Medical Device Link.

Originally Published MEM Spring 2006

BATTERY TECHNOLOGY

A higher-energy-density technology can deliver more power than alternatives despite a smaller footprint.

Robin Tichy

Medical equipment manufacturers have joined the trend toward greater product mobility demonstrated by other industries, such as consumer electronics. In doing this, they are supporting the healthcare industry push to deliver faster response and increased lifesaving and monitoring capabilities in the field, as well as in fixed-base medical facilities. Beyond portability, there is added urgency for medtech manufacturers to deliver highly reliable devices when human lives hang in the balance. If a consumer's cellular telephone fails to work, that is annoying. But if a portable heart-monitoring device or infusion pump stops working because of a dead battery, the end-user—and patient—faces a far more serious problem.

Not too many years ago, medical professionals were unable to take lifesaving equipment out into the field; there simply were no such portable devices. Today, however, a wide variety of monitoring devices, ultrasound equipment, and infusion pumps are being transported to, and stored at, remote locations—even battlefields. Portable equipment is getting increasingly mobile. Thanks to the implementation of technologies such as lithium-ion (Li-ion) batteries, bulky defibrillators that weighed 50 lb and strained professionals' muscular resources have been replaced by lighter, more compact units that are also user-friendly.

Patient mobility also is becoming more important. Today's patients may move from the radiation department to the intensive care unit, from an ambulance to the emergency room, or from one hospital to another via ambulance ride. The proliferation of portable home-use devices and mobile monitoring equipment, which allow patients to dwell with loved ones instead of in a healthcare facility, is a related phenomenon. In the end, portable medical devices must be truly and fully portable to provide optimal service.

The need for smaller, lighter medical devices thus is growing in importance. Industry movement toward untethered products is driving the increase in interest in higher-energy-density, smarter battery-pack options. And now, medical equipment designers can exploit the innovations in Li-ion battery technology that have been instigated by the laptop computer and cellular phone industries.

Li-ion batteries offer many advantages for portable medical equipment applications, comparing favorably in many cases with the older technologies. Their advantages include a much higher energy density, lighter weight, longer cycle life, superior capacity retention, and endurance of a broader range of ambient temperatures.

Having a unique chemistry, Li-ion technology presents different design constraints than the older battery technologies such as nickel–metal hydride (NiMH), nickel-cadmium (NiCd), and sealed lead-acid (SLA). At the same time, medical equipment has more-stringent operating requirements than consumer electronics in some ways; dependability is paramount, and this leads to the need for a robust battery-pack design featuring very accurate fuel gauging and reliable cells.

Considerations for the design of a portable power system that merges the requirements of a medical device with the unique attributes of Li-ion technology are outlined in this article. The distinctive properties and capabilities of Li-ion batteries are contrasted with the characteristics of other battery chemistries.

Energy Density and Voltage

Figure 1. The energy density of Li-ion is far greater than those of the nickel-based chemistries. (click to enlarge)

The chief advantage of Li-ion cell technology is the pronounced increase in energy density it offers. For their size and weight, Li-ion cells store and deliver more energy than other rechargeable batteries. Energy density is measured both volumetrically and gravimetrically. Li-ion technology now can provide a volumetric energy density of almost 500 Wh/L and a gravimetric energy density of 200 Wh/kg (see Figure 1).

Compared with the older technologies, Li-ion is able to deliver more power with a smaller footprint and less weight. Li-ion battery cells operate at higher voltages than other rechargeables, typically about 3.7 V versus 1.2 V for NiCd or NiMH. This voltage level means that a single Li-ion cell often can be used where multiple cells are required when the older-technology batteries are used. The higher the energy density of the batteries used in the design of a portable device, the more the product's size can be decreased and its convenience increased. Shrinking the battery-pack compartment allows additional device capabilities to be designed into the same product (see Figure 2).

Self-Discharge

Figure 2. Li-ion cells come in two shapes, cylindrical and prismatic, and they can come in a variety of sizes and capacities. (click to enlarge)

Rechargeable batteries lose capacity over time. This phenomenon is referred to as self-discharge. However, with proper storage, most of its lost capacity can usually be restored to a rechargeable battery.

All batteries should be stored at room temperature (25°C or cooler) for maximum capacity retention. To ensure the optimal performance of SLA batteries, end-users should keep them at cool temperatures and charged as near to 100% capacity as possible at all times. At 25°C, a sealed lead battery will self-discharge about 20% after six months; but after six months at 40°C, this self-discharge factor increases to about 30%. NiMH cells carry similar recommendations, to avoid any deactivation of the reactant materials during long-term storage. The self-discharge rate at 25°C for both NiCd and NiMH technologies is about 20% in the first month, with dramatic slowing subsequently.

By contrast, Li-ion technology delivers the best cycle-life performance when batteries are stored at a 30–50% state of charge. The self-discharge on Li-ion cells stored at 25°C is only about 10% after six months.

Rate Capabilities

In considering which chemistry to choose, it is important to think about what the design's inrush current and maximum discharge rate will be in the end unit. Starting out cells or packs at high rates causes a voltage drop. If the design does not accommodate this, the unit may shut down due to insufficient voltage.

High-rate NiCd batteries can take 2C continuous-discharge rates (two times the rated capacity of the battery) and sometimes more, depending on the cell recipe and its internal impedance. Many SLA batteries can take 3C continuous discharge and more. Most Li-ion cells can handle only a 1C continuous-discharge rate, but new cells using this technology have emerged that accommodate extremely high discharge rates up to 80 A for 30 seconds. These compete very favorably with both NiCd and SLA batteries.

