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Addressing Power Needs for Compact Medical Devices

Tue, 08/06/2013 - 3:41pm
Alex Bynum, Business Development Manager, Lithium Battery Division, Saft

Medical technology continues to grow smaller, yet remains robust, delivering an array of features that help enhance the level of care provided. One challenge is in powering these devices while keeping in mind the trend toward miniaturization. This article looks at lithium technology options and reviews the offerings of each type for medical devices.

Saft Lithium solutions offer high power, compact designThe medical field is constantly evolving and with this progression, there is an increasing demand for the tools and equipment used by medical professionals to be more compact without sacrificing the power or functionality of the device. Whether it is the efficient management of data collected using computer carts in a hospital, achieving patient comfort at home using a respirator, or an automated external defibrillator on a commercial aircraft, reducing the overall size and weight of devices is a key objective of today’s medical technology.

Similarly, wireless sensors very often enable the use of portable devices in the medical setting and are directly impacted by battery performance. In these applications, battery selection is critical for reliable performance, patient comfort, and security.

Design Considerations
Ultimately, the size and design of a medical device battery are determined by the requirements of the application. Although high energy density is an enabler for reducing the size of a battery and overall device, there are many additional factors to consider.

For example, will the application be for one-time use or many uses? Is it disposable or reusable? A device that is reusable and requiring many uses may be better suited by a rechargeable solution whereas a device that will serve for a one-time, disposable use is better suited by a primary battery.

Once the decision for a primary or rechargeable solution is made, there are a host of other questions and industry considerations. Operational voltage, energy, and runtime requirements are typically known early in the design process. However, operational loads and environments for the application may change as the device’s use scenario evolves and matures. The following list provides a range of the requirements that can impact the development of a solution for transport, storage, use, maintenance, and end-of-life disposal within the medical setting.

  • Product Life Cycle: Transportation, storage, commissioning, modeling, operation, maintenance, decommissioning, disposal, warranty
  • Battery: Voltages, load profile, energy, electrical interface, mechanical interface, weight, volume, container, pressure, thermal management, marking, packaging
  • Charging: Electrical values, weight, volume, integrated or external, communication
  • Environment: Operational temperature, storage temperature, thermal cycling, humidity, immersion, shock, vibration
  • Qualification: Safety, transportation, local and international standards

Today, primary lithium and rechargeable lithium-ion products provide the highest energy densities available; however, primary lithium can have more than twice the energy density of lithium-ion rechargeable. Therefore, it is important to explore the fundamental differences in commonly used primary batteries where the smallest solution can be achieved.

Lithium iron disulfide batteries, like those found in retail stores, have a slightly higher operating voltage (i.e., 1.8 V) compared to traditional alkaline (i.e., 1.5 V) but are still significantly lower than other lithium primary chemistries, like lithium-thionyl chloride at 3.6 V. This reduced voltage translates into a reduced gravimetric energy density often stated in Watt-hour/kg.

Although readily available with a low price, the commercially available cells do not serve as an optimal performance solution. For instance, a typical AA-size lithium-iron disulfide product (LiFeS2) has an energy density of 297 Wh/kg compared to traditional AA alkaline manganese dioxide at 143 Wh/kg1. However, the equivalent AA-size lithium-thionyl chloride LS14500 cell from Saft (www.saftbatteries.com) has an energy density exceeding 550 Wh/kg, which is nearly twice the LiFeS2 and four times the alkaline manganese dioxide. This improved energy density with increased operating voltage per cell can result in a smaller battery or significantly increased energy in the same footprint.

Options in Primary Lithium
There are multiple primary lithium chemistries in a range of sizes and constructions available today with high operating voltages and ranges of performances. Three common chemistries provided by Saft are lithium-sulfur dioxide (Li-SO2), lithium-manganese dioxide (Li-MnO2), and the previously mentioned lithium-thionyl chloride (Li-SOCl2). To make the optimal selection, the basic characteristics of the cell construction and chemistries should be considered.

Cell Construction
The ability of a battery to deliver energy or power is related to the cell construction. Two common cylindrical cell constructions are bobbin and spiral wound. Bobbin cells are designed with the highest amount of active material or lithium per volume. Achieving this high amount of lithium metal is key for energy but comes at a penalty to power capability due to the low surface area between cathode and anode. Spiral wound cells, on the other hand, have a high surface area between cathode and anode but also require additional non active material, like the separator that reduces the effective volume available for the lithium metal reducing capacity. In addition to the two-cell constructions, hybrid systems are available, combining the high energy density bobbin construction with a capacitor for power capability.

Chemistry Comparison
Li-SOCl2 (Liquid cathode)
This option is ideal for long-term applications that require a life of five to more than 20 years. With unrivaled nominal capacities, they offer very low self-discharge for extended operating life at 3.6 V with the ability to operate in extreme low and high temperatures.

  • Bobbin or spiral construction
  • Wide operating temperature: -60°C/85, 150°C for specialty products
  • Unrivalled nominal capacities
  • Self-discharge <1% per year

Li-SO2 (Liquid cathode)
This design provides 3.0 V with superior pulse capacity, as high as 60 A in some cell constructions, and power at cold temperatures, as low as -40°C. Typical cells are spiral construction with continuous current capabilities in the 0.1 to 5.0 A range.

  • Spiral construction
  • Operating temperature: -60°C/70°C
  • Excellent capacity above 1A
  • Superior pulse capability
  • Superior power at -40°C

Li-MnO2 (lithium-manganese dioxide)
This cell range operates at 3.0 V with minimum voltage delay and high capacity even at high discharge rates. It offers excellent resistance to passivation even after long-term storage in uncontrolled temperatures. Intrinsically safe cells for ATEX applications are also available.

  • Spiral construction
  • Operating temperature -40°C/70°C
  • Non-corrosive electrolyte
  • Good pulse capability
  • Excellent voltage start-up after long-term storage
  • Good capacity at high current and low temperature

Comparison of lithium technologiesConclusion
As technology and requirements rapidly evolve within the medical world, practical devices that offer compact design and optimal performance can be a matter of life and death. Lithium batteries provide the highest density energy of the solutions available, and offer the scalability necessary for design applications. As with any solution, the design concept must consider the whole picture. The good news is that companies like Saft are answering the call and driving research to meet the needs of every setting.

1 Lithium Iron Disulfide Handbook and Application Manual (Version Li4.01)

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