Modern product applications using rechargeable batteries typically have built-in sensors and a battery management system (BMS) circuitry. The BMS monitors the voltage, current, and temperature of the rechargeable battery system, whether it's a single cell, a module (a group of cells), or a battery pack (a group of modules). Monitoring battery voltage and current is often insufficient to determine the battery's health condition.
Monitoring battery temperature can warn you of potential defects and quickly isolate the location of a fault. The BMS monitors the battery pack to keep the operating temperature within the optimal range. Batteries that are too hot will degrade or malfunction. Batteries that are too cold will experience sluggish performance due to slower internal electrochemical reactions, thus reducing their overall performance.
This article focuses on common temperature-related battery issues and shows you how testing instruments can help build better battery-powered applications.
Common problems in battery temperature monitoring
When monitoring battery temperature, attention should be paid to issues such as thermal imbalance, hot spots in the battery pack, and low performance and capacity.
Thermal imbalance generated during use
Large-scale applications typically use battery packs connected in series and parallel. Strategically placed thermal sensors throughout the battery pack detect temperature changes. Thermal imbalance in large battery packs usually begins with non-uniformity in the battery cells affecting their charging and discharging voltages. Over time, this non-uniformity accelerates, causing some battery cells to overcharge or over-discharge, leading to battery overheating.
Battery balancing uses a battery management system (BMS) to balance the voltage and state of charge (SOC) between batteries when fully charged, minimizing thermal imbalances. Battery manufacturers can also select battery packs with very similar open-circuit voltages to build battery packs, further minimizing SOC variations.
Product application design can also cause thermal imbalance, for example, the battery pack's cooling system may not be effective enough for certain external environments.
Battery pack hotspot
Monitoring battery temperature helps detect hot spots. Depending on the importance of the battery application, sometimes strategically placing a few sensors within the battery pack is sufficient. However, in applications requiring critical performance, a temperature sensor is placed on each battery module.
Hot spots often occur in the weaker battery cells within a battery pack. These weaker cells are more susceptible to excessive stress and gradually degrade. Therefore, they tend to get hotter during operation than normal, high-performing battery cells because they struggle to keep up with the performance of the better cells.
Hot spots can also warn you of potential damage to battery cells or modules. Physical impacts to the battery pack can puncture or deform the internal structure of battery cells, such as electrodes or polymer separators. If this occurs and no intervention is taken, the damage to the battery cell can worsen and lead to thermal runaway. This can result in fire and explosion. Therefore, detecting hot spots, locating faulty cells, and replacing them quickly is crucial.
Other causes of hot spots include poor terminal connections, defects in heat dissipation components, and short circuits in external cables.
Low battery performance and low capacity
Monitoring battery temperature can also be an active closed-loop process to keep the battery pack operating within its optimal charging and discharging temperature range.
Cold weather can cause battery performance to decline because the electrochemical reaction rate slows down. As a result, the battery's capacity will decrease significantly, and it may even stop working altogether.
A bigger problem is that the battery system is operating at temperatures higher than the manufacturer's specifications. Battery life will be shortened, and weaker batteries may deviate more from the performance of better batteries. As a result, thermal imbalances and hot spots begin to appear.
An essential independent testing device for monitoring battery temperature.
Many commercially available battery management systems are suitable for a wide range of applications, from IoT devices to high-voltage automotive applications. Basic functions include overcurrent protection, overvoltage protection, overcharge protection, overheat protection, undervoltage protection, battery balancing, SOC (State of Charge), and state of health.
However, in your application, there are many valid reasons to purchase separate testing equipment to monitor battery temperature.
Independent testing and verification system
Having a dedicated test and verification system (such as a Modular Data Acquisition (DAQ) system) helps verify that your BMS is functioning correctly. It also helps verify the overall integration of the application. A standalone DAQ system can perform the following operations:
• Use various types of temperature sensors, such as thermocouples, thermistors, and resistance temperature detectors (RTDs), for more accurate measurements. Using thermistors or RTDs, you can achieve temperature accuracy of ≤1 °C.
• Measurement temperature range is -150 °C to 1,820 °C
• Measurement applications implement more points than BMS. You can verify whether BMS has missed any critical locations.
• Perform measurements at shorter intervals without consuming hardware resources of the BMS and applications. This can help you find the optimal interval settings for your BMS monitoring system.
External redundancy of mission-critical applications
Another key reason for having an independent testing system is to provide redundancy for mission-critical applications. Medical devices that monitor and control the function of vital organs cannot withstand unexpected power outages during operation. Another example is large-scale energy storage systems that power critical building functions such as IT, telecommunications, and medical equipment.
A standalone DAQ system can perform the following operations:
It can provide independent alarms and emergency secondary shutdown to prevent battery system meltdown or fire.
