Power Solutions: Leader of the Pack

Author: 
Shana Leonard

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Five Tips for Optimizing Li-Ion Battery Pack Design

Advances in battery chemistry have allowed medical systems to cast off the shackles of power cords and become portable. Consequently, this mobile movement is also freeing patients from the tethers of bulky consoles as well as enabling the development of innovative portable medical systems for field use and point-of-care diagnostics. But with this newfound freedom comes new concerns about battery reliability and safety for life-critical applications. In response, custom battery pack manufacturers are leading the charge to optimize power solutions through the design of more-intelligent battery packs that incorporate enhanced safety features and battery-management electronics.

A Matter of Chemistry
Mirroring a ubiquitous trend in the consumer electronics industry, the medical device industry is laboring to produce smaller, lighter-weight designs while maintaining—or even improving—product performance. And while other factors have played a role in product miniaturization, the driving force behind increasingly portable systems in both industries has been advancements in rechargeable lithium-ion (Li-ion) cell chemistries.

“A lot of medical OEMs have always had backup power in their systems, so something like a patient monitor or infusion pump has historically had a lead-acid backup battery,” explains Robin Tichy, marketing manager at battery pack manufacturer Micro Power Electronics (Beaverton, OR). “Because the patient is becoming more mobile in the hospital setting, the backup batteries are no longer being used just as backup power but as rechargeable power. You see a shift from a sealed lead-acid backup battery to a primary lithium-ion battery for increased portability. It’s a shift not just from tethered to untethered [systems], but from older battery technology to newer technology.” Tichy adds that almost 100% of Micro Power’s new battery pack designs are based on lithium-ion chemistries.

House of Batteries
Battery packs from House of Batteries can feature intelligent electronics for safe, reliable portable medical systems.

Chief among Li-ion’s advantages is that it offers higher energy density for its size and weight than older rechargeable chemistries, such as nickel-metal hydride and nickel cadmium. A higher energy density translates into a longer run time, which is, of course, a desirable characteristic for portable medical applications. Providing a volumetric energy density of almost 500 Wh/L and a gravimetric energy density of 200 Wh/kg, Li-ion essentially packs more power into a smaller form factor.

“More products are emerging that couldn’t have been developed before due to the increase in energy and decrease in size from the newer rechargeable chemistries,” notes Mel Weis, vice president of custom medical battery pack provider House of Batteries (Fountain Valley, CA).

Although high energy density is the primary benefit of Li-ion battery cells, it’s not the only one. Li-ion cells operate at a higher voltage than other chemistries, which enables designers to replace older nickel-based batteries with smaller Li-ion batteries. “Nickel-metal hydride and nickel cadmium usually run at about the same voltage as an alkaline battery: 1.2 to 1.5 V, requiring more cells to achieve a similar pack voltage,” says George Cintra, director of engineering and technology at custom battery power and wireless sensing technology specialist Electrochem Solutions Inc. (Clarence, NY). “Li-ion, on average, runs at about 3.7 V. That makes it a lot more attractive; it will run more efficiently and adds to the run time.” Li-ion batteries also boast a cycle life of about 500 cycles, no memory effect, and lower self-discharge than nickel-based chemistries.

Taking Safety Precautions
Despite their significant contribution to product miniaturization, however, Li-ion batteries have been the subject of some scrutiny and criticism in the past several years. Spontaneously combusting laptops in 2006, for example, were attributed to short circuits caused by metal flakes in Sony’s lithium-ion cells that were generated during the manufacturing process. As a result, these headline-grabbing incidents sparked widespread recalls and debate as to the safety of lithium-ion batteries. In the wake of these dangerous incidents, however, Li-ion cell and battery pack manufacturers alike have taken additional safety precautions and improved protection circuitry to prevent such events.

Electrochem Solutions Inc.
A range of rechargeable chemistries offered by Electrochem, including lithium-ion and nickel-metal hydride, are suited for use in portable medical applications. However, lithium-ion cells provide a higher energy density in a smaller footprint.

Ensuring safety in a lithium-ion battery pack requires a multipronged approach involving protection measures designed into the cell, battery pack, and charging system, according to Cintra. Cells, for example, may include such safety features as a pressure- and temperature-activated current interruption method, overcurrent protection, and a thermal-shutdown separator. “Cell manufacturers [incorporate] materials such as shutdown separators that will melt when a cell reaches a certain temperature, essentially shutting down the reaction,” Cintra explains. “They also have a temperature cutoff device inside the cell called a PTC. So, if the cell reaches a certain temperature, that device will break the circuit.”

For further hazard protection, prismatic cells may be designed with pressure vents scored into the side of the case, Cintra notes. These safety vents, he says, help to prevent a buildup of pressure and resulting catastrophic event.

