set is almost the same as Li-ion, so the safety hazards are similar and care must be taken to ensure that the packaging is not compromised with the inclusion of some rigid enclosure or support in battery pack design. Swelling issues have plagued Li-polymer cell manufacturers and concerned portable product designers, but these issues have mostly been overcome. The expected swelling is now usually about 6%, similar to Li-ion prismatic cells, and this swelling should be factored into the design of the battery pack enclosure. One potential safety improvement afforded by the packaging is that excess gas is not likely to build up to explosive pressures; it is allowed to escape in small amounts.

Early in its development, Li-polymer technology had problems with internal resistance, leading to low maximum discharge rates and, subsequently, longer charge times compared to more mature technologies. However, Li-polymer cells have the potential to be made very thin and many are based on LiMn2 O4 cathode materials. This cathode material has a three dimensional structure, which lends itself to good ionic conductivity. Short path lengths are achievable with thin designs, so the rate capability available with Li-polymer batteries has the potential to equal or surpass conventional Li-ion batteries.

The thin profile is the major advantage for Li-polymer cells. These cells are being manufactured extremely thin. Li-polymer cells are usually available in custom sizes. While some are quite large, most commonly the applications are small, single cells, as thin as 2 mm. Recent improvements in Li-polymer cells have expanded their reach to other applications. The energy density is rising and may soon exceed that of other Li-ion cells. Improvements in lot capacity uniformity have made multiple cell configurations a possibility, so higher voltage multi-cell packs can be based on Li-polymer cells. The choices available to a designer of portable products continue to expand, and Li-polymer cells are another option in designing for a mobile world.

Portable/military Li-polymer benefits

So where can portable or military applications benefit from the advantages of Li-polymer cells? One area that can benefit from these cells is land warrior devices. These are a new generation of network-centric devices (handheld or wearable) that improve the degree of information shared between the infantry soldier and his/her operations center. Examples of these devices are handheld radios, GPS receivers, night-vision goggles, handheld computers, and wireless local area network (WLAN) transceivers.

As military equipment manufacturers shrink the electronics within these devices, they can produce more ergonomic and portable devices. Thinner circuit boards lead to thinner devices, and thinner devices can be coupled with very thin Li-polymer batteries. This allows a handheld or holstered device to be thinned down to a 20-mm device, which can now be worn or strapped onto an arm for non-intrusive operation. Li-polymer cells, as thin as a few millimeters, facilitate wearable devices that enable better freedom of movement in the field.

Li-polymer cells are available in a wide variety of dimensional form factors. This allows equipment manufacturers to select a cell that maximizes the capacity given the space allocated for the battery. If an existing form factor is not available for your requirements, the cost of getting a custom cell configuration has dropped dramatically in the last few years. This allows equipment manufacturers to get custom or even esoteric shapes, such as circular (and thin) shapes or rectangular shapes with rounded corners. If an OEM decides to get custom cell designed for their portable device, they can design a cell that provides maximum capacity given the space allocated for the battery.

Another recent variant introduced into the Li-polymer industry is the high-rate Li-polymer cells. These high-rate cells, available from vendors like Kokam, can support discharge rates 20 to 40 times the capacity level of the cell. As an example, a cell rated for 2 A-hr capacity could support 80-A current draws for short durations. These types of cells are great for powering electromechanical devices, such as unmanned aerial/ground/underwater vehicles.

As noted above, Li-polymer cells have made great strides with industrial and consumer products, and have been designed into military devices. Many unique characteristics of Li-polymer cells make them a good choice for handheld and portable applications.

For more on Li-polymer batteries, visit http://electronicproducts-com-develop.go-vip.net/power.asp.