Cycle Life

The cycle life of a battery is the number of recharging cycles the battery can go through before it reaches the point where it can be recharged only to a defined percentage of its original capacity. Cycle life for a lead-acid battery is between 250 and 500 cycles, depending on the quality of the manufacturer's products and the depth of discharge (down at most to 60% of rated capacity). NiCd, NiMH, and Li-ion batteries are usually good for 500–700 charge-discharge cycles, down only to 80% of their rated capacity. In all cases, regardless of the battery chemistry used, the deeper the depth of discharges, the fewer the number of cycles the user can expect from the battery.

Charging Differences

Li-ion batteries require a different charging regimen than the older battery chemistries. For an SLA battery it is best to have either a constant-voltage charge, commonly at a tenth-of-rated-capacity (C/10) rate and taking 14–16 hours, or a trickle, or float, charge at a rate of C/20 to C/30. The recommendation for NiCd batteries is to terminate the charge with a negative delta-voltage charge, where the charger is looking for peak voltage. NiMH cells, because of their exothermic nature, require the use of a temperature gauge in their charge regime, for which a ³T/³t method is preferred. Specially made quick-charge NiCd and NiMH cells can be charged at a C/2–C/3 rate within perhaps 4–6 hours. Very-low-impedance nickel-based cells designated as rapid-charge can be charged at a 1C rate in about an hour. Finally, for Li-ion cells, a constant-current/constant-voltage (CC/CV) method is recommended.

Typically, 60–75 minutes of charging at 1C to 4.1 V is sufficient to bring a Li-ion–powered device from a depleted energy state up to an 80–90% state of charge. With the other technologies, unless the cells are specifically made for high-current charging, getting up to the same 80–90% charge might require charging for several additional hours. To obtain the remaining 10–20% of capacity, the Li-ion battery is slow-charged for an additional 4–5 hours to 4.2 V. This charging method offers two benefits. Users enjoy the advantage of a close-to-full charge in a very short period of time, and the voltage at the end of the charge is virtually guaranteed not to exceed 4.2 V.

It should be noted that Li-ion cycle life can be uniquely extended if the battery is charged only to 4.1 V rather than 4.2 V; however, per-cycle capacity will be decreased. The battery is used as a backup device in some medical applications, and so is kept constantly charged to ensure that it is ready for use at all times. Li-ion chemistry does not take well to trickle charging; a Li-ion battery should not be kept on a constant float charge. However, there are ways to minimize the potential for Li-ion overcharging effectively, without harming the battery or compromising the medical device. One approach is to make sure that the battery is drained at least 20% before instigating recharge, and then to follow a standard charging regime. The improvement in energy density that Li-ion technology offers in comparison with SLA chemistry is dramatic enough in many cases to warrant the derating of Li-ion from full charge.

Safety Circuitry

Figure 3. Battery packs designed for Li-ion cells require electronics for safety. A fuel gauge and charging circuit may also be included inside the pack. (click to enlarge)

Each form of battery technology has its own set of safety considerations. Good battery design necessitates that NiCd battery packs have some sort of current-interrupt device to prevent catastrophic failure. NiMH chemistry, because it is exothermic, demands that the battery incorporate a heat-sensing device that communicates with the charger to prevent overcharge, as well as a current-interrupt device for the pack itself. In Li-ion packs, lithium metal may be generated in the event of overvoltage. This possibility dictates that a safety circuit be employed that will keep the battery within a specified voltage range for both charge and discharge (see Figure 3).

Although SLA batteries do not generally require external safety components, many medical equipment manufacturers insist that a nonresettable fuse be somewhere in or around the battery. Because most SLA batteries are constructed with protruding positive and negative tabs, if there is no fuse then the battery can easily short out when placed on a metal table, which are plentiful in healthcare facilities. These batteries may be subjected to other shorting hazards as well. If this happens, the unit can become volatile. Li-ion packs, however, may be shorted out with little danger; the safety circuit they are required to have protects the battery.

Putting a safety circuit on a battery increases the cost and uses more space in the device. Designers must accept the fact that such considerations are among the trade-offs to be negotiated in choosing among the major battery chemistries. Overall, Li-ion batteries save pack size and weight and deliver more power, despite their safety circuit requirements.

Fuel Gauging

As more medical device manufacturers embrace Li-ion technology, battery management features will become more commonplace in the industry. Fuel gauges can provide the end-user with such information as the expected run time for the battery. When management features are built in, battery capacity assessment and recharging decisions can be executed with far more certainty.

Designers using Li-ion cells have myriad options when it comes to battery management. For instance, some fuel gauges available for Li-ion batteries include communication features that report on the number of charge and discharge cycles that have occurred. This kind of information could be vital for critical medical device applications. There are two basic types of fuel-gauging methods: voltage based and coulomb counting. Solutions that combine these techniques provide 99% accuracy.

High-Temperature Tolerance

Li-ion batteries outperform their counterparts in high-temperature conditions ranging up to 40°–45°C. SLA and NiMH batteries do not perform well in higher-heat situations. This could become a factor in emergency vehicle applications, where medical responders are unable to store their portable devices in a cool environment.

Conclusion

Total cost and total performance should be evaluated to determine the best overall power option for a portable-device application. The higher voltage characteristic of Li-ion technology can result in fewer cells being needed, and hence bring pack costs down to a par with the nickel technologies. In addition, Li-ion cell suppliers are introducing new materials to bring down the cost of these cells.

The benefits of smaller size, lighter weight, greater power, longer cycle life and endurance, higher voltages, and better heat endurance add up to make Li-ion batteries potentially advantageous. Manufacturers of medical electronics can exploit these attributes to expand the market for their devices and ultimately bring therapeutic and other benefits to customers, medical professionals, and patients.

Robin Tichy, PhD, is product marketing engineer at Micro Power Electronics Inc. (Hillsboro, OR). She can be reached at rtichy@micro-power.com.

Copyright ©2006 Medical Electronics Manufacturing