When your IoT application involves connecting thousands or even millions of devices, which need to last for five, ten or even twenty years without intervention or maintenance, then selecting the right battery is of critical importance. You need to be sure that once installed and operational, the battery will provide the required energy throughout the planned lifetime of the application. You want a battery that you can just fit and forget, safe in the knowledge that it will do its job.
Here are 5 questions to consider when selecting the right battery for your device:
The amount of data to be transmitted, the distance of transmission, and the type of network all influence the power consumption of an IoT device. For example, the energy consumption for one transmission per day for a long-range wide area network (LoRaWAN) is typically around 130 mAh per year, which can be supplied by an AA or A size cell over 10 years. A narrow-band IoT (NB- IoT) consumes more, around 250 to 300 mAh per year, and a cellular 2G/3G GSM consumption can exceed 4000 mAh per year, which would require a complex multi-D size cell solution to last for 10 years.
As the amount of data collected and processed increases, so too is the shift towards 4G and 5G cellular networks, which improve the power consumption of devices.
The anticipated lifetime is a crucial component of your IoT business case since the cost of the battery and its eventual replacement often represent the most significant costs of the IoT device. Battery longevity is therefore key to optimize the total cost of ownership (TCO). It is directly related to the level and duration of the stress inflicted on the battery by the application and is therefore specific to a given project. The objective is to match the power requirements of the application as closely as possible to the capacity of the battery while taking into account the various factors affecting battery life.
Most people think that rechargeable batteries last the longest, because they can be charged, discharged and recharged again and again. However, it all depends on the type of application and the location of the device. For many industrial applications the cost or complexity of recharging or replacing batteries means that single use batteries last the longest and are the most cost effective.
In the case of smart metering for example, a water or energy utility company may have millions of meters transmitting data on a regular basis. Often located in places that are difficult to access, with no connection to the electric grid for safety reasons, and no alternative source of energy, primary batteries are the only solution. Utility companies need to be certain that once installed, the battery will allow the meters to operate as designed, without maintenance or intervention for up to 25 years. The battery lifetime is critical for their business model because the cost of replacement would be too high. In this case, Saft’s primary lithium thionyl chloride batteries (Li-SOCl2) offer a high nominal voltage (3.6 V) and high energy density to last their entire lifetime.
Cost is also an important factor when considering primary versus secondary batteries. Rechargeable li-ion batteries have more components and are more costly to manufacture, but when connected to low-cost renewable energy sources, such as PV solar panels, the total cost of ownership becomes more attractive. Saft’s lithium-ion renewable batteries boast a very long lifetime with a high cycle count (up to 2,800 times with only a 30% capacity loss) requiring low maintenance even in harsh conditions. Offering high nominal voltage and capacity, they can be charged and discharged over a wide temperature range, even at low temperatures, which makes them ideal in outdoor conditions.
When considering the difference between bobbin or spiral cell construction, it is important to understand the difference between energy and power. Batteries store energy, the more energy is stored, the greater the capacity, the more work can be done over the lifetime of the battery. Power on the other hand, is the speed at which the stored energy can be used. As an analogy, consider the difference between a marathon and a sprint. Going fast requires more power.
Bobbin cells (High Energy) are designed specifically for long term (5 to 20+ years) applications. The construction is designed to maximize the quantity of raw materials embedded in the casing to generate maximum energy. These cells are particularly well suited for low power wide area (LPWA) applications requiring very low continuous or moderate pulsed currents (typically in the 5-150 mA range) such as metering devices or parking sensor applications. Their ability to withstand broad fluctuations of pressure, temperature (from - 60°C to + 150°C) and harsh mechanical environments make them ideal for use in remote locations and extreme environments such as trackers.
Spiral cells (High Power) are designed for applications requiring continuous currents and very high pulses, (typically in the 50 mA-2 A range). The spiral construction means that the exchange surface between the raw materials is maximized to increase power. The offset is reduced energy density and a shorter operating life (2 to 10+ years). Some specific ranges can operate at very high temperatures found in oil and gas applications, for example.
To maximize energy and power a number of hybrid solutions have been developed which combine a primary bobbin cell assembled in parallel with a lithium-ion capacitor to act as a pulse support component. These are specially designed for applications requiring performance stability and high pulse capacity, over a wide temperature range and for a lifetime of more than 10 years.
Temperature is the main factor impacting the battery’s power consumption. And while Lithium-Thionyl Chloride (Li-SOCl2) batteries provide the widest operating temperature range, from -60°C up to +150°C, this does not mean that the battery performance remains the same across the range. At high temperature the cell’s chemical reactions are stronger, internal resistance is lower which increases the battery’s ability to deliver high energy. This results in faster discharge and a corresponding loss of battery life.
At low temperature the chemical reactions are less efficient, internal resistance of the cell 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 lower efficiency of the electronics and therefore higher consumption. Moreover, liquid cathode systems such as Li-SOCl2 electrochemistry are subjected to passivation phenomenon (low voltage reading under current pulse). Under temperature fluctuation in the field and specific use cases this phenomenon could lead to anticipated end-of-life of a device before using all the energy available in the battery.
These chemical reactions, in response to variations in temperature need to be taken into account when calculating the estimated battery lifetime in the field.
Saft LSH high temperature range is specially designed for operating in extreme conditions with temperatures as high as +150°C. They are ideally suited for measurement while drilling applications in the oil and gas industry for example.
Lithium-Manganese Dioxide (Li-MnO2) batteries provide high power and high energy with no passivation. With a nominal voltage of 3.0 V, Saft’s LM/M range offers a good trade-off between energy and power for applications allowing a cut-off voltage below 2.5 V. The high surface area of their spiral electrodes allows maximum current pulse capability for optimized operation from -40°C to +85°C, making them suitable for smart metering devices requiring high pulses, but also parking sensors and smart farming applications.
So… to cut a long story short! Batteries are the ideal “fit and forget” solutions for IoT applications, but there is no “one-size-fits-all” solution. There are many parameters that need to be considered at the design phase of your IoT device. Lifetime, power needs and operating temperature are the most important factors to ensure that your batteries are fit for purpose.
To help you make your choice, why don’t you submit your use case consumption profile to our application engineers for a personalized recommendation?
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