Imaging is one of the first medical applications to adopt these exceptionally thin batteries.
New Digital Radiography (DR) equipment captures DR images using conventional x-ray equipment and procedures. Wireless cassette-size DR detectors can capture digital images and transmit them to a DR system’s capture console. Untethered detectors also enable cordless positioning, which results in greater patient comfort and faster exams. Typical detectors are 14 in. by 17 in. and only .05 in. thick. Due to their small size, these DR detectors require batteries that are less than 5 mm thick. As such, new DR detectors are one of the first medical devices to universally adopt exceptionally thin lithium polymer batteries.
This article provides an overview of lithium polymer batteries used in DR detectors. It discusses their advantages and disadvantages as well as known issues regarding their use.
Lithium-ion (Li-ion) cells come in three basic form factors: cylindrical, prismatic, and flat lithium polymer cells. The 18650 cell, used in most notebook computers, is the most commonly used Li-ion cell, with several million manufactured per month. The 18650 (“18” refers to the 18-mm cell diameter, “65” to its 65-mm length, and “0” to the fact that it is cylindrical) offers the lowest cost per watt-hour of any Li-ion cell.
Lithium polymer cells, sometimes called laminate cells, are available in custom footprint sizes. They can be very thin or quite large, depending on their intended use. The primary advantage of lithium polymer batteries is the variety of form factors available. With respect to cell construction, lithium polymer cells can either be rolled or stacked like a deck of cards. They are packaged in a flexible coffee-bag-like material, and their positive and negative terminals protrude from the cell as tabs.
Lithium polymer batteries can be made very thin, down to around half a millimeter. However, in batteries at the bottom of this range, much of the space is wasted on the packaging, so cells typically range from 2 to 6 mm thick. Many DR detectors utilize cells only a few millimeters thick.
The length and width can be made quite large. Cell capacities can range from 50 mAh for a small cell, such as for a Bluetooth headset, to 10 Ah or more for an electric vehicle battery. Typical cells used in DR detectors support 2–3 Ah. Figure 1 shows two polymer cells typically used in medical battery packs.
Figure 1. Two polymer cells typically used in medical battery packs.
In recent years, lithium polymer cells have been embraced by consumer equipment manufacturers. Most mobile phones and Apple iPod and iPad products utilize polymer cells. The new ultrabooks coming on the market use them as well. This consumer demand for polymer cells has substantially reduced the cost per watt-hour for polymer cells.
The absence of a metal can in lithium polymer cells allows for more flexibility in size depending on the requirement of an application. This variety exists for several reasons. First, from a components supply standpoint, the laminate material is slit to different widths, so can suppliers are not required to produce new tooling to manufacture new can sizes. Also, the heat-sealing process is easily modified, compared with crimping for cylindrical cells.
New variants of polymer cells have been introduced into the marketplace. Kokam offers polymer cells that deliver high currents, while Leydon Energy offers cells that operate in high-temperature environments. These new variants expand that range of applications where polymer cells are a natural choice.
Lithium polymer is extremely thin, although this feature comes at a cost. Lithium polymer cells are more expensive per watt-hour compared with other types of Li-ion cells for a couple of reasons. First, the high-quality laminate material and the special tabs that allow sealing against the bag are expensive. Secondly, the lower speed of manufacturing increases both labor and overhead costs.
Another disadvantage is that the soft packaging on polymer cells is easily punc¬tured and has more swelling than metal cans. Figure 2 shows the construction of a typical lithium polymer cell.
Figure 2. The construction of a typical lithium polymer cell.
Lithium polymer cells also have less volumetric energy density than cylindrical cells. This is because cylindrical cells do not bulge due to their extremely strong shape, so very high electrode densities can be used. Also, the selection of materials is easier for cylindrical cells because the small amounts of gas they produce have no effect on their performance or shape.
The same is not true of lithium polymer cells. However, this disadvantage in energy density can be overcome by the advantage in packing density. In addition to the space lost between cells, cylindrical cells are a fixed size—mostly 18 mm in diameter—so they may not be able to make use of all the space available in an application.
Lithium polymer cells tend to have better cycle life than cylindrical cells because they are not so tightly constrained, allowing the electrodes to expand and contract more freely during cycling. Figure 3 presents a cycle life chart of a 1-C charge, 1-C discharge cycle life of a 2.7 Ah cell. It still retains 90% of its capacity after 500 cycles. Newer designs coming can achieve 95% capacity after 500 cycles and should exceed 1000 cycles.
Figure 3. a cycle life chart of a 1-C charge, 1-C discharge cycle life of a 2.7 Ah cell.
Puncturing a cell is a much larger risk for a lithium polymer cell compared with a steel or aluminum can. A punctured cell can cause an internal short circuit, which will cause the cell to get hot. Even if it does not short the cell, a leak may allow in moisture, eventually causing the cell to self-discharge and die. The cell may also swell from the anode reaction with moisture. Special care must be made in handling the cells and in the pack design to avoid sharp objects that could come into contact with the cells.
Edge shorting is another issue that is often overlooked. The aluminum layer in the packaging is conducting, so if it is exposed at the cut edges of the package, it can short out components that are put in contact with it. In addition, there are internal corrosion reactions in the cell that can occur if the tabs to the aluminum layer are shorted. This could happen if the tabs are bent over the edge of the packaging. Again, careful handling and good pack design is required.
Overdischarge damage is an issue for all Li-ion cells, but the resultant gassing in lithium polymer cells is more obvious. When the cell voltage drops too low (~1.5 V), reactions at the anode start to produce gas. As the voltage continues to drop under 1 V, copper from the anode current collector starts to dissolve and will short out the cell. Over-discharge should be prevented by the battery management system (BMS) within the battery pack.
Overcharge is similar. Gassing occurs at the cathode as the electrolyte starts to decompose at high voltage (~4.6 V). Cylindrical cells have integral pressure activated current interrupt devices (CIDs) to stop the overcharge when the gas pressure builds. Polymer cells do not have a CID. An external thermal fuse is usually added for overcharge protection within the battery pack, in addition to similar control logic within the charger. An external short circuit can cause swelling due to heat and overdischarge. Cylindrical cells have an integral positive thermal coefficient (PTC), a device that expands and creates high impedance when it is heated or self-heats due to the high current during an external short circuit. Polymer cells do not have this integral PTC, so an external PTC or thermal fuse can be added for shorting protection.
Battery packs made for DR detectors are not a simple configuration of cells. They are carefully engineered products with many safety features. The main components of a battery pack include the cells (the primary energy source), the printed circuit board, or BMS (the intelligence of the system), the plastic enclosure, external contacts, and insulation. Only recently have medical applications, such as DR, begun using lithium polymer cells within their battery packs. To date, there has been only one production launch of this type of product, but the buzz in the industry is that we should expect several of the competitors to follow suite.
Robin Sarah Tichy is a marketing manager at Electrochem Solutions Inc. (Clarence, NY), formerly Micro Power Electronics Inc., where she specializes in the battery and charger industry. Prior to joining Electrochem, she worked in the semiconductor, nanotechnology, and MEMS verticals for companies such as Hewlett Packard and Sematech. Tichy received a PhD from the University of Texas for her work in solid oxide fuel cells.