Portable power for land warrior equipment

Soldiers are becoming more reliant on mobile power sources with the recent modernization of military armament. The Land Warrior defense initiative provides an electronic system that improves situational awareness and communications for soldiers by equipping them with personal computing capabilities and connecting them to centralized command via a wireless network.



The complete range of portable devices specified by the Land Warrior program requiring portable power includes a wireless local area network (WLAN) transceiver, a head-mounted display (HMD), a multi-band inter-team radio (MBITR), a global positioning system (GPS) receiver, a daylight video scope (DVS) and a wearable computer. Supplemental equipment under assessment include chemical detection devices and portable air purifying respirators. In addition to the requirements of commercial battery packs, there are a number of incremental characteristics or design considerations affiliated with ruggedized battery packs.









SELECT THE OPTIMAL CELL CHEMISTRY

Demands on battery technology have required the use of more reactive materials, and therefore, active safety circuits are required to ensure that certain battery chemistries are kept in a stable condition. With careful design, incidents involving battery rupture or explosion are very rare. However, under certain conditions (such as extreme temperatures or puncture), which are more likely in battlefield or emergency services, the battery pack integrity can be breached and subsequently expose the user to harmful chemicals or even flames.



Portable rechargeable cell chemistries include alkaline, nickel cadmium (NiCd), nickel metal hydride (NiMH) and lithium ion (Li-ion). Li-ion provides the highest energy/density for portable or mobile applications. Lithium primary cells are disposable and designed for single use applications. Lithium primary requires minimal protection circuitry because it is used only once and never recharged.







Rechargeable Li-ion requires the greatest degree of protection including a thermal shutdown separator and exhaust vents (within each cell) to vent internal pressure; an external safety circuit, which prevents over-voltage during charge and under-voltage during discharge; and a thermal sensor, which prevents thermal runaway. However, with the appropriate level of safety designed into a Li-ion pack, Li-ion offers the most attractive method of portable battery power.





HIGH TEMPERATURE OPERATION

Temperature extremes, especially high temperature, can pose design challenges for battery packs. Milspec requirements may specify extended operating temperatures, up to 80°C. However, most commercial Li-ion cells are specified to operate from 20° to +60°C, so thermal monitoring and heat dissipation within the battery pack is critical for high temperature operation.



First, when current is introduced (i.e. charge) or removed (i.e. discharge) from a battery at high rate, there is an associated temperature increase, which can be dangerous. The pack circuitry should use a thermal sensor to disconnect the cells at a specified temperature. This eliminates thermal runaway and overheating.







Second, placement of circuitry within the pack is critical. The circuit board may have heat generating components, such as a field effect transistor (FET), and improper placement may result in the FET heating the cells. The application of heat to select cells within a pack erodes the longevity and safety of that pack. Third, packs can be designed with vent holes to dissipate generated heat or exhaust vented gases from cells.



A final consideration is the position of the pack in relation to any heat generating components, such as high performance processors, operating within the host device. Uneven heating may cause the cells to behave differently from their companions in the pack, thus shortening the pack life and compromising safety. If these design techniques cannot safely extend a rechargeable Li-ion pack up to the target (high) temperature, consider using Lithium primary cells (disposable) to power the device since Lithium primary cells’ operational range is 40° to +80°C with very low self-discharge.





COLD TEMPERATURE OPERATION

Military requirements may specify extended operating temperatures down to 40°C. Rechargeable lithium ion operates at 20° to +60°C. When challenged with this requirement, there are several design options to maximize electrical output at low temperatures.



First, a heater embedded with the pack can warms cells prior to use. The embedded heater can be powered from the main cells within the pack or an external source (charger, another battery pack). Embedded heaters can heat cells, reduce electrolyte viscosity, and reduce voltage droop or delay prior to use.



Second, the host device can be designed to pulse discharge cells prior to primary discharge, selfwarming the cells via the I2R heating effect. This technique is applicable when the duty cycle is predictable and cyclical (i.e. periodic transmission of GPS position report), rather than a random or haphazard duty cycle (i.e., handheld radio transmission). Third, super-capacitors embedded within the pack can provide immediate energy to the host device while cells warm up to their optimal electrical performance. Fourth, using high pressure vent holes will relieve and exhaust warm air only after a specific pressure has been reached within the pack.



Typically, vent holes are unobstructed openings that can expel potential gas vented from cells (when the cells are stressed or misused). High pressure vent holes retain heat generated within the pack. Finally, if these design techniques cannot extend operations of a rechargeable Li-ion pack down to the target (low) temperature, again consider using Lithium primary cells to power the device. When assessing lithium primary formulations, lithium manganese dioxide (Li/MnO2) provides less voltage drop than lithium sulfur dioxide (Li/SO2) and lithium thionyl chloride (Li/SOCl2) in cold temperatures.





WITHSTANDING SHOCK AND VIBRATION

Military requirements typically specify strict shock and vibration requirements. Many design techniques are available to improve the ruggedness of the battery pack. One of the most basic techniques is selecting mil-spec components for inclusion on the printed circuit board assembly (PCBA). In addition, strategic placement of PCBA with the battery pack improves ruggedness. Ensuring adequate clearance and airflow around the PCBA and using fixed mounts to support the PCBA with the pack reduces damage under stressful conditions.



In the manufacturing process, quality resistance welds among components (PCBA, cells, connectors) should meet IPC-610 soldering standards. In addition, multiple pull tests of at least 7lb. should be repeated throughout the manufacturing process. Finally, nickel strips of adequate gauge, minimally 5mm wide, should be used to solder onto battery terminals.



Another consideration is the composite material and wall thickness of the battery pack enclosure. Polycarbonate (PC), acrylonitrile butadiene styrene (ABS), PC/ABS blends, and fire resistant (FR) additives can provide a durable enclosure. Within the pack, insulation can absorb shock and vibration for the PCBA and cells. Typical insulating material includes vulcanized rubber, polyaramid paper, or polyimide film. Insulation should be puncture resistant, withstand temperature changes without shrinkage and deformation, and be applied with adequate thickness.





CONCLUSION

These techniques are among several methods used to design ruggedized battery packs for military applications. As additional electronics improve the information supplied to the Land Warriors in the field, battery packs need to be lightweight, rugged, and high capacity. Using Li-ion or Lithium primary chemistries within custom battery packs ensures the Land Warrior has realtime access to critical information.

About the author Jeffrey VanZwol is marketing manager at Micro Power Electronics. He is responsible for developing and supporting Micro Power’s military equipment market, as well as determining Micro Power’s strategic technology direction and investments.

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