Power Electronics

Designing Battery Packs for Thermal Extremes

While battery packs enable individual cell capacities to be scaled in series/parallel configurations to suit portable applications, they must also provide thermal management in extreme environments.

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).

Temperature Effects

Much like humans, temperature affects a battery's performance during both rest and work. How Li-ion cells react and perform depend on the temperature under which they are operating. Many of the principles that hold true for lithium also apply to other rechargeable chemistries such as nickel cadmium, NiMH and alkaline.

The performance of rechargeable Li-ion chemistry starts to suffer as the temperature drops below 0°C, causing the internal impedance of the battery to increase. The result of this effect is shown in Fig. 2, where a 2-A load causes the voltage to droop. This voltage droop is more pronounced at -20°C, and the electrolytic material within the cell will freeze with further temperature declines. Cell capacity is also reduced during the lower temperatures. If these cells are used or stored at or below -50°C, irreparable damage may occur under certain conditions to internal separators within the cells, making the cells a safety hazard.

If extremely low temperature requirements eliminate Li-ion as a viable chemistry, one should consider using lithium-primary cells, because they operate down to -40°C. Cells based on Li/MnO2 chemistry use a solid cathode, while 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. Voltage delay is related to the growth of a cell's passivation layer and is exacerbated when the cell is stored at a high temperature and then a load is placed on it at a low temperature. As shown in Fig. 3, this condition results in the possibility that a liquid-cathode battery may not perform at low temperatures when required. The Li/MnO2 system is less susceptible to the voltage-delay phenomenon at lower temperatures.

Storage temperature affects the subsequent performance of Li-ion cells. As Fig. 4 shows, under optimal storage conditions of 20°C, a fully charged Li-ion cell has a natural self-discharge of 1% per month. However, with an elevated storage temperature of 60°C or a 12-month period, the capacity naturally discharges down to 40% of the original capacity. This drastic self-discharge substantially limits the run time of the cells after storage.

Additionally, after storage this fully charged cell would only have a recoverable capacity of 70% of the original capacity. Although not shown in Fig. 4, a cell stored at 60°C for 12 months at 50% state-of-charge would have a recoverable capacity of 90%. This demonstrates the wisdom of storing cells at a 40% to 50% state-of charge during transport and prior to regular use.

Extremely high temperature operation provides equal challenges for cells based on lithium chemistry. As mentioned previously, the upper range of safe operation for Li-ion and lithium-primary cells is 60°C. Cells provide energy through the electrochemical shuttling of Li-ions between the anode and cathode materials. However, at high discharge rates this chemical reaction generates heat, and the effects of this heat must be factored into a sound battery-pack design. The effect of the generated heat is compounded in a multicell pack.

To demonstrate the typical temperature rise in a multicell pack, Micro Power has assembled a pack and monitored the temperature rise using environmental and electrical test chambers. This testing should be performed on packs that must operate near the upper or lower boundaries of the cell specifications. The pack assembled for the test was a 4S6P (four cells in series, six strings in parallel) Li-ion pack using 18650 (18-mm diameter, 65-mm length) 2.4-Ah cells, resulting in a pack that provides 14.4 V and 14.4 Ah of capacity.

Testing was conducted at a condition of 145-W discharge at 45°C ambient temperature. Thermistors were placed within the pack core and around the outside edge of the cells. The pack was wrapped in packing material to simulate a plastic enclosure, and an additional thermistor was placed on the outside of the packing material to capture the temperature outside the simulated enclosure. Temperature and performance data were recorded on automated Maccor battery-testing equipment.

Results of the testing are presented in Fig. 5. The TCORE thermistor placed within the cells in the center of the pack registered a core temperature of 65°C. The two thermistors (T1 and T2), placed at the edge of the assembled cells, registered a perimeter temperature of 64°C and 65°C. Note that the temperature at the core and edge of the pack is similar. The cylindrical 18650 cells used in this test are efficient at distributing heat generated from the center of the cell to the cell exterior.

Additionally, these cells are resistant to absorbing heat from external sources. Hence, cells located in the center of the pack assembly and surrounded by other cells had a similar temperature to cells located on the edge of the pack, which have fewer neighboring cells. Fig. 6 shows a thermal image of the pack during the test, and one can see that the temperature of the interior and edge cells are similar. Finally, the TCASE thermistor outside the packing material registered a temperature of 54°C.

This test demonstrates that one can expect up to a 20°C rise in temperature when the pack is in operation, which may result in a 9°C temperature rise of the pack enclosure. Note that the temperature changes are dependent on the amount of current drawn from the pack (i.e. greater current results in greater heat generation). These temperature increases, both within and outside the pack, must be factored into the design of the battery pack and portable device.

This level of testing and analysis should be performed on all pack designs that must operate in low- or high-temperature conditions. Intimate knowledge of the usage profile allows the pack to be designed in a manner that avoids thermal stress to the pack.

Temperature extremes, especially high temperature, may pose design challenges for battery packs. Mil-spec requirements may specify extended operating temperatures up to 80°C. However, most commercial Li-ion cells are specified to operate from -20°C to +60°C; therefore, thermal monitoring and heat dissipation within the battery pack is critical for high-temperature operation. If a pack will be used in high-temperature environments, then specific design principles should be applied to that pack.

First, when charge is introduced (via charging current) or removed (during discharge) from a battery at high rates, 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, the placement of circuitry within the pack is critical. The circuit board may have heat-generating components, such as a FET, and improper placement could 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 may be designed with vent holes to dissipate generated heat or exhaust vented gases from cells. Multiple vent holes may be used to increase airflow in and out of the pack.

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, one should consider using lithium-primary cells to power the device because lithium-primary cells have an operational range of -40°C to +80°C with very low self-discharge.

Environmental requirements may specify extended operating temperatures down to -40°C. Rechargeable Li-ions operate at -20° to +60°C. When challenged with this requirement, there are several design options to maximize electrical output at low temperatures.

A heater embedded with the pack can warm cells prior to use. The embedded heater can be powered from the main cells within the pack or by an external source such as a charger or another battery pack. Embedded heaters can heat cells, reduce electrolyte viscosity and lower voltage droop or delay prior to use.

The host device may be designed to pulse discharge cells prior to primary discharge; this self-warms the cells via the I2R heating effect. This technique is applicable when the duty cycle is predictable and cyclical (i.e., periodic transmission of telemetry report), rather than a random or haphazard duty cycle (i.e., handheld radio transmission).

Super capacitors embedded within a pack may provide immediate energy to the host device while cells warm up to their optimal electrical performance.

Typically, vent holes are unobstructed openings that expel potential gas vented from cells (when the cells are stressed or misused). High-pressure vent holes may relieve and exhaust warm air only after a specific pressure has been reached within the pack. As long as the pressure is not increased to a dangerous level with the pack, high-pressure vent holes can retain heat generated within the pack.

Lastly, if these design techniques cannot extend operations of a rechargeable Li-ion pack down to the target (low) temperature, one should again consider using lithium-primary cells to power the device. When assessing lithium-primary formulations, Li/MnO2 provides less voltage droop than Li/SO2 and Li/SOCl2 in cold temperatures.

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