Modern product applications running on rechargeable batteries typically have built-in sensors and battery management system (BMS) circuitries. A BMS monitors a rechargeable battery system’s voltage, current, and temperature, whether a single cell, a module (a group of cells), or a battery pack (a group of modules). Monitoring the voltage and current flowing from the batteries is usually not enough to determine battery health.
Monitoring battery temperature can warn you of potential defects and quickly isolate fault locations. A BMS monitors battery packs to keep operating temperatures within an optimal range. A battery that’s too hot will degrade or malfunction. One that’s too cold will perform sluggishly from slower internal electrochemical reactions, reducing its capabilities.
This paper highlights common temperature-related battery issues and shows you how test instruments help build better battery-operated applications.
Common Issues Found in Monitoring Battery Temperature
Thermal imbalance, battery-pack hotspots, and low performance and capacity are areas to look out for when monitoring battery temperature.
Thermal imbalance caused by use
Large-scale applications typically use battery packs with modules wired in series and parallel connections. Thermal sensors placed strategically across a battery pack detect temperature variations. Large battery-pack thermal imbalance usually starts with the non-uniformities of battery cells affecting their charging and discharging voltages. Over time, the non-uniformity variation accelerates, with some cells overcharging or overdischarging, causing the batteries to overheat disproportionately.
Cell balancing using a BMS to equalize voltages and state of charge (SOC) among the cells at a full charge can minimize thermal imbalance. Battery manufacturers can also select batches of battery cells with very close open-circuit voltage to build battery packs, minimizing the SOC variations.
Product application design can also cause thermal imbalance. For example, the cooling system of battery packs is not effective enough for certain external environments.
Monitoring battery temperatures helps you detect hotspots. Depending on how critical the battery application is, sometimes a few sensors strategically located across a battery pack are sufficient. However, in applications that require critical performance, a temperature sensor is placed on each battery-pack module.
Hotspots tend to happen on weak battery cells in a battery pack. Weak battery cells are susceptible to overstress and gradually degrade. Thus, they get hotter during operation than normal good cells because they struggle to keep up with the performance of good cells.
Hotspots can also warn you of potential damage to battery cells or modules. A physical impact on the battery pack can puncture or deform the battery cell’s internal structure, such as the electrodes or polymer separator. If that happens and no intervention occurs, the battery cell damage can degrade and potentially cause a thermal runaway. Fire and explosion may result. Hence, it is important to detect hotspots, locate the faulty cells, and quickly replace them.
Other causes of hotspots include poor terminal connections, heat dissipation component defects, and external cable shorts.
Low battery performance and usage capacity
Monitoring battery temperatures can also be a proactive closed-loop process to keep the battery packs operating in the optimum charging and discharging temperature ranges.
Frigid temperatures cause sluggish battery performance because of slower electrochemical reactions. Thus, battery usage capacity will drop significantly, and the battery may even stop operating.
The bigger concern is when the battery system operates at temperatures above the manufacturer’s specification. Battery life will degrade, and weaker batteries may deviate more from the good ones. Hence, thermal imbalance and hotspots start to show up.
Essential Independent Test Equipment to Monitor Battery Temperature
Many commercialized battery management systems are available for all kinds of applications, from Internet of Things devices to high-voltage automotive applications. Essential features include overcurrent protection, overvoltage protection, overcharge protection, overtemperature protection, undervoltage protection, cell balancing, SOC, and state of health.
However, there are many good reasons to acquire independent test equipment to monitor battery temperature in your applications.
Having an independent test validation system, such as a modular data acquisition (DAQ) system, helps validate that your BMS is performing properly. It also helps validate the overall integrated system of your application. An independent DAQ system can do the following:
External redundancy for mission-critical applications
Another key reason for having an independent test system is to provide redundancy for mission-critical applications. Medical devices that monitor and control vital organ functions cannot afford unscheduled power interruptions during operations. Another example is large energy storage systems that power essential building functions such as IT, telecommunications, and medical equipment.
An independent DAQ system can do the following:
Versatility and flexibility to expand in large-scale projects
A DAQ system is the best choice as independent test equipment to monitor temperature because it is highly versatile. Many modern DAQ systems have built-in high-resolution, 6.5-digit multimeter instruments. They also come with various solid-state, armature, and reed-switching multiplexer modules to monitor more than 100 channels of temperature points. In addition, since the DAQ has a built-in digital multimeter, it can measure other signals besides temperatures, such as AC / DC voltage and current, resistance, and capacitance.
DAQ systems are modular, as shown in Figure 1, allowing for the expansion of channels for temperature monitoring. The DAQ system allows you to add modules to scale up accordingly when your project grows. Hence, you do not have to invest in new systems, saving precious development time.
click for full size image Figure 1. Keysight 34980A data acquisition switch / measure unit (SMU). (Source: Keysight Technologies)
Test equipment to help build better battery-operated applications
Once you understand the sources of battery failures, you can use battery emulation software to predict drops in battery capacity.
