Technological advancements are making medical devices increasingly feature-rich and miniaturized: two performance characteristics that are inherently conflicting, thus requiring increasingly sophisticated battery power management solutions.
Battery-powered devices span the entire medical spectrum, from surgical drills and power tools, to automatic external defibrillators (AEDs), robotic inspection systems, infusion pumps, bone growth stimulators and other wearable devices, glucose monitors, blood oxygen meters, cauterizers, RFID asset tracking tags, and other remote wireless devices.
Application-specific requirements dictate the choice of power supply, including:
- Reliability: patient wellness depends on procedure outcome
- High power-to-size ratio: keeping the medical device small, lightweight and ergonomic for ease of use and accuracy
- Long shelf life: making sure the instrument in in working order even after prolonged storage without having to recharge or replace the battery
- High temperature survivability: for autoclave sterilization
- Cold temperature operability: for reliable operation in the cold chain
- Ability to supply high pulses: extra power needed to run motors and communications circuits.
Consumer or Industrial Grade?
Certain devices will continue to be powered by consumer grade alkaline and rechargeable batteries. However, industrial grade lithium primary batteries are increasingly being utilized in advanced medical equipment, as lithium chemistry offers the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of any available chemistry. Lithium cells have a nominal open circuit voltage of between 1.7 and 3.9V. Their electrolyte is also non-aqueous, permitting certain cells to operate in extreme temperatures.
A Wide Choice of Primary Lithium Chemistries
As Table 1 shows, several primary lithium battery chemistries are available. For example, lithium manganese dioxide (LiMNO2) batteries are commonly used to power hand-held glucose monitors. These cells are inexpensive, easily replaced, and good enough for most in-home applications.
Lithium sulfur dioxide (LiSO2) batteries deliver high pulses, especially at low temperatures, but add bulk due to their low energy density. These batteries also have high annual self-discharge rates.
Bobbin-type lithium thionyl chloride (LiSOCL2) cells feature the highest energy density, highest capacity, and lowest self-discharge rate, which is ideal for use in long-life applications that require small amounts of current. Bobbin-type LiSOCL2 cells can also operate at extreme temperatures (-80°C to 125°C), making them suitable for autoclave sterilization. Specially modified bobbin-type LiSOCL2 batteries can withstand temperatures as low as -80°C (with certain cells surviving prolonged testing at -100°C), which is required for monitoring frozen tissue samples, pharmaceuticals, and transplant organs transported in the cold chain.
However, due to their low rate design, standard bobbin-type LiSOCL2 batteries cannot deliver the high pulses needed to power motorized instruments. One economical solution is to combine a standard bobbin-type LiSOCL2 battery with a patented hybrid layer capacitor (HLC) that stores the energy and delivers periodic high pulses to applications such as automatic external defibrillators (AEDs). Use of an HLC offers a far less bulky, and less complex solution than supercapacitors.
Specialized Batteries Deliver High Rate Current
Applications that require continuous high rate power, such as surgical drills and power tools, can benefit from an innovative lithium metal oxide battery that delivers high voltage, instant activation, and exceptionally long shelf life even in extreme temperatures.
Constructed with a carbon-based anode, a multi metal oxide cathode, and an organic electrolyte, these batteries can deliver up to a 20-year operating life with an annual self-discharge rate of less than 1 percent per year. These small but powerful cells feature a nominal voltage of 4V and up to 2 Wh of energy, with a discharge capacity of 135 mAh to 500 mAh, capable of handling 5A continuous loads and 15A maximum pulses. They also offer a wide temperature range (-40°C to 85°C), and a hermetic seal.
Medical Grade Rechargeable Lithium-Ion Chemistries
Consumer grade Lithium-ion (Li-ion) rechargeable batteries are not manufactured to medical grade standards, as their high self-discharge rate may result in low battery availability if the cell is not properly recharged before the procedure. Consumer rechargeable batteries also have a relatively low power-to-size ratio, thus requiring the use of very large battery packs to deliver the high pulses required to drive surgical power tools. Consumer grade Li-ion batteries also have crimped seals that may leak, and cannot handle the high temperature and humidity associated with autoclave sterilization.
A medical grade rechargeable Li-ion battery was recently developed that can operate for up to 20 years and 5,000 recharge cycles. This battery can draw up to 15A of continuous current from a small AA size cell, and has an extremely low selfdischarge rate, thus allowing the instrument to sit on the shelf for extended periods and still be ready for instantaneous use when called upon. These ruggedized batteries also feature a hermetic seal and an extended temperature range.