Li-ion batteries provide reliable back-up for critical care portable medical devices.By Jeffrey VanZwolThe United States Food and Drug Administration (FDA) provides three categories for the classification of medical devices. Class I devices are defined as non-life sustaining; these products are the least complicated and their failure poses little risk. An example of this device is an illuminating ear scope. Class II devices are more complicated and present more risk than Class I, though are also non-life
| Medical battery packs include cells, a printed circuit board or battery |
management unit, and
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sustaining. Examples of these devices include endoscopes or patient monitors. Class III devices sustain or support life, so that their failure is life threatening. Examples of these devices include infusion pumps and ventilators. Because Class III devices sustain or support life, they need to have fail-safe mechanisms, such as a battery for back-up power in the event of power failure. Many of these devices are used during or after surgeries and have been, for the most part, stationary, so the size or weight of the battery is not of primary concern. The cost and indiscriminate charging needs of sealed lead acid batteries have made them the standard choice to perform this function.
However, care providers are demanding increasingly mobile apparatus. This flexibility can result in significant cost savings and increased quality of care. Equipment can be brought to the patient and they can be ambulatory sooner after surgery. Ventilators, for example, help patients with heart disease breathe and help patients recovering from the effects of anesthesia and sedatives given during surgery. The hospital units typically are sophisticated devices that offer such features as central monitoring. The units come with a number of built-in alarm systems to alert care-givers of impending or immediate life-threatening changes in patients' respiration, and they must have a back-up power source. If the apparatus is to be mobile, the battery should be as light and small as possible, making Li-ion batteries the best solution.
For devices such as portable ventilators, Li-ion batteries offer many attractive advantages over other rechargeable chemistries, including a much higher energy density, lighter weight, longer cycle-life, superior capacity retention, broader ambient-temperature endurance and higher current tolerance. The energy density for Li-ion batteries has increased over the tears by stuffing more and more material in the same size cell. Li-ion is more environmentally friendly than the other chemistries, especially when designing Restriction of Hazardous Substances (RoHS) compliant products. Li-ion has decreased in cost because of the economies of scale provided by consumer products, such as laptop and cell phones. The most cost effective Li-ion cells are offered in a common 18mm diameter and 65mm long form factor. Often a Li-ion battery is at cost parity with a Nickel-based battery because the higher operating voltage (3.6 V vs. 1.2 V), which enables a lower cell count within the pack.
Li-ion cells are the base component of a battery pack. The main components of a typical battery pack include the cells providing the primary energy source; the printed circuit board or battery management unit supporting state-of-charge monitoring, protection circuitry and a serial communications bus; and a custom plastic enclosure. All these elements can be customized when designing a battery pack that integrates with a medical device.
Battery packs for medical devices should incorporate redundant safety systems and reliable protection circuitry because even minor problems can prove life threatening. The stable condition of the cells is normally maintained with a safety circuit. All Li-ion batteries must be protected against over- and under-voltage, as well as short-circuiting.
Continuing with the portable ventilator example, a valuable smart battery pack feature for life-support devices is the ability to monitor its status, accurately predict its remaining run time, communicate its state-of-charge and operational status to the host device. Fuel gauges monitor the number of coulombs being transferred and opportunistically calibrate with the open circuit voltage of the Li-ion pack. This allows the end-user to intelligently manage device use and avoid unexpected failures or shutdowns. This operational information can be delivered to the host device via a one- or two-wire serial communication bus such as System Management Bus (SMBus) or Inter-Integrated Circuit (I
2C).
High voltage power, greater than 12 V, is achieved with a high-count string of cells in series. High-count strings of Li-ion cells lend themselves to imbalance of the cells, leading to drastically reduced runtimes. New power management technology allows the balancing of up to 100 Li-ion cells in series strings. This “cell balancing” technology resides on a printed circuit board or battery management unit.Battery packs must adhere to stringent Department of Transportation (DoT) and United Nations (UN) regulations. These guidelines state that a battery pack exceeding eight grams of equivalent lithium content must be shipped as Class 9 hazardous material. Shipping guidelines for Class 9 materials impose additional fees and regulations on the device manufacturer, so it is always advantageous to stay under the Class 9 limit. If a specific device requires a battery pack that may be classified as Class 9, a common work-around is to design two smaller independent battery systems that comply with UN/DoT regulations. Both these packs can be inserted in the devices, and electrically joined in series or parallel through the host device electronics. If they are joined in parallel in the host device, a benefit of this approach is that a nearly-depleted pack may be hot-swapped while the other is powering the device.
Battery packs must be manufactured to be compliant with all of the relevant standards and regulations. The FDA’s testing requirements are for the complete medical device, inclusive of the battery pack. In addition, the pack must withstand extremes in heat and cold, and withstand rigorous drop testing as required by UN/DoT. The legislation on materials restrictions must be monitored because the heavy metals in some cells, such as Nickel Cadmium, will be restricted. The European RoHS directive does not currently apply to medical devices or 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.
Like the medical device, the battery pack must be designed with mechanics that are adapted for the specific environment of the medical device. And 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.
Jeffrey VanZwol is marketing manager at Micro Power Electronics, a manufacturer of high-performance, mission-critical battery packs.ADVERTISEMENT