How do temperature, age, and discharge rate affect battery run time?
Age, temperature, and the discharge current rate can all drastically affect battery run time. Grasping the magnitude of these factors is essential for designing consumer electronic and IoT devices. The internet is full of negative device reviews due to poor battery performance, with the underlying cause being age, temperature, or battery drain rate. Therefore, battery-operated device manufacturers must design solutions and select batteries considering these factors.
In this blog post, I will explore why age, temperature, and discharge rate impact battery characteristics and, consequently, run time. In addition, I have created battery models to show the real-world impact on battery parameters.
You can create battery models with profiling software. A model maps out the open circuit voltage (Voc) and internal resistance (Ri) versus the state of charge. It is crucial to map out these characteristics so that the model accurately reflects the battery's real-world performance. These characteristics can vary significantly at different states of charge. Figure 1 is an example of a typical plot.
Figure 1. Battery model mapping out the Voc and Ri of the battery
Age
Each time you cycle a battery, some of its active materials are consumed, which can reduce the battery's overall capacity. This reduction means the battery can hold less charge and provide less energy during subsequent cycles.
Also, during charging and discharging cycles, the active materials inside the battery undergo physical and chemical changes that cause the battery resistance to increase over time. Plus, as the active materials degrade or break down, the formation of byproducts can cause resistance to build up.
Figure 2 is the plot of a Lithium Ion Battery with a 2.8 Ah capacity. The open circuit voltage drops from 4.2V to 3V as it is discharged, and the internal resistance fluctuates around 330 milliohms.
In Figure 3, you can see the model of the same battery after it has been cycled numerous times. Its capacity has dropped from 2.8 Ah to 1.7 Ah. Also, the battery's internal resistance has more than doubled and exhibits large spikes at higher charge levels. Both reduced capacity and increased resistance will significantly shorten the battery run time of any device using the aged battery.
Figure 2: Lithium-ion battery model generated using the E36731A battery emulator and profiler
Temperature
A battery’s performance can vary depending on temperature. A battery's internal resistance elevates at cooler temperatures, inhibiting its ability to conduct current. This increase happens due to a slowdown in the movement of ions, their transition rates, and the overall electrochemical reactions occurring between the battery's electrodes and electrolytes. These effects result in amplified internal resistance, which hinders both the charging and discharging processes.
In Figure 4, you can see a Lithium-Ion battery model captured at zero degrees C. The same battery used to create Figures 2 and 3 was used to generate the model. The battery's internal resistance increased by 42%, which will significantly affect the run time of any device using the battery. The battery's capacity also decreased slightly from 2.82 Ah to 2.68 Ah.
Conversely, when a battery is charged or discharged at higher temperatures, the heat accelerates the internal electrochemical reactions, lessens its internal resistance, and enhances its performance and storage capacity. However, extended exposure to elevated temperatures leads to rapid aging and diminishes battery life.
Current Discharge Rate
The rate at which a battery is discharged can also affect its characteristics. When you discharge a battery at a high rate (i.e., a large current is drawn quickly), its effective capacity can decrease. The reasons behind this are multi-factorial and tied to changes in chemical reactions and impacts tied to the battery's internal resistance.
For example, all batteries have some internal resistance, resulting in energy being lost as heat. The faster you draw current, the more heat is produced and the more energy is wasted, thus reducing the battery's run time.
Below you can see models (Figures 5 and 6) of an identical nickel-cadmium (Ni-Cd) battery discharged at different rates. The capacity decreases from 1.41 Ah to 1.22 Ah when the discharge rate increases from 100 mA to 500 mA.
Conclusion
The critical influence of factors like age, temperature, and discharge rate on battery performance underscores the need to analyze current drain to validate actual battery run time. Performing such tests with physical batteries can be prohibitively time-consuming and impractical. Instead, creating battery models can speed testing and provide 'known good' models for batteries under specific aging and temperature conditions.
The alternative approach, maintaining a ‘suitcase’ of batteries at various ages and regular testing in temperature chambers, is not an efficient solution and is often unfeasible. These testing challenges often result in discrepancies between real-world battery performance and advertised claims.
You're invited to watch the Live from the Lab webinar, "Power for IoT," for a more comprehensive exploration of this subject matter. The webinar demonstrates the development of battery models under different conditions, and I and other Keysight experts discuss the varying impacts. Furthermore, you will see how you can employ these models for battery emulation and the rapid analysis of current drain.