OEMs are requesting rechargeable lithium-ion (Li-ion) battery packs for portable devices that operate in extreme hot and cold environments. Many portable radios are used in very cold (–40°C) environments, and many medical devices need battery packs that operate after exposure to 137°C steam sterilization, both exceeding the limits of conventional Li-ion technology. Presenting similar challenges, some medical devices and radios are required to operate in wet and even explosive environments.
In The Cold
Conventional Li-ion chemistry starts to suffer as the temperature drops below 0°C and the internal impedance of the battery increases. The result of this effect is “voltage droop,” which becomes more pronounced at –20°C or lower. Cell capacity is also reduced during these lower temperatures. If these cells are used or stored at –50°C, irreparable damage may occur under certain conditions to internal separators within the cells, making the cells a safety hazard.
Luckily, cell vendors have refined the material formulation to improve the lower-temperature performance. They balance the blend of their mixed metal-oxide formulations (predominantly nickel, aluminum, manganese, and cobalt) to deliver power below the conventional –20°C limit while maintaining competitive price points. Boston Power offers cells that specify and support moderate current delivery at –40°C. Saft offers a more specialized line of Li-ion cells that are optimized for low-temperature performance.
If the combination of higher current and lower temperatures eliminates Li-ion as a viable chemistry, one can consider utilizing lithium primary cells as they operate down to –40°C. Primary lithium cells can deliver more current at lower temperature. Cells based on lithium/manganese-dioxide (Li/MnO2) chemistry use a solid cathode, while the lithium/sulfur-dioxide (Li/SO2) cells use a liquid cathode. Liquid-cathode systems suffer from a “voltage delay” phenomenon, which causes the resulting voltage to be momentarily suppressed when a load is applied, particularly after extended periods of storage. Saft and Ultralife are major suppliers of primary lithium cells.
In The Heat
Many surgical instrument manufacturers wish to sterilize their tools and battery packs using steam sterilization. The sterilization process uses pressurized steam heated to 137°C. The exposure to this temperature can range for three to 30 minutes. The traditional chemistries for these surgical battery packs were nickel-based, such as nickel-cadmium (NiCad) or nickel metal hydride (NiMH).
The upper limit for battery storage without permanently damaging a lithium cell, though, can range from 70°C to 90°C. Some cells will experience thermal runaway with prolonged exposure to 137°C, while others will not. However, cell vendors have raised the limit of high-temperature tolerance—that is, brief exposure to temperatures greater than 100°C without a reduction in cycle life—by balancing the blend of their mixed metal-oxide and iron-phosphate formulations. These new cell varieties, combined with innovative packaging and insulation techniques by the pack manufacturers, are bringing Li-ion batteries to the forefront of this market.
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