Designing Battery Packs for Thermal Extremes

Jul 1, 2006 12:00 PM
By Jeff VanZwol, Marketing Manager, Micro Power Electronics, Beaverton, Ore.

Battery packs for ruggedized portable devices must operate in both extreme hot and cold environments. Many of these devices, such as handheld radios, telemetry monitors, weather stations, test equipment, missiles, rockets and satellites, are used in harsh environments. There are unique design considerations and techniques that must be considered when designing and manufacturing a pack that will be operated in extreme environments from -40°C to +80°C.

The main components of a typical battery pack are shown in Fig. 1. The cells serve as the primary energy source. The printed circuit board provides the intelligence of the system for advanced functions such as fuel-gauge calculations on remaining cell capacity, protection circuitry, thermal sensors used to monitor internal pack temperature, LEDs that indicate pack or cell status, and a serial data communications bus that communicates with the host device. A custom plastic enclosure is typically produced in an injection mold. External contacts provide a physical electrical interface, and insulation is used to absorb external shock, as well as retain or dissipate heat generated within the pack.

All of these elements can be customized when designing a battery pack for high- or low-temperature operation. However, the cells are the elements critically affected by extreme temperatures.

Optimizing Battery Chemistry

Advances in battery technology have led to increased energy densities over the last few decades. More reactive materials have been employed to achieve these advances, and active safety circuits are now required to ensure that certain battery chemistries are kept in a stable condition. With careful design, incidents involving battery rupture or explosion are rare. Nevertheless, it should be recognized that under certain conditions, such as high temperature or punctured cells, the pack integrity can be breached and, subsequently, expose the user to harmful chemicals or even flames. Therefore, each rechargeable chemistry has its own set of risks in addition to its desirable attributes.

For example, sealed lead acid (SLA) cells use concentrated sulfuric acid electrolyte and toxic heavy metal electrodes, and provide a nominal voltage of 1.5 V. SLA cells are cost-effective, but are too bulky and heavy for most portable applications. SLA cells have a wide operating temperature, ranging from -40°C to +70°C.

Nickel metal hydride (NiMH) cells include a nominal voltage of 1.25 V, 500 duty cycles per lifetime, an optimal load current of less than 0.5 C, an average energy density of 100 Wh/kg, a charge time of less than 4 hours, a typical discharge rate of approximately 30% per month when in storage and a rigid form factor. NiMH cells operate effectively between -20°C and +60°C.

Lithium-ion (Li-ion) cell characteristics include a nominal voltage of 3.6 V, 1000 duty cycles per lifetime, a rate load current of less than 4 C, an average energy density of 160 Wh/kg, a charge time of less than 4 hours and a typical discharge rate of approximately 1% to 3% per month when in storage. Li-ion cells operate effectively between -20°C and +60°C. However, new chemical formulations are extending that range to -30°C and +80°C.

Among these chemistries, 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 that prevents overvoltage during charge and undervoltage during discharge, and a thermal sensor that 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. Many of the portable devices using the older chemistries have migrated to Li-ion in recent years.

If rechargeable chemistries cannot perform in the required temperature ranges, disposable, one-time-use lithium cells — known as lithium-primary cells — should be considered. These lithium-primary cells feature a nominal voltage of 3.6 V, an optimal load current of less than 5 C, an average energy density of 160 Wh/kg and a negligible self-discharge rate supporting years of storage. The operating temperature for lithium-primary cells ranges from -40°C to +80°C. Examples of lithium-primary chemistries include lithium thionyl chloride (Li/SOCl₂), lithium sulfur dioxide (Li/SO₂) and lithium manganese dioxide (Li/MnO₂).

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