The impact of the temperature on your IoT application’s power consumption
The impact of the temperature on your IoT application’s power consumption
Did you know that temperature is the main factor impacting your battery’s power consumption?
Like humans, batteries work best at room temperature. Chemical reactions are affected by temperature and since a battery relies on a chemical reaction to provide energy, a slight change in temperature affects the battery’s capacity and service life. And not just that of the battery! Temperature is an important parameter for almost all internal components and generates side reactions. The consumption of your electronic components will also be affected by temperature!
When you plan to deploy a device over a long lifetime, the slightest Ampere can count. A drift of 1 µA might not seem much, but over a year this would represent 8,76 mAh or almost 90 mAh over a 10-year period. For a small battery that contains 1,2 Ah such as the LS 14250, this would represent almost 7,5% of the nominal capacity of your battery over 10 years, a huge impact on the device’s lifespan.
This is why anticipating and planning all possible and unimaginable temperature conditions will help you chose the right battery technology for your device and accurately calculate your device’s lifetime.
The impact of high temperatures on a battery
The summer and warmer temperatures generally make you feel energetic, but if it gets too hot, chances are that you’ll feel a little more passive, 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.
But two other phenomena are enhanced by an increase of the battery’s temperature: self-discharge and passivation.
The 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 while in use (material consumption and addition of the components leak current or the device’s current leaks whilst in sleep mode). This means that the battery will have a decreased capacity.
Both self-discharge phenomena are accelerated by the elevation of temperature 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 for 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. Batteries are not constant voltage generators and the thicker the layer is, the more it hinders the establishment of the current, like limescale that accumulates on a pipe and prevents water from flowing through. 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:
Let’s illustrate these points with the datasheet of our LS 14500 cell.
In this graph, the battery’s available capacity measured in Ampere-hours (Ah) is indicated on the Y axis. The rate of discharge is indicated in milliamperes (mA) on the X axis. The temperature is indicated by the curve in the middle of the graph. (Find out about how to read a battery’s datasheet in our article)
To see the impact of temperature on battery’s self-discharge —which is only visible under low discharge currents— you’ll need to look at discharge currents that are lower than the nominal current (here, 2mA for the LS 14500).
The difference of available capacity between the curve at 20°C and the curves at higher temperatures will give you an inkling of this impact on the self-discharge.
In this example, at 20°C, the available capacity is of 2.6 Ah, at 55°C, it drops at 2.3 Ah.
At 70°C, it reaches 2.1 Ah.
The association of low discharge rate and high temperature is even worst as the slope of the curve shows it under lower discharge rates.
So, when using a battery at a high temperature, you should consider these possible effects:
• Lower internal resistance
• Higher ion mobility in the electrolyte
• Higher energy
• Higher self-discharge
• Lower capacity
• Higher risk of passivation
The impact of low temperatures on a battery
When it’s cold outside, your body needs more time to get started and you’ll need warming up before you can do any sport. Batteries also experience this peculiarity. At low temperature, the chemical reactions in the cell are less efficient, 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.
You might think that since the self-discharge and the passivation layer evolution are less important at cold temperatures and the components will consume less too, this might make up for it! Well, yes and no.
At cold temperature, the passivation will indeed grow more slowly, but the layer will be more compact with smaller crystals. Moreover, the electrolyte viscosity is higher and both electrochemical and diffusion reactions are slowed down. Which means that the effect of passivation could be even more visible, especially under high current draw.
You can find out which type of applications are likely to be disturbed by passivation in our article The 7 most common pitfalls about passivation (and how to avoid them)
Let’s look again at our LS 14500 cell datasheet. This time, we’ll look at the “Voltage plateau vs. current temperature (at mid-discharge)” graph.
At -20°C, or at -40°C, there is almost no self-discharge, but it is also more difficult for the ions to move forward from one electrode to the other. So, the battery voltage decreases very fast. Moreover, if the current is too low (less than 1mA), it cannot warm up the chemistry thanks to the Joule’s effect. This warming can work if the currents are higher, but the battery is not powerful enough at such temperature and its capacity will decrease accordingly.
So, when a battery is being used at a low temperature, you should consider these possible effects:
• Higher internal resistance
• Lower power performance,
• Lower capacity,
• Lower ion mobility in the electrolyte,
• Lower self-discharge.
How to anticipate 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, make sure to know the consumption profile of your application in all 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 you to select a battery, Wisebatt for Saft can help you to create a virtual prototype and simulate its consumption, whilst Deutsche Telekom’s IoT Solution Optimizer can help you to model the complete system.
And once you have a real prototype in place, Qoitec Otii solution can help you measure in real time consumption of your device at various temperatures, up to the measurement of the firmware and hardware operations without expensive test means.
But whichever the stage of your development, your best assets are our engineers, who will happily recommend you the best battery for your operating conditions and give you an estimate of its lifetime in the field. Get in touch!
Find out more about Saft batteries operating at a wide temperature range
Saft offers various battery chemistries that can operate in wide temperature ranges:
• Li-SOCl2: from -60°C to +85°C and even +150°C
• Li-SO2: from -60°C to +70°C
• Li-MnO2: from -40°C to +85°C
• Li-ion: from -40°C to 60°C and even +85°C
Batteries that stand the test of extreme temperatures. Read our case studies.