Lithium Coin-cell battery
In space-constrained IoT applications that need to transfer a small amount of data at regular intervals or based on some event, Lithium coin-cell batteries are being used widely. Some examples can be Beacons that need to transmit few bytes every few seconds or a wireless sensor node that needs to measure sensor output and then transmit it. Coin-cell batteries have very small capacity. In early days, these were primarily used for applications that used to consume very small amount of power continuously, for example, a wrist watch. However, IoT applications are very different when it comes to load current pattern. In IoT applications, device may consume tens of mA for several milliseconds or even seconds based on the system requirement, and thus heavily loading the battery for that duration.
Though batteries come with a typical capacity for example around 230 mAh for a CR2032 coin battery; this rating is valid for 15 k ohm load/1.9 mA current output with 2.0 V cutoff voltage. As load increases, battery capacity reduces. For instance, Figure 1 shows load vs capacity graph for CR2032 from one of the manufacturers’ datasheets1 (at 2 V cutoff voltage). It can be seen that capacity starts going down as load current starts going beyond 0.2 mA (at 20° C). As, IoT applications consume much higher peak current than 0.2 mA, these batteries offer lesser than their typical capacity. Another factor that impacts battery capacity is load current duty cycle. A shorter duty cycle results in better capacity compared to a high duty cycle drain. So, to get the best out of battery it is important to keep peak current and peak current duration as low as possible as it directly impacts battery capacity.
Battery’s internal resistance also plays an important role in the usability of the battery capacity. Internal resistance increases as battery capacity is utilized. In the later stages of battery’s life, due to high internal resistance, loaded voltage may come down below the required voltage to operate the system.
Another important factor that decides the usable capacity of battery is the minimum operating voltage of the system. As battery drains out, voltage decreases. So, lower the minimum operating voltage, more battery capacity can be used. Though most battery types like Zinc Carbon and Alkaline batteries have slow decrease in output as battery drains, Lithium coin-cell batteries have abrupt decrease in voltage when battery capacity is almost used.
In next installment of this series, we will talk about factors that impact power consumption in BLE connected IoT applications and techniques to optimize power.
Co-authored article by Sachin Gupta and Vikas Vadlamudi.