Battery failure mechanisms and concerns
You can analyze the root cause of battery failures by physically cross-sectioning them. However, electrical measurements offer signs that can help predict failures before they happen (Figure 2).
click for full size image Figure 2. Internal battery failure mechanisms over time. (Source: Keysight Technologies)
One source of failure comes from lithium plating or dendrite growth on the anode electrode.[1] This growth typically occurs from overcharging batteries through many cycles, causing lithium deposits on the anode. Over time, this may cause an electrical short across the two battery electrodes. It is difficult to monitor this electrical short as it happens quickly — in milliseconds of a voltage drop.
Another source is degradation of the electrode showing oxide buildup or microcracks from charge and discharge cycle fatigue and repetitive chemical reactions of the electrolyte.[2]
Internal battery separator failure causing an electrical short[3] is another source of failure. A separator failure can come from a physical impact or puncture of a battery or exposure to very high temperatures. A material defect during manufacturing can also cause failure.
Aging and a drop-in battery capacity are not serious failures requiring immediate intervention. However, these factors are concerning to battery application users. Open-circuit voltage measurement itself is not a good indicator of battery capacity. The internal resistance of aging batteries increases over time, but you cannot take a snapshot resistance measurement and make an immediate capacity degradation conclusion. Temperature, SOC, and discharge rate affect internal battery resistance.
Battery failures are complex because of electrochemical reactions and batteries’ exposure to physical variables such as temperature and mechanical stress. The method of charging is another factor. Therefore, no single battery test instrument can provide a definitive diagnostic solution for battery failures.
However, test equipment solutions are available to meet your needs, depending on your application, power usage requirements, capacity, and production cycle (R&D, compliance testing, or production).
Let us explore test equipment tools to help you better substantiate battery life and the effects of temperature on it.
Battery emulation to validate battery performance, including effects of temperature
You can use battery emulation software to better understand and predict drops in battery capacity over time. In addition, battery emulation software can predict the impact of temperature on battery life.
Before you emulate a battery, you must first profile it. You need to understand the amount of energy the battery can store and supply as a battery discharges over time. The open-circuit voltage and internal resistance vary as the battery discharges.
Therefore, it is crucial to map these out so that battery profiles accurately reflect the real-world performance of the battery. Figure 3 is an example of a typical plot. An engineer can obtain a battery profile by using battery modeling software or receiving a profile from a battery supplier. A profile created by modeling software reflects the current consumption for a specific device and is more accurate than a battery supplier’s generic profile. Battery profiles are the basis for the software to emulate the battery.
click for full size image Figure 3. Battery profile created with Keysight BV9210B / 11B PathWave BenchVue advanced battery test and emulation software. (Source: Keysight Technologies)
It is critical to consider the effect of temperature on battery life. Figure 4 shows how temperature can affect the capacity curves of a battery. Profiles generated at different temperature values enable you to better predict the impact of temperature on battery life.
click for full size image Figure 4. 1,000 mAh Li-ion cell, 3 V cutoff voltage — temperature variation. (Source: Keysight Technologies)
Once you have developed battery profiles, you can use battery emulation software to cycle batteries to determine loss of capacity and battery life reduction. Battery performance can decline significantly over a lifetime of charging and discharging. That is why it is vital to simulate battery cycling. Battery test and emulation software offers an easy solution to accomplish this. The software must support arbitrary waveform generation and data logging. Also, the ability to create varying charging and discharging waveforms for a battery is valuable.
Engineers can combine multiple disparate charging and discharging sequences to simulate complex cycling profiles. They can then confirm how a battery’s performance degrades over time. Emulation software solutions enable engineers to make, for example, up to 1,000 cycle operations to determine the battery’s age effect and reliability under sequence test conditions (see Figure 5).
click for full size image Figure 5. Battery cycling testing using BV9210B / 11B software. (Source: Keysight Technologies)
Keysight’s BV9210B / 11B PathWave BenchVue advanced battery test and emulation software, along with the N6705C DC power analyzer and the N6781A or N6785A SMU modules, can perform battery profiling, battery emulation, current drain analysis, and battery cycle testing.
Having an independent test system to monitor battery health and temperature is indispensable. It helps you detect potential issues such as thermal imbalance, hotspots, and changes in ambient temperatures that can affect the overall performance of your battery system, even if you already have a BMS.
This independent battery test system can serve as a test validation system and an external redundancy safety system. It expands to meet all your battery test system needs. Furthermore, this independent system helps in troubleshooting battery failures and issues. With a few additional setups and battery software applications, you can use it as a battery emulator to help build better battery-operated applications.
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