Lithium-Ion Cells - A Prescription for Best Performance in Portable Medical Devices

The era of battery-powered medical devices has improved life-saving care from the surgery room, to the patient's home and from the emergency room, to the combat zone. These devices are valued for their mobility, their ability to power up immediately, and their compatibility in sterile environments.

Medical endusers desire more powerful and more reliable portable devices; medical-device manufacturers are eager to comply. OEMs are creating more compact devices that don't sacrifice run-time, working with battery designers on custom battery packs designed specifically for an application rather than accepting the compromises inherent with off-the-shelf batteries. These batteries are tailored for the desired size, weight and performance needs, and can offer safety features and tailored functions such as runtime indicators, authentication functions and data logging.

Despite the rapidly growing acceptance of battery powered products, medical professionals still want assurance that the product they are using will perform reliably over a predictable period. They want to know that the pack is safe. They want to know if the battery has sufficient remaining charge to complete the procedure or if it needs to be swapped for a fresh pack. They also want to know if the pack is old and needs to be replaced. These concerns have been prompted by individual reports that some portable devices have abruptly run low and others have not recharged consistently. Such concerns have proved expensive in terms of medical-device protocol. Partially discharged batteries get swapped for fully recharged packs even when the original pack can still provide consistent power for hours. Battery packs are often retired long before they reach the end of their actual service life.

This lithium ion pack with a fuel gauge and cell balancing is used in a prosthetic foot.

When designing battery powered devices, several factors need to be considered. They include safety, battery life, safe and efficient charging, accuracy of the fuel gauge, impact of high-temperature sterilization, and complying with regulations.

Safety, a Primary Concern

A primary concern, not surprisingly, is safety. If the battery is making contact with a patient, it cannot present a shock hazard, issue unwanted heat or create a reaction with materials already present in the patient's body or his surroundings. Moreover, ports that might be accessed by medical personnel or patients must be protected against short circuits, electrostatic discharge and other electromagnetic interference.

Because of its high energy density, Lithium-ion (Li-ion) is the battery chemistry of choice for today's portable medical equipment. However, with Li-ion there is a trade off in terms of an increased need for safety measures. That's why several safety mechanisms are built into Li-ion battery cells and packs. Cell construction may include a thermal-shutdown separator, pressure-activated current interrupter, pressure vent, or over-current protection. Protection in the battery pack includes an electronic safety circuit, which protects the cells from excessive charge or discharge voltage, or current and over-discharge. Additional pack protection may also include fuses or other current interrupt devices that activate at unsafe currents or temperatures. During the pack construction process, care is taken to avoid obstructing pressure vents, insulating tape is used to prevent potential external short circuits and heat generating components are placed away from the cells.

Regarding Battery Life

Every time a battery is used, its recoverable capacity decreases slightly, depending on the depth of the discharge. As the battery cycles, it can no longer deliver 100% of its rated capacity. Consider this: On its 100th charging cycle, a battery that registers fully-charged would likely hold only 90% to 95% of the charge it held when new. Device designers should take into account this degradation in the battery capacity relative to the promised runtime of the device. A battery is generally considered near the end of its life when fully charged is really equal to only 80% of its capacity when new. In the case of most batteries, that cycle life is between 300 and 500 full cycles.

Designers have the option of manipulating several parameters to increase battery life. Reducing the charge voltage extends cycle life with only a small penalty in capacity. If cycle life or the need to keep the product fully charged when not in use is critical, reducing the charge voltage on Li-ion rechargeable batteries to 4.0-4.1V/cell, instead of the typical 4.2V/cell can significantly increase the cycle and calendar life.

Balanced Charging

Generally, Li-ion batteries are charged using a regulated voltage with limited current. During the early portion of the charge cycle, current is constant and voltage rises to its regulated value. After reaching it, current gradually decreases as the battery reaches full capacity. Charging typically ends when the current falls below a predetermined level.

It is important that all cells in a multicell pack remain balanced with respect to capacity and state-of-charge. That's because charge is limited by the cell most full, while discharge is limited by the cell least full. In short, one cell's state-of-charge limits the performance of the entire battery pack. Manufacturers must grade and match cells according to capacity, state-of-charge and impedance, especially for Li-ion packs. Cell age, temperature and use (or abuse) are all factors in this voltage balance.

Cell balancing during charge and discharge gets the most from a pack. There are two main ways to balance cells. One way, resistive cell balancing, reduces the charge to the cell with the highest state of charge by shunting charge around it until the remaining cells catch up. The second, cell balancing by charge transfer, applies circuitry to move charge from one cell to another. This can actually raise the effective capacity of a battery pack beyond that of its weakest cell, though it is more complex and costlier than resistive cell balancing.

