High-capacity batteries provide greater levels of energy density at a smaller size. This design characteristic allows devices to obtain the power they need for long periods without giving up the necessary space to larger battery packs. Many high-capacity battery packs can allow for faster recharging cycles while providing increased performance.
With faster charging cycles and higher levels of energy density in a smaller volume unit, it's vital to ensure that these battery systems are safe to use. Special design considerations for high-capacity batteries focus on implementing safety technologies within the pack as well as rigorous testing protocols.
High-Capacity Battery Chemistry Safety Standards
When it comes to selecting cell chemistry for high-capacity batteries, manufacturers turn to lithium-based batteries in their design. Lithium-based batteries can provide from 150 wh/kg to 700 wh/kg of specific energy density depending on the materials, such as lithium-ion, lithium-cobalt, lithium manganese, and lithium phosphate.
Due to lithium metals being unstable, these batteries in particular require mandatory protection circuits installed to provide monitoring and safety control features. They also must undergo testing and certification as national and international organizations have established a range of safety measures for both standard batteries and high-capacity batteries, such as:
- Safety certifications: UL 2595 has general requirements for appliances that are battery powered; and UL 1642 standards for lithium batteries.
- International safety standards IEC 62133 requirements for secondary sealed cells for portable applications; and IEC 61508 standards for functional safety.
- UN classifications regarding the transportation of dangerous goods such as UN 3480 for lithium-ion batteries; and UN 3481 standard for lithium-ion batteries that are packed with equipment.
Failures with High-Capacity Batteries
Failures experienced with high-capacity batteries deal with voltage, current, temperature, short circuits, and degradation. Addressing these main issues can ensure that batteries pass testing and will remain safe for the life of the product.
High-capacity battery pack during testing.
Voltage failures focus on over-voltage and under-voltage. Overvoltage of lithium-based batteries leads to lithium plating, overheating, and the creation of dendrites that can cause a short circuit. The under-voltage of cells may cause material breakdowns of both the electrode and anode which will cause capacity loss.
Battery current issues may involve the device exceeding load limits and creating a short circuit. Other issues could involve the use of an inappropriate charger that could create an overcurrent, or having the battery pack terminals exposed that may create a short-circuit discharge.
A factor that should never be overlooked is extreme temperatures. Due to high-capacity batteries carrying a larger density packed inside a smaller housing, this may create excessively high temperatures when placed into use. Without proper cooling or venting capabilities, this failure may result in thermal runaway. Temperatures may spike during overcharging and undercharging.
Fail-Safe Mechanisms
Fail-safe mechanisms focus on monitoring the current, voltage, and temperature of the high-capacity batteries, comparing the measurements to safety limits, and instituting controls to bring the battery pack back within safe operating levels or cutting power to prevent damage, lost capacity, or devastating losses to equipment.
When controlling temperatures, NTC thermistors are placed into high-capacity battery systems. If temperatures that pass over the NTC thermistor are too hot or too cold due to over-charging/discharging or under-charging/discharging, the protection system can constantly monitor the temperatures and take automatic action.
Sense resistors are also installed to provide current protection. Resistors measure the current voltage and compare it to established thresholds to detect overcharging as well as short-circuit discharges. FETs are also designed into high-capacity battery systems to monitor the power that travels between the load and the power source. FET can provide cell balancing and current management as well as provide communication with monitoring devices whenever safety protection controls are put into use.
Testing for High-Capacity Batteries
Battery cell manufacturers will perform a variety of tests such as capacity testing, stress testing, and cycle life testing. Capacity testing checks to see how long the battery takes to discharge at a specified constant current during a determined time. Stress testing evaluates how the high-capacity battery handles extreme temperatures, peak power demands, and other stress factors. Lifecycle testing simulates real-world applications and the constant charging and discharging cycles to see how the cell capacity is impacted over a length of time.
In addition to these tests, the batteries will also undergo temperature monitoring, internal resistance monitoring, and voltage monitoring to determine performance parameters, check cells for potential issues with capacity, and evaluate materials for degradation.
Summary
National and international organizations, as well as specific industries, may require specific certification for high-capacity batteries to meet safety standards. These certifications will cover both the operation of the battery system inside the device during operation, as well as when the batteries are transported already installed in the device when the batteries are packaged with the device, or when the cells are shipped by themselves. Understanding the failures that may occur with high-capacity cells can help determine what safety controls and protections are needed for the battery pack.
Key Takeaways
- Safety Regulations and Certifications: High-capacity battery systems must comply with strict safety certifications such as UL 2595, IEC 62133, and UN 3480 to ensure they meet national and international safety standards for performance, transport, and reliability.
- Common Failure Modes: Voltage fluctuations, excessive current draw, and thermal issues are the primary causes of battery failures. Overvoltage can lead to lithium plating and dendrite formation, while under-voltage can degrade electrode materials and reduce capacity over time.
- Fail-Safe Mechanisms for Protection: Built-in safety features such as NTC thermistors, sense resistors, and FETs continuously monitor voltage, current, and temperature to detect unsafe conditions and take automatic corrective actions, such as shutting down the battery to prevent catastrophic failure.
- Comprehensive Testing Methods: High-capacity batteries undergo rigorous testing, including capacity, stress, and cycle life evaluations, to simulate real-world conditions and assess performance under varying temperature extremes, peak power demands, and long-term usage.
- Balancing Energy Density and Thermal Management: As energy density increases, so does the risk of overheating. Effective thermal management strategies, such as heat dissipation designs and advanced cooling mechanisms, are crucial to preventing thermal runaway and ensuring long-term battery safety.