• It can provide a backup monitoring and control system if the main system fails or loses communication.
Its versatility and flexibility make it suitable for expansion in large projects.
DAQ systems are the best choice for standalone test equipment for monitoring temperature due to their versatility. Many modern DAQ systems incorporate a high-resolution, 6.5-digit multimeter. They also come equipped with a variety of solid-state, armature, and reed switch multiplexer modules, allowing monitoring of temperature points over 100 channels. Furthermore, because the DAQ has a built-in digital multimeter, it can measure other signals besides temperature, such as AC/DC voltage and current, resistance, and capacitance.
The DAQ system is modular, allowing for the expansion of temperature monitoring channels. The DAQ system allows you to add modules to scale as your project grows. Therefore, you don't need to invest in a new system, saving valuable development time.
Testing equipment helps build better battery-powered applications.
Once you understand the cause of the battery failure, you can use battery simulation software to predict the decline in battery capacity.
Battery failure mechanisms and problems
You can analyze the root cause of a battery failure by physically dissecting it. However, electrical measurements can provide some clues that help predict failures before they occur.
One cause of the failure is lithium deposition or dendrite growth on the anode electrode. This growth typically occurs after the battery has been overcharged multiple times, leading to lithium deposition on the anode. Over time, this can cause an electrical short circuit between the two battery electrodes. This electrical short circuit is difficult to monitor because it occurs very quickly—the voltage drop takes only a few milliseconds.
Another reason is electrode degradation, which results in oxide buildup or microcracks due to fatigue from charge and discharge cycles and repeated chemical reactions of the electrolyte.
Another source of failure is an electrical short circuit caused by a faulty internal separator. Separator failure can result from physical impact or puncture to the battery, or exposure to extremely high temperatures. Material defects during the manufacturing process can also lead to failure.
Battery aging and capacity degradation are not serious faults requiring immediate intervention. However, these factors are concerning for users of battery applications. Open-circuit voltage measurements are not a good indicator of battery capacity. The internal resistance of an aging battery increases over time, but you cannot immediately conclude capacity degradation from snapshot resistance measurements. Temperature, state of charge (SOC), and discharge rate all affect battery internal resistance.
Battery failures are highly complex because batteries undergo electrochemical reactions and are affected by physical variables such as temperature and mechanical stress. The charging method is another factor. Therefore, no single battery testing instrument can provide a definitive diagnostic solution for battery failures.
However, depending on your application, power requirements, capacity, and production cycle (R&D, compliance testing, or production), a test equipment solution can meet your needs.
Let's explore testing equipment tools to help you better verify battery life and its impact on temperature.
Battery simulation is used to verify battery performance, including the effects of temperature.
You can use battery simulation software to better understand and predict how battery capacity decreases over time. Furthermore, battery simulation software can predict the impact of temperature on battery life.
Before simulating a battery, you must first analyze it. You need to understand the energy a battery can store and provide during a period of discharge. The open-circuit voltage and internal resistance change as the battery discharges.
Therefore, creating these charts is crucial so that battery profiles accurately reflect the battery's actual performance. Engineers can obtain battery profiles using battery modeling software or by acquiring profiles from battery suppliers. Profiles created by modeling software reflect the current consumption of a specific device and are more accurate than generic profiles from battery suppliers. Battery profiles form the basis for software simulations of the battery. Considering the impact of temperature on battery life is essential.
After developing a battery profile, you can cycle the battery using battery simulation software to determine capacity loss and shortened battery life. Battery performance degrades significantly during charging and discharging. This is why simulating battery cycling is crucial. Battery testing and simulation software provides a simple solution for this. The software must support arbitrary waveform generation and data logging. Furthermore, the ability to create different charging and discharging waveforms for the battery is also valuable.
Engineers can combine multiple different charge and discharge sequences to simulate complex cycling profiles. They can then determine how battery performance degrades over time. For example, simulation software solutions enable engineers to perform up to 1,000 cycles to determine the aging effects and reliability of batteries under sequential test conditions.
Keysight BV9210B / 11B PathWave BenchVue advanced battery testing and simulation software, together with the N6705C DC power analyzer and the N6781A or N6785A SMU module, can perform battery analysis, battery simulation, current consumption analysis, and battery cycle testing.
Summary
Having a dedicated testing system to monitor battery health and temperature is essential. It can help you detect potential problems such as thermal imbalance, hot spots, and changes in ambient temperature, which can affect the overall performance of the battery system, even if you already have a BMS.
This standalone battery testing system can be used as a test verification system and an external redundant safety system. It is scalable to meet all your battery testing system needs. Furthermore, this standalone system helps troubleshoot battery faults and problems. With some additional settings and battery software applications, you can use it as a battery simulator to help build better battery-powered applications.