As for the battery packs, electronic safety circuitry is required to protect the Li-ion cells from excessive charge or discharge current and voltage. However, some Li-ion battery pack manufacturers go above and beyond basic requirements in order to ensure the utmost safety. “We have multiple levels of safety in the pack,” Weis says. “We can protect against every external fault, whether that's in the charging system or the product application itself. We incorporate that with every Li-ion rechargeable battery pack we design.” A battery pack may also include additional overcurrent or high-temperature fuses or current-interrupter components.

Battery pack design and layout are also critical safety considerations, however. Engineers need to be cognizant of the pressure vents, for instance, to avoid blocking them and thus negating their intended function. Likewise, heat-generating components in the battery-management circuit must be situated away from the Li-ion cells.

The last piece of the safety puzzle, according to Cintra, is protection at the charger level. “So-called smart chargers will monitor the temperature while charging and the amount of current that’s delivered as well,” he says. “When you look at safety, it’s truly a systems approach.”

Gauging Capacity
In life-support or life-critical medical systems, it is imperative that a battery does not unexpectedly fail. “A battery is actually a consumable part, so it will have an expected finite lifetime. The cycle life of a Li-ion battery is defined in the industry as when the battery reaches 80% of its original capacity,” Tichy of Micro Power explains.

To avoid unexpected battery failure, ensure user confidence in product lifetime, and prevent the waste of functional batteries by overly cautious users, battery packs can be designed with intelligent battery-management electronics that support increased reliability. Also referred to as a gas gauge or state-of-charge indicator, a fuel gauge, for example, is a chip that can be integrated into the battery pack that calculates and communicates to the end-user how much capacity remains. Information such as the remaining number of cycles and run time, as well as how much charge time is left and the age of the battery, can be provided from the battery pack to the host device, according to Weis.

“A lot of medical products like infusion pumps or ventilators that are used in life-support applications have very specific requirements around gas-gauge accuracy from a countdown of say 30 minutes until you have a dead battery. So, they can give off warnings to the user,” Tichy adds.

And while fuel gauges offer valuable data, their accuracy, until recently, was considered questionable. Initially, fuel gauges calculated a battery’s capacity through voltage measurement; however, this method was deemed somewhat inaccurate because of the flat discharge-voltage profiles associated with nickel-based chemistries. The subsequent emergence of coulomb counting, Tichy states, proved more accurate than voltage measurements but required calibration as capacity diminished. Coulomb counting determines capacity by monitoring the current flow entering and exiting the battery.

Neither of these fuel-gauging methods was traditionally compatible with Li-ion chemistries, however. Luckily, a breakthrough in gas-gauge technology has changed that. Combining the best of both methods, a new fuel-gauge technology suitable for lithium-ion battery packs provides 99% accuracy if implemented correctly. “Scientists have developed algorithms based on an understanding of the chemistry that, as they track the current, voltage, and temperature, can reliably estimate the run time or time to end,” Cintra comments. “The [technology] will also store real-time information to adjust the algorithm as the battery ages. Plus, as you charge and discharge the battery pack many times, it does start to get depleted. So, there’s a calculation in there as well that takes that into account.”

Adds Tichy: “The new gas gauge monitors the number of coulombs being transferred and opportunistically calibrates with the open-circuit voltage of the Li-ion pack. These features allow the end-user to intelligently manage device use and avoid unexpected failures or shutdowns.”

A Cell Balancing Act
Like fuel gauging, cell balancing is an ‘intelligent’ battery-management option that has become increasingly common in battery packs for the portable medical device market. “Cell balancing monitors every group of cells that is in series during the life of the product to keep them all balanced, providing the longest life possible,” explains Weis.

A multicell Li-ion battery pack should consist of cells from the same manufacturer and production lot to ensure that the batteries’ capacities are closely matched at the outset. Over time, as the cell repeatedly charges and discharges, however, various external factors can affect an individual cell’s capacity, Cintra states. Proximity of the battery pack to heat-generating electronic components within the portable system, for instance, can affect the capacity of the cell or cells closest to the components, thereby creating imbalance within the pack over time.

And unequal charges within a given battery pack can affect overall power performance and life. Ultimately, Cintra notes, a power solution is only as strong as the weakest cell in the pack. Cell-balancing electronics can assist in mitigating this risk by keeping the charges of a battery pack’s various cells in check. This type of battery management monitors such factors as age, current, and temperature and then prompts an appropriate action to correct cell imbalances within the pack.

“The purpose is so that you don’t get into a situation with Li-ion specifically in which you overcharge or discharge an individual cell repeatedly, which could lead to performance and/or safety issues,” Cintra says. “Through the circuitry, [cell-balancing electronics] can divert current from one side to the other, depending on the state of that cell.”  

Providing such intelligent functionality, cell balancing, fuel gauging, safety features, and the implementation of other battery-management electronics into rechargeable Li-ion battery packs are enabling smarter, safer power solutions for portable medical systems. “I would say that about 90% of the design projects we get are for rechargeable lithium [battery packs] with some intelligence,” concludes Weis. “The more information users have throughout the life of the product, the better they’re going to know what to do with the battery pack to maximize the performance of the device.”