Bernard Ang, Brian Whitaker, Keysight Technologies
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 over discharging, causing the batteries to overheat disproportionately. Cell balancing using a battery management system (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 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. Other causes of hotspots include poor terminal connections, heat dissipation component defects, and external cable shorts.
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Figure 2: Internal battery failure mechanisms over time
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.
Independent test equipment
Many commercialized battery management systems are available for all kinds of 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.
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Figure 3: Battery profile created with Keysight BV9210B / 11B PathWave BenchVue advanced battery test and emulation software
Having an independent test validation system, such as a modular data acquisition (DAQ) system, helps validate that the BMS is performing properly. It also helps validate the overall integrated system of the application. An independent DAQ system can:
· Measure more accurately with many types of temperature sensors, such as thermocouples, thermistors, and resistance temperature detectors (RTDs). Thermistors or RTDs can achieve temperature accuracies of ≤0.1 °C.
· Measure temperature ranges from -150 °C to 1,820 °C.
· Measure more points than the BMS implementation in the application, validating that the BMS is not missing out on any key locations.
· Measure in much shorter intervals without taxing the BMS and application’s hardware resources to find the best interval setting for the BMS monitoring system.
Another key reason for having an independent test system is to provide redundancy for mission-critical applications. It can provide an independent alarm and emergency secondary switch-off to prevent battery system meltdown or fire, or it can also provide a backup monitoring and control system if the primary system malfunctions or loses communication.
A DAQ system is the best choice to monitor temperature as 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 also modular, allowing for the expansion of channels for temperature monitoring.
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Figure 4: 1,000 mAh Li-ion cell, 3 V cutoff voltage — temperature variation
Building better battery-operated applications
Once you understand the sources of battery failures, you can use battery emulation software to predict drops in battery capacity. Then 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. One source of failure comes from lithium plating or dendrite growth on the anode electrode. 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. Internal battery separator failure causing an electrical short 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 all needs, depending on the application, power usage requirements, capacity, and production cycle (R&D, compliance testing, or production).
Battery emulation to validate battery performance
Battery emulation software can predict drops in battery capacity over time. In addition, battery emulation software can predict the impact of temperature on battery life. Before a battery is emulated it must be profiled to understand the amount of energy it can store and supply as it 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. 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 the better prediction of the impact of temperature on battery life.
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Figure 5. Battery cycling testing using BV9210B / 11B software
Once battery profiles have been developed, battery emulation software can be used to cycle batteries to determine loss of capacity and battery life reduction. Battery performance can decline significantly over a lifetime of charging and discharging. 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. Multiple disparate charging and discharging sequences can be combined 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).
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.