Originally Published MEM Fall 2005
Product safety and performance reliability depend on the mobile-equipment designer making informed component decisions and then testing the design under real-world conditions.
The demand for portable medical devices is on the rise. Advances in battery-based power systems have allowed traditionally tethered devices to become mobile. Such medical devices as automated external defibrillators (AEDs) and x-ray, magnetic resonance imaging, and ultrasound equipment are no longer confined to hospitals and are used now in the field.
However, the fact that these devices can be used in novel settings does not diminish their criticality. They are lifesaving tools whose every element needs to be designed with special attention to safety and reliability. With regard to the battery pack, the designer must create an optimal balance among the individual cells, other pack elements, and the operating requirements of the host device. This is a complex undertaking. Battery pack design principles specific to portable medical devices are the subject of this article. The discussion covers the creation of a battery usage profile, the selection of a cell chemistry and cell manufacturer, and pack design for safety and reliability.
The Usage Profile
|Figure 1. A typical power system for a portable medical device.
(click to enlarge)
A typical portable power system is illustrated in Figure 1. Its design should minimize hazards occurring as a result of reasonable use and foreseeable misuse. Rather than relying on specification sheets to determine the appropriate cell technology for an application, it is better—in fact, important—to evaluate cells on the basis of a real-world use profile. Such a profile would include the following factors:
The performance data appearing in product specification sheets are collected, unfortunately, under conditions rarely experienced in the real world: room temperature and a low continuous discharge rate. By contrast, AEDs typically have high pulse currents and operate in two modes, a low-current mode while the defibrillator monitors the heart rhythm and a short period of higher current while the capacitors charge. The heat encountered by the battery pack can be caused in part by internal impedance or voltage drops during cycling, but the device may also be subjected to extreme ambient temperatures of –20° to 60°C during storage in northern winters or summer sun.
Cell manufacturers' specification sheets may not contain enough information to enable a power system for a life-sustaining application to be designed for the necessary reliability. It is crucial to design the battery pack, and test its performance, according to the manner in which it will be used.
Higher levels of functionality and telemetric capability characterize many new medical devices. These new devices require more energy than older, now outmoded, devices did. Adding size and weight to the equipment in an effort to increase available power is precluded, however, by the exigencies of designing battery packs for mobile units. Thus, technological advances in battery lifetime and capabilities have been in demand.
Cell Chemistries. With these advances come some risks. Portable rechargeable-cell chemistries all involve safety concerns. Sealed lead-acid (SLA) cells have a concentrated sulfuric acid electrolyte with toxic heavy metal electrodes. They can potentially generate hydrogen during charge, which can lead to explosion. In addition, most countries are trying to minimize lead content in medical devices.
Nickel cadmium (NiCd) cells have an electrolyte with toxic heavy metal electrodes that also can generate hydrogen during charge. The other nickel-based technology, nickel—metal hydride (NiMH) cells, features a caustic electrolyte and the potential for thermal runaway during discharge, as well as the potential for hydrogen generation during charge.
|Figure 2. The volumetric and gravimetric energy densities of principal rechargeable-cell chemistries.
(click to enlarge)
Lithium-ion (Li-ion) cells have a volatile solvent electrolyte and carry the potential for fire because of the lithium-metal plating. The greatest degree of protection is necessary. Also, Li-ion cells have the highest energy density by both weight and volume (see Figure 2). As long as the appropriate level of safety is designed into a Li-ion pack, this chemistry offers the most attractive source of portable battery power.
Li-ion generally offers operating voltages of 3 to 4.2 V per cell and cell capacities from 130 mAh to 5.6 Ah, at C/5. Battery discharge rates are designated as a fraction (per hour) of the rated capacity and cycle life of more than 500 cycles down to 80% of rated capacity. With a nominal operating voltage within the range of 3.6 to 3.8 V, only one rechargeable lithium-chemistry cell is required for a 3-V operating system. Previous nickel-based technologies, on the other hand, which operate at 1.2 V, require three cells for a 3-V operating system. Some rechargeable lithium technologies currently available provide energy densities above 500 Wh/L and can handle continuous discharge rates of 100 C.
Supplier Evaluation. Li-ion technology requires a serious level of safety protection. In addition, counterfeit batteries have become increasingly available. Consequently, choosing a reputable supplier is imperative. It is important to look for certain safety measures. These include a thermal-shutdown separator, which physically separates the anode and cathode materials and is made from material that increases internal resistance when cell temperature rises above a shutdown point; exhaust vents; and an overcurrent protector.
Now the biggest payoff in battery design comes with matching the right cell, chemistry, and manufacturer with the operating profile of the device. Not all cells are equal, and small variations in the particle size distribution, mixing, formulation, and coating of active materials can have a huge effect on performance. Therefore, product designers are strongly urged to consider several key factors when evaluating cell manufacturers.
First, they must determine whether it makes more sense to seek certification in Li-ion technology or to outsource system integration. Certification is time consuming and expensive. Before marketing cells to a distributor or OEM customer, Li-ion cell manufacturers require that the purchaser undergo a rigorous and lengthy certification process verifying its competence with the technology. Unless a manufacturer is planning to purchase a large volume of cells on an ongoing basis, the company should consider working with a precertified battery system supplier.
Second, designers should look for quality indicators, such as consistent core chemistry. Mismatched cells can compromise the entire battery pack; the cell with the lowest capacity will reach the low-voltage cutoff threshold sooner than the others and cause the battery to shut down even though energy remains. Typically, cell vendors running fully automated production lines deliver the best product consistency. Cells purchased from such a supplier for incorporation in a battery pack are more likely to be evenly matched, or balanced.
