The prediction of the life-time of battery-powered IoT devices and wireless sensor networks is almost exclusively based on the assumption that the total charge in a battery (i.e. the mA-h) can be linearly consumed in time. This is not the case in reality. Batteries are complex electro-chemical systems and their discharge behavior depends heavily on the timing and intensity of the applied load. There is very little empirical data or reliable models available for these kinds of batteries and loads that are typically used in IoT sensor nodes for very long operational time, 5 -10 years.
We characterize the inexpensive CR2032 Li-coin cells using carefully controlled synthetic loads and a wide range of IoT-typical load parameters. We observe that actual lifetimes can differ from predicted linear ones by almost a factor of three. Furthermore, loads with similar average currents can vary significantly in the amount of capacity of the battery they can utilize. We conclude that short duration loads generally are faring better than sustained loads which was not anticipated. We suggest a better prediction model, that captures the non-linear short duration behavior, which can be implemented in constrained IoT devices.