Testing battery safety using an independent battery testing system

Modern product applications using rechargeable batteries often have built-in sensors and battery management system (BMS) circuitry. The BMS monitors the voltage, current, and temperature of the rechargeable battery system, whether it is a single cell, a module (a group of cells), or a pack (a group of modules). In general, monitoring the voltage and current of a cell is usually not enough to determine the health of the battery.

Monitoring battery temperature can warn you of potential defects and quickly determine the location of a fault. The BMS monitors the battery pack to keep the operating temperature within an optimal range. A battery that is too hot can fail. But a battery that is too cold can also degrade due to slower internal electrochemical reactions, which can reduce its performance.

This white paper highlights common temperature-related battery issues and shows you how test instrumentation can help you build better battery-powered applications.

Common Challenges of Monitoring Battery Temperature

Thermal imbalance, battery pack hot spots, low performance and charge are areas of concern when monitoring battery temperature.

When use causes thermal imbalance

Large-scale applications typically use battery packs where modules are connected in series and parallel. Strategically placed thermal sensors in the battery pack detect temperature changes. Thermal imbalance in large battery packs typically begins with the inhomogeneity of the battery cells affecting their charge and discharge voltages. Over time, the inhomogeneity changes accelerate and some battery cells become overcharged or over-discharged, causing the battery to overheat.

You can use a BMS to balance the cells, equalizing the voltage and state of charge (SOC) between cells when fully charged, thereby minimizing thermal imbalance. Battery manufacturers can also build battery packs by selecting cells with very close open circuit voltages and minimize SOC variations.

Thermal imbalances can also be caused by product design, for example if the battery pack's cooling system is not effective enough for certain harsh external environments.

Battery Pack Hot Spots

Monitoring battery temperature helps detect hot spots. Depending on the criticality of the battery application, sometimes just a few sensors strategically placed in the battery pack are sufficient. However, in applications that require critical performance, such as electric vehicles, you can place a temperature sensor on each battery pack module.

Hot spots tend to appear on weak cells in a battery pack. Weak cells tend to be overstressed and gradually degrade. As a result, they run hotter than normal good cells because they have a hard time keeping up with the performance of good cells.

Hot spots can also warn you of possible cell or module damage. Physical impact to the battery pack can puncture or deform the internal structure of the battery cell, such as the electrodes or polymer separators. If this happens and no intervention is taken, the battery cell damage can worsen and potentially lead to thermal runaway. This can cause fires and explosions, so it is important to detect hot spots, find the faulty cells, and replace them quickly.

Other causes of hot spots include poor terminal connections, defective heat sink components, and external cable shorts.

Low battery performance and usage capacity

Monitoring battery temperature is an active closed-loop process to ensure the battery pack operates within the optimal charging and discharging temperature range.

Cold weather causes battery performance to deteriorate because the electrochemical reactions slow down. As a result, the battery's usable capacity drops significantly or it may even stop functioning.

The bigger problem is when the battery system is operated at temperatures above the manufacturer's specifications. Battery life is shortened, and weaker performing cells may deviate significantly from the better performing cells. At this point, thermal imbalances and hot spots begin to appear.

Independent test equipment monitors battery temperature

Many commercial battery management systems are available for a variety of applications, from Internet of Things (IoT) devices to high-voltage automotive applications. Basic functions include over-current protection, over-voltage protection, over-charge protection, over-temperature protection, under-voltage protection, cell balancing, SOC, and SOH.

However, there are many good reasons to purchase stand-alone test equipment to monitor battery temperature in your application.

Benefits of independent testing to verify the system

Having a standalone test verification system, such as a modular data acquisition (DAQ) system, helps verify that your BMS is functioning properly. It also helps verify the overall integrated system of your application. A standalone DAQ system can do the following:

More accurate measurements are made using various types of temperature sensors such as thermocouples, thermistors, and resistance temperature detectors (RTDs). Using a thermistor or RTD, a temperature accuracy of ≤ 0.1 °C can be achieved.

The measuring temperature range is -150 °C to 1820 °C. This allows you to monitor both the internal battery system and the external ambient temperature simultaneously.

Measure more points in your application than your BMS implementation to verify that your BMS has not missed any critical locations.

Taking measurements at shorter intervals does not tax the hardware resources of the BMS and application. Shorter intervals can help you find the optimal interval settings for your BMS monitoring system.

Get external redundancy for mission-critical applications

One key reason to have an independent test system is to provide redundancy for mission-critical applications. For example, medical devices that monitor and control the functions of vital organs cannot withstand unexpected power outages during operation. Another example is large energy storage systems that power critical building functions such as IT, telecommunications, and medical equipment.

A stand-alone DAQ system can do the following:

Provides independent alarm and emergency secondary shutdown to prevent battery system melt or fire.

Provides backup monitoring and control system in case of failure or loss of communications with the primary system.

The highly versatile DAQ system is the best choice for monitoring temperature in stand-alone test equipment. In addition, it is flexible enough to accommodate larger projects.

Many modern DAQ systems have high-resolution, 6.5-digit multimeter instruments built in. They also come with a variety of solid-state, armature, and reed switch multiplexer modules to monitor temperature points on over 100 channels. In addition, because the DAQ has a built-in digital multimeter (DMM), it can measure other signals besides temperature, such as AC/DC voltage and current, resistance, and capacitance.

The DAQ system shown in Figure 1 is modular, allowing for expansion of channels for temperature monitoring. The system allows you to expand by adding modules as your project expands. This means you don’t have to invest in a new system and can save valuable development time.

Figure 1: Keysight 34980A Data Acquisition Switch/Measure Unit (SMU)

Test equipment helps build better battery-powered applications

Once you understand the causes of battery failure, you can use battery simulation software to predict the decline in battery capacity.

Battery failure mechanisms and issues

You can analyze the root cause of battery failures by physically dissecting them, but electrical measurements can provide signs that help predict failures before they occur.

One cause of failure is lithium deposition, or dendrite growth, on the anode electrode. This growth is usually caused by overcharging the battery over many cycles, which causes lithium to deposit on the anode. Over time, this can cause a short circuit between the two battery electrodes. This short circuit is difficult to monitor because it occurs quickly - within milliseconds after the voltage drops.

Another source of failure is electrode degradation, where the electrodes develop oxide buildup or microcracks due to fatigue from charge and discharge cycles and repeated chemical reactions of the electrolyte.

Electrical shorts can also be caused by a separator failure inside the battery. Separator failure can be caused by a physical impact or puncture of the battery or exposure to extremely high temperatures. In addition, material defects during the manufacturing process can also cause failure.

Battery aging and capacity degradation are not critical failures that require immediate intervention. However, these factors are worrisome for battery application users. Open circuit voltage measurements alone are not a good indicator of battery capacity. The internal resistance of an aged battery increases over time, but you cannot immediately conclude capacity degradation from a snapshot of resistance measurements. Temperature, SOC, and discharge rate affect battery internal resistance. Figure 2 shows several key battery failure mechanisms that can occur over time in a battery cell.

Figure 2: Internal battery failure mechanisms over time

Battery failure is complex because batteries undergo electrochemical reactions and are exposed to physical variables such as temperature and mechanical stress. The battery charging method is another factor. For example, if a battery is frequently fast-charged, its temperature will be much higher than normal charging and it will degrade faster over time.

No single battery test instrument can provide a definitive diagnostic solution for battery failure. However, depending on your application, power requirements, capacity, and production cycle (R&D, compliance testing, or production), there are a variety of test equipment solutions that can meet your needs. See the Learn More section for links to test equipment that can meet your diagnostic needs.