Finally, product designers must determine the appropriate number of separator layers for their application. The electrodes in Li-ion cells are kept apart by a porous polymer separator. Cells developed for price-sensitive applications often use a single layer of porous polyethylene (PE), which, if the temperature deviates far upward, melts and closes its pores. In theory, this condition stops any further electrochemical reactions. But in practice, the melted PE offers little or no mechanical barrier to hard shorts between the electrodes. It is important that Li-ion cells use multilayer separators consisting of porous PE laminated between elements of porous polypropylene. Porous PE melts at a higher temperature and maintains mechanical integrity after the PE may have melted.
Materials specifications have been developed that hold identified impurities to levels below critical limits, but the proliferation of portable handheld devices has spawned many aftermarket battery-pack suppliers, and some suppliers may not adhere to rigorous standards. Lack of quality may lead to unsafe conditions, but regardless, the safety measures built into the cell should be considered the last barrier to an unsafe condition.
Safe, Reliable Pack Design
Cell selection is only the first consideration in pack design. A number of safety design and manufacturing quality factors must be taken into account in the build of the battery pack. Not only do the cells in a pack need to be of high quality and the same chemistry, but they must be balanced so that the variance in capacity is less than 2%.
The development of leading-edge battery technologies presents several design challenges. Older SLA batteries do not generally require safety components other than a fuse in or around the battery, but because NiCd, NiMH, and Li-ion batteries can be hazardous when overstressed, extra precaution must be taken during the design phase to ensure that cells of these types are used in an appropriate manner. Therefore, the pack consists of not only the cells but also the printed circuit board, which provides the intelligence of the system for advanced functions involving the fuel gauge, protection circuitry, thermal sensors, and communication devices.
Fuel Gauges and Communication. Smart battery systems—that is, battery systems that can communicate back to the host—are the preferred choice in mission-critical applications. A smart system can monitor its operational status, accurately predict its remaining run time, and communicate its status to the host device. Smart batteries generally contain a communication device that might include a cycle counter to count the number of charge-and-discharge cycles for warranty purposes. A smart system may also have a fuel gauge to track the remaining run time per cycle and report the battery's state of health, or both.
|A cutaway view of a typical battery pack.
(click to enlarge)
Voltage-based fuel gauging relies on periodic voltage measurements of the battery pack. This method works best on a cell technology with a sloping discharge curve. It has its limitations, however; it estimates only relative capacity and gives capacity information only outside of product use.
The one-wire method can determine battery capacity by monitoring the amount of current that is input to, or removed from, a rechargeable battery. The integrated circuit monitors a voltage drop across a sense resistor connected in series between the negative battery terminal and ground in order to determine the charge-and-discharge activity of the battery.
The two-wire, or coulomb-counting, method is the most accurate. It allows estimation of absolute capacity and gives capacity information while the product is in use. The most common protocols for the two-wire method are InterIntegrated Circuit (I2C) and System Management Bus Standard (SMBus).
Protection Circuitry. A safety circuit, separate in function and purpose from any fuel-gauging capability within the battery pack, should be used to protect the pack from external stressors, such as overcharging, overdischarging, and short-circuiting. Li-ion chemistry can be volatile on overcharge. Because of its volatility, a safety circuit must be added to prevent the battery from exceeding 4.25 V per cell. Each parallel string of cells in a Li-ion pack requires independent voltage monitoring. The more cells that are connected in series, the more complex the protection circuit becomes. Four cells in series is the practical limit for commercial applications. For high-cell-count packs, an active balancing circuit may be advisable.
Thermal Sensors. The safety circuit should include thermal sensors to protect against excessively high or low operating temperatures, especially in the case of NiMH batteries, which are exothermic in nature and pose a heating danger. NiMH packs must include devices that prevent overheating, or devices that prevent overcurrent conditions that can cause overheating.
Temperature extremes and high discharge rates can lower the performance, reliability, and even capacity of a battery system of any chemistry. If temperature, discharge rate, and battery age are not compensated, an inaccurate fuel gauge can leave as much as 30% of available battery capacity unused.
Other battery pack components include the plastic enclosure, external contacts, and insulation. All pack components must be designed with regard for the safety of the device user and patient. For example, to enable safety devices to trip at the appropriate time, heating has to be even throughout the pack and gases must be allowed to vent in the most extreme events. Major causes of battery-pack field defects are cold, fractured, or missing solder joints; these may create an electrical connection that manages to pass a functional test but that breaks in the field when vibration or heat is encountered.
Healthcare providers and hospital patients alike are becoming more and more dependent on portable medical devices. The safety and reliabili- ty of the power systems for these mission-critical devices are ensured by careful design. Unfortunately, an increasing number of reports of field- related battery problems indicates that equipment manufacturers are too often encountering design issues that they may not have the tools to solve or the awareness to recognize.
It is necessary to look carefully at the demands the portable device places on the battery system during real-world use, and to quantify its effect on the system's capacity, cycle life, reliability, safety, and durability. Strict cell-selection criteria, fuel gauges for reporting battery capacity level, the use of capacitors to support high-pulse duty cycles, and communication interfaces between packs and host devices are all essential design considerations for many medical device battery packs. However, while advanced features such as fuel gauging can make the end-user's life easier, poor fuel-gauge accuracy can limit the performance of the battery system. Thus the need for designing with care.
Portable devices that have well-designed battery packs can perform with the reliability and ease of use formerly associated only with tethered devices, while their mobility makes possible a ubiquity of application that is changing the nature of patient care.
Robin Tichy, PhD, is product marketing engineer at Micro Power Electronics Inc. (Hillsboro, OR). She can be reached at firstname.lastname@example.org.
Copyright ©2005 Medical Electronics Manufacturing