Rechargeable Battery Technology for Portable Medical Applications
By Dr. Robin Sarah Tichy
The pack consists of the cellsthe primary energy sourceas well as the printed circuit board, a plastic enclosure, and a status indicator.
At a Glance
- Lithium-ion technology
- Battery pack components
- Safety considerations
- Smart systems
Li-Ion TechnologyBattery technology struggles to meet the demands of the marketplace for greater energy density. State-of-the-art Lithium-ion (Li-ion) advantages include a much higher energy density, lighter weight, longer cycle-life, superior capacity retention, and broader ambient-temperature endurance. In addition, Li-ion cells offer the longest life-time and shelf-life and can tolerate much higher currents, important features for medical devices. The batteries offer many attractive advantages for portable medical equipment applications over the older technologies, but over the last 10 years, the fundamental materials on which Li-ion is based have not changed.
Li-ion batteries are available in a variety of shapes and sizes to fulfill an array of needs.
The new cells will originally be offered in the common 18-mm diameter and 65-mm long size, but eventually, they will be offered in a variety of shapes and sizes. LiMn2O4 from E-one Moli Energy and the nano-material based cells from A123 have extremely high rate capabilities; the moli 28 V pack enables two and a half times the work of an 18 V NiCd. These high drain rate cells have been designed specifically for the makers of cordless power tools, but surgical tools have similar requirements. The lithium metal phosphate technology developed by Valence has an inherent safety beyond the other chemistries, increasing the possibility of Li-ion cells being used in very large battery packs, such as those used for mobile X-ray units.
The Battery PackDesign engineers are making great strides in power management technologies that complement the novel battery chemistries. Battery system engineers and specialists work with design engineers and power management experts to maximize the available power for medical products. Their goals are to help the designer choose the best cell chemistry for the application and then to ensure that every drop of available power is utilized.
A standard battery pack consists of the cells, which are the primary energy source; the printed circuit board; a plastic enclosure; and LED status indicators. Other components of a battery pack include the plastic enclosure, external contacts, and insulation.Designing for safety and performance is a balancing act. You want to avoid nuisance tripping, yet you must avoid dangerous situations. An example of the challenges faced is easily imagined in the implementation of the new high-drain-rate cells. How does one distinguish between a high current pulse and a dangerous short? Knowing the true usage profile is essential to incorporating appropriate safety features and parameters.
The printed circuit board is the “intelligence” of the system for advanced functions such as the fuel gauge, protection circuitry, thermal sensors, and a serial data communications bus. The pack circuitry should use a thermal sensor to disconnect the cells at a specified temperature and prevent thermal runaway and overheating. Placement of circuitry within the pack is critical so the cells are not exposed to excess heat and are given room to swell. Smart battery systems are the preferred choice in mission-critical applications, such as the medical market. A valuable smart battery pack feature to users of mission-critical applications is the pack’s ability to monitor its status, accurately predict its remaining run time, and communicate its operational status to the host device. Traditional fuel gauges either monitored the voltage or the capacity, and the accuracy was quite limited. A new gas gauge monitors the number of coulombs being transferred and opportunistically calibrates with the open circuit voltage. Texas Instruments claims an accuracy of 99% with its Impedance Track technology, one example of this new type of gauge. These features allow the end-user to intelligently manage device use and avoid unexpected failures or shutdowns. A smart battery pack can also give feedback on its usage history, which is convenient for traceability and warranty issues.
Additionally, battery systems for medical and industrial applications must incorporate redundant safety systems and reliable protection circuitry. Even minor problems tend to influence how users perceive the value of a medical device. For example, any battery problem that occurs during field use can cause an increase in the number of service calls, warranty costs, and downtime, wasting the customer’s time and money. As a result, a number of safety design and manufacturing quality factors must be taken into consideration in the build of the battery pack. Active safety circuits are necessary to ensure that certain battery chemistries are kept in a stable condition. This is normally done with a safety circuit or battery management unit. All Li-ion batteries must be protected against over- and under-voltage, as well as short-circuiting. Like its host device, the battery pack must be designed with mechanics that are adapted for the specific environment of the medical device. As a final safety precaution, packs should be designed with vents to dissipate generated heat or exhaust vented gasses from cells in the unlikely event of failure.
Battery packs must be manufactured to be compliant with all the relevant standards and regulations. The pack must withstand extremes in heat and cold and survive rigorous drop testing as required by UnDOT. Vibration leading to creep in susceptible points, such as solder joints, is another typical concern. The constantly evolving legislation on materials restrictions must be monitored because the heavy metals in some cells are likely to be restricted. Currently, the European RoHS (restriction of hazardous substances) directive does not apply to batteries. They are governed by the European Battery Directive 91/157/EEC which includes requirements such as the ability to remove the battery for recycling.
ConclusionSmart battery systems are complex, sophisticated power sources that enable many advances. Products such as left ventricular assist devices and automated external defibrillators enable patients to survive events that might otherwise have proved fatal. Surgeons benefit from greater flexibility when traditionally tethered surgical devices can be untethered, such as orthopedic power tools or endoscopic devices. An experienced battery system manufacturer can ensure that the packs are designed properly and offer a seamless enhancement of a product for the doctor or patient.
OnlineFor additional information on the technologies and products discussed in this article, see the following websites:
- www.micro-power.com 
- www.valence.com 
- www.molienergy.com 
- www.a123systems.com 
- www.texasinstruments.com