Temperature and batteries for IoT - what you need to know
All about batteries - September 13, 2024
Many IoT applications are deployed in outdoor locations – whether they’re tracking, sensing, or otherwise. This can mean they are exposed to challenging environmental conditions and, with reliability ever-important, these applications and its components need to be able to withstand a range of temperatures.
This is perhaps more important now than ever before, with the impact of ongoing climate change causing more extreme temperatures across the globe. Indeed, the Copernicus Climate Change Service reported that the 2024 summer was Earth's warmest on record.
It’s not only relevant for outdoor applications, of course. Smart devices in buildings play a vital role for security, safety and day-to-day operations, and they need to maintain performance regardless of the climate.
Whether a device operates in hot or cold (or variable) environments, the temperature is an important parameter for choosing almost all internal components. Here, we explore the impact of temperature on a battery.
The significance of temperature
Temperature is the main factor that impacts a battery’s power consumption.
Batteries work best at moderate room temperatures. That’s because chemical reactions are affected by temperature and, since a battery relies on a chemical reaction to provide energy, even a small change in temperature can affect a battery’s capacity and service life.
Small changes in capacity might not seem all that big of a problem – but over the course of a battery’s lifespan it adds up. A drift of just 1 µA (ampere) might not seem much, but over a ten-year period it could tally almost 90 mAh (milliampere-hour).
If we apply this to one of Saft’s smaller cells often used in IoT applications – the LS 14250 – this would represent 7.5% of the nominal capacity over ten years.
That’s why anticipating and planning for temperature conditions is important before selecting the optimum battery technology for a device.
The impact of high temperatures on a battery
You know what it’s like on a really hot day. Chances are that you’ll feel a little more passive and less inclined to undertake physical effort. Similarly, at high temperature, a cell’s chemical reactions are stronger and the internal resistance is lower, which increases the battery’s ability to deliver high energy. This, in turn, results in faster discharge and a corresponding loss of battery life.
Two other phenomena are enhanced when a battery operates at high temperature: self-discharge and passivation.
Self-discharge is an internal chemical reaction consuming anode and cathode materials during storage (even though there is no connection to any external circuit) or through current leakage while in use or in sleep mode. It means that the battery will have a decreased capacity. This phenomenon is accelerated by elevated temperatures since the cells’ chemical metabolism will be higher and the leak from currents of the components will be stronger.
Passivation is a surface reaction that occurs spontaneously onto lithium metal surfaces in all primary lithium batteries, particularly those with a liquid cathode such as Lithium-Thionyl chloride (Li-SOCl2 ).
A solid protecting layer, the “passivation layer” is built, preventing the cells from discharging on their own and enabling their long shelf life. But at high temperatures, the passivation layer grows thicker, and bigger crystals build up. It won’t necessarily impact drastically the battery’s available energy but it may have some detrimental consequences for battery operation. The internal resistance of the cell, also called the impedance, is enhanced due to the presence of the passivation layer. This impedance causes the battery to be unable to deliver the required power to the device.
Self-discharge and passivation therefore need to be carefully managed if you plan to deploy your device in high temperatures.
The impact of low temperatures on a battery
At low temperature, the chemical reactions in the cell are less efficient and molecules slow down. As a result, internal resistance of the battery increases and it won’t be able to deliver the same level of power. It can deliver the current but at a lower voltage level, resulting in a lower efficiency of the electronics and therefore higher consumption.
Passivation is different at cold temperatures too. It the passivation layer will grow more slowly, but it will be more compact with smaller crystals. Moreover, the electrolyte viscosity is higher and both electrochemical and diffusion reactions are slowed down. This means that the effect of passivation
Planning for the effect of temperature on your battery’s lifetime
Even the slightest deviation in temperature can reduce your battery’s capacity by half if not properly anticipated. So, it is important to know the consumption profile of an application in all operating conditions (standby, sleep mode, active, etc.) and that you associate these conditions to the warmest and coldest temperatures the device will be exposed to during storage, transport or in operation.
If you are at an early stage of your project, the IoT Smart Selector can help guide you towards selecting a battery that meets the requirements of your application.
Developers will often use computer modelling to assess how a battery, and other components, will respond when deployed in operating conditions.
But whatever the stage of your development, the best advice comes from expert engineers who know how to match an application’s profile to the right battery.
