Designing Battery Packs for Harsh Environment Mission-Critical Devices
Many portable military electronic devices are used in extreme environments and are designed to comply with the MIL-STD-810 environmental test specification. The device and battery pack need to survive high or cold temperatures, shock or vibration, or even submersion underwater. Typical devices include handheld radios, laptops and personal protection equipment.
Battery packs in these devices serve a critical role, as in most cases, they are the sole source of power. The energy density in these packs is typically as high as possible, given the space available, to maximize device run times. This intensifies the need to control and protect the stored energy where a failure could result in catastrophic events such as thermal runaway. This article presents design considerations and techniques for designing and manufacturing a battery pack that operates in extreme temperatures, absorbs shock and vibration, and maintains a watertight seal.
Operating temperature is a critical parameter for battery packs as it impacts internal cell chemistry, electrical components and the circuit board itself. Figure 1 shows the discharge characteristics chart for a Panasonic UF553450Z Li-ion cell, which illustrates variation in discharge capacity for the cell over different operating temperatures.
It can be seen that the discharge capacity reduces as the temperature reduces. At an operating temperature of -20°C, the capacity reduces sharply, so that a device with a battery pack that had 100% state of charge (SOC) at room temperature, may stop working after just minutes of operation. Conversely, the capacity increases with higher temperatures, but there is the risk of heating and cell venting.
Various techniques are available to limit the impact of temperature on battery pack performance while increasing safety. Multiple temperature sensors can be positioned at various locations in battery packs to monitor the temperature of cells. Data from these sensors can be used by the Battery Management System (BMS) to provide multiple levels of protection during charging and discharging. The BMS ensures protection by controlling the charge (CHG) and discharge (DSG) FETs as required. The thresholds for protection need to be carefully set in the BMS by considering the limitations of the cell and the operating conditions for the battery pack based on application.
There needs to be similar protections for cells at cold temperature. The primary risk occurs when charging cells at cold temperatures. The datasheet for cells specifies the acceptable temperature range to charge, which can be programmed in the configuration file of the BMS. The BMS can stop charging when temperature falls below the minimum allowed range. It can also activate a heater to heat the cells to the acceptable range.
Cell layout plays a critical role in ensuring battery packs are safe for harsh environmental conditions. Isolating individual cells with fuses at both positive and negative terminals ensures that each cell is protected as soon as they are placed into the pack. In the event of a cell venting, it vents out from positive terminal. Thus, having a cell layout with positive terminals of cells not facing each other minimizes the chance of having a venting cell affect other cells and causing a catastrophic chain reaction. This makes PCB layout challenging but reduces the possibility of an unsafe event.
It is also desirable to have the positive terminal of the highest potential cell in the cell stack be on the edge of the pack and facing out. Figure 2 shows an example of the cell layout of a 4S4P pack. It can be seen that the positive terminals of the cells do not face each other, and the positive terminal of the 4th cell faces out from the pack.
Qualification plays an important role in verifying that battery packs can survive extreme operating conditions. Military standards are often referenced to generate test procedures and ensure testing is conducted per the standards. Testing can be run by external test agencies with the experience and the equipment required to perform the tests. Figure 3 shows the temperature profile of a temperature chamber in which one of the battery packs for military applications was tested. The pack remained operational over the entire duration.
Protections play a critical role in battery packs so that they can handle extreme events in a safe manner. They protect the pack from over- and undervoltage, over-current and over-temperature, during both charging and discharging of the pack.
The pack requires redundant protections at multiple levels so that in case one of them fails, the pack is still protected. Depending on the component that provides protection, they can be temporary or permanent. For example, a protection provided using FETs is temporary in that it can be removed once the conditions are safe, but one that is provided using a chemical fuse is permanent. Following is a list of components that can be used for protections:
Ideal diode at charge and discharge pins
BMS along with CHG and DSG FETs
Secondary protection chip along with chemical fuse
Individual cell fuses
Shock and Vibration
Battery packs used in harsh environments are exposed to various levels of shock and vibration. As an example, a common occurrence is one where the operator drops the pack while handling. It is important to make sure every piece of equipment inside the battery pack is constrained from dislocating and creating shorts.
Padding the inside of the battery pack with materials such as foam is one way to prevent components from moving. For any connection joint, one should design in redundant points of constraint so that if one fails, the joints are still held on by backup connection joints. Additionally, for circuit board components, applying epoxy or potting material helps in preventing them from coming loose and creating shorts.
Utilizing components that have been tested and certified is a simple step to prevent water ingress. The components include connectors, user interface buttons, displays, etc. Reducing the number of parts that make up the battery pack housing reduces the points of failure and the regions to seal. Potting and applying epoxy on connectors are methods to make non-certified components watertight.
Extensive testing is required after assembly to verify that the pack remains water resistant. For certain standards, environmental conditions may need to be simulated as they are difficult to test for otherwise. Even with all the steps taken to prevent water ingression, it is advisable to use alternate methods of internal sealing in the event of a water ingression.
Steps taken to prevent water ingression also help in preventing dust from getting inside battery packs. One of the serious risks with dust ingression is settling and the creation of shorts between electrical components, which pose a risk for device failure.
In addition to the above considerations, designers have the freedom to not utilize 100% of the capacity of cells. The cells can be limited to reduced capacity, such as 80%, so that the energy density of the pack is less, which reduces the severity of an unwanted safety event, such as thermal venting. Filling battery packs with flame retardant material also helps in preventing the heat and flame generated in a cell from spreading to other cells. Finally, a thorough Design Failure Mode and Effect Analysis (DFMEA) considering environmental conditions as causes of failure must always be conducted, as this can bring out more failure modes that could occur with the battery pack.
I would like to acknowledge Kevin Zwart, Senior Mechanical Engineer at Inventus Power, for his contributions towards design considerations for underwater immersion and vibration.
Panasonic Corporation (2018). UF553450Z Datasheet.
This article was written by Anvin Joe Manadan, Senior Electrical Engineer, Inventus Power (Woodridge, IL). For more information, visit here .