Although cell balancing can be an important tool in maximizing pack performance, overusing it is not advised. Cell balancing is intended to correct minor deviations. A significantly out-ofbalance cell might be a sign of a serious problem developing inside the cell. That might present a safety hazard if the laggard cell is continually forced into balance with others.

Accuracy of the Fuel Gauge

Previous battery state-of-charge indicators or fuel gauges have proved unreliable, and conscientious medical professionals will not undertake even simple procedures with a portable monitor or other device registering a half-full battery. An accurate state-of-charge indicator has been hard to design because of the difficulty in accurately determining a battery's state-of-charge while in use. Predicting a battery's remaining run-time has improved significantly in recent years. Credit that to advances in the micro processing capability of battery electronics that tracks several variables. The first fuel gauges measured voltage as a means to gauge the remaining charge. This method was not very accurate and as a result not sufficiently reliable for use on medical products. A significantly greater level of accuracy was gained by tracking the current flow in and out of the battery over time by counting coulombs or milliamp hours. This method greatly improved accuracy; however, it did not reflect the effect of aging, self discharge or storage at high temperature.

To adjust for these conditions the fuel gauge needed to relearn or be recalibrated to correct errors that had built up over time. The recalibration involved relearning the actual full-charge capacity due to the loss in capacity with age and stress, and then adjusting that calculation for the existing present discharge rate and temperature. Relearning capacity was previously accomplished by fully discharging the pack so that it could measure its capacity. But that required fully charging the battery and discharging it to nearly empty. Relearning is not regularly scheduled or practical in most busy medical environments. It must be done off-line with controlled charge and discharge cycles. Not surprisingly, relearning is often ignored and users incorrectly assume the displayed capacity is still valid. New technology developed by Texas Instruments has enabled the design of a highly accurate fuel gauge with a self-learning mechanism that accounts for the change in cell impedance and the change in capacity due to aging. Using this technology, the fuel gauge will provide accuracy within 1% and does not require manual recalibration.

Today's improved fuel gauges can actually measure the state of health of the cells in a pack.

Properly implementing sophisticated fuel-gauge ICs lets knowledgeable battery-pack designers develop power sources that are safe, abuse-tolerant and accurate in terms of predicting remaining run-time. That's important to the medical professional. When a fuel gauge says 50%, he or she can be confident that the device still has half of its battery capacity remaining.

Some hospitals and clinics replace batteries on a strict time schedule – every six, nine, 12, or 18 months – but they often throw away good batteries. Recent developments now let batteries send more accurate information to users, such as identifying good batteries from bad. Today's improved fuel gauges can actually measure state of health of the cells in the pack to determine when it is time to replace the pack on an individual basis. And new functions can disable the battery when it is no longer reliable. The number of recharge cycles can be synced with the battery pack's warranty; with that function, users will know when the pack really needs to be replaced.

Sterilization

Battery packs are used in sterile environments, including operating rooms, and must withstand sterilizations. That's no minor challenge, since autoclave temperatures and pressures can damage cells. Some manufacturers of portable medical devices that must be sterilized have opted to use single-use battery packs.

NiMH packs, when they are sealed and insulated, withstand autoclave sterilization, but sterilization reduces the batteries' cycle life. Alternative sterilization techniques are strongly advised for Li-ion battery packs. They include chemical cleaning at lower temperatures or peroxide-gas systems in a vacuum at room temperature. These lower temperature chemical treatment alternatives require proper selection of compatible battery materials that do not degrade when exposed to these environments.

Regulatory Issues

About two years ago Underwriters Laboratory (UL), the organization that oversees electronic device safety, changed how it enforced certain requirements regarding UL 60601 approvals, one of the standards for medical electrical equipment. One change requires that battery packs comply with UL 2054 (the battery pack standard) before certifying the host medical device for UL60601. UL 2054 addresses safety of commercial and household battery packs and says devices must withstand short-circuit and abusive-charge tests at 25°C and 55°C.

UL regulations for medical devices, especially those used near patients, are more stringent than those for other devices. Consider one example: Devices that can be touched by patients must prevent access to any voltage source over 0.1V. Other standards permit access to voltages as high as 60V.

With regard to lithium-based batteries, all must be shipping certified by passing the rigorous UN-T transportation tests, which assure the battery will hold up despite high altitude (low external pressure), severe thermal cycling, vibration, shock, short circuit and overcharge. These tests also indicate that they do not present hazards in normal use.

Battery packs must also be tested to assure medical manufacturers that the packs will withstand real-life conditions. Any new design should go through a series of stress tests to mimic foreseeable use and misuse scenarios common in medical environments. Cells must be tested because, in nearly all cases, required discharge rates will differ from that used by the cell manufacturer for their data, or they will be operated in temperatures outside the typical roomtemperature range. Information from such sophisticated testing improves beta testing and loops back to device designers to ensure a more reliable final product.