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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In The Water
Most ruggedized equipment is specified to withstand 30 minutes of immersion in 3 m of water. To ensure a watertight seal between the two halves of the plastic pack enclosure, ultrasonic welding is recommended to join plastic case surfaces. Unlike alternative methods of sealing enclosures, such as snap-tight seals, watertight seals are possible with ultrasonic welding.
Ultrasonic welding ensures the enclosure is resistant to shock or impact, as the resultant joint strength can match the strength of the welded material. If the battery enclosure design does not accommodate ultrasonic welding due to wall thickness or the inability to create an acceptable weld joint, then adhesives can be used to seal the pack.
Humidity often penetrates the seals of the battery enclosure around the contacts. Even if the contacts are insert-molded in the battery enclosure, vapor can still enter the enclosure around the contacts. One common technique for sealing contacts is to place potting compound, such as polyurethane or silicone, on the interior of the enclosure behind the contacts. This barrier will prevent any penetration of fluid into the enclosure interior.
Intrinsically safe batteries have long been the domain of nickel cells. Portable radios need them for explosive gas environments such as oil refineries, mines, grain elevators, and fuel handling at airports. Surgical equipment needs them for sterilization techniques using ethylene oxide (EtO).
These batteries prevent excessive heat buildup, eliminate the risk of an electric spark on equipment failure, and have limits on the current discharged by the battery pack. Many of the radio and surgical equipment manufacturers have started migrating to lithium polymer, prismatic, and cylindrical cells.
The challenge with lithium chemistry, relative to nickel chemistry, is that the energy/density is much higher, and thus more volatile. Additionally, the lithium-polymer cell is more fragile than the metal-encased cylindrical and prismatic cells, so polymer cells require more supportive pack enclosures. Again, packaging is critical to develop Li-ion packs for an intrinsically safe environment.
ROBIN SARAH TICHY is a technical marketing manager with Micro Power Electronics Inc. She has a doctorate of philosphy from the University of Texas for her work in solid-oxide fuel cells.