Batteries for the IoT

A battery is a system which stores chemical energy and converts it into electrical energy thanks to an electrochemical reaction. It serves as the primary source of energy for powering an IoT device. There are two fundamental types of batteries, each of which can be further divided into sub-groups based on their chemistry:

• Primary batteries, which are non rechargeable
• Secondary batteries, which can be recharged and reused several times

Batteries are often referred to as "electrochemical cells." When one connects the battery to an external circuit, a reduction–oxidation reaction is triggered, releasing energy in the form of an electrical current.

When a battery supplies electrical power, its positive pole is called the "cathode" (an electron taker / oxidizing agent) and its negative pole is referred to as the "anode" (an electron provider / reducing agent). The stronger the oxidation or reduction power of the chemistry used, the greater the difference of potential, or voltage, between both poles of the battery. In this sense, the cell supplies current to the IoT device by transferring electrons from the negative pole over an external electrical circuit to its positive pole. Positive ions are transferred from anode to cathode through a porous separator, which is inserted between both electrodes. This internal component mainly acts as electronic insulator. The entire system is furthermore immersed in an ionically conductive electrolyte transporting ions formed at the anode to the cathode side, within a sealed can.

Primary batteries are intended for single use (or "disposable") and cannot be re-charged. During their discharge, the electron provider (the anode) is irreversibly consumed.

The most common example of primary batteries are alkaline type; however, in the Low Power Wide Area (LPWA) environment, where one usually tries to achieve very long battery lifetime and reduce maintenance cycles for IoT applications, it is recommended to use primary lithium chemistries, e.g.:

• Lithium-Thionyl chloride (Li-SOCl₂), where lithium is the anode and thionyl chloride is the cathode
• Lithium-Manganese dioxide (Li-MnO₂), where lithium is the anode and manganese dioxide is the cathode

Depending on the chemistry used, one can leverage different performance characteristics, such as the cell's nominal voltage. In addition to the battery chemistry, one should also consider the different internal constructions of the cell:

• Bobbin construction: This is the classic construction type showing high capacity and energy density. Bobbin type batteries are optimally used during several years with low currents (µA to a few mA) and limited pulse currents (from 5 to 50 mA in certain conditions).
• Spiral construction: This design brings more electrodes surface, leading to higher current capability, thus ideally used for power and pulse applications.

The Lithium-Thionyl chloride technology has been created and developed in the mid-sixties, primarily for military devices (radios). Its processability and performance repeatability has continually been improved since that time, therefore it can be considered as a mature technology.

The Lithium-Thionyl chloride primary (non-rechargeable) electrochemistry offers the best choice for long duration applications, since it combines:

• high energy density,
• wide operating temperature range (from - 60°C up to + 85°C),
• low self-discharge (from less than 1 % up to 3 % in storage at + 20°C).

The technology has been widely adopted for powering electronic devices, particularly communicating devices, thanks to its high operating voltage (3.6 V vs 1.5 V for alkaline systems), which remains very stable during the discharge, thus the battery use.

The Lithium-Thionyl chloride cells exists with two different construction types: bobbin and spiral designs. Whilst the bobbin type is suited for low drain currents, limited pulses and several years lifetime, the spiral type cells are ideally powering medium to high pulse applications, such as IoT devices with LPWA communications.

The Lithium-Manganese dioxide technology is a mature technology, thanks to its more than 30 year-history. This technology has been widely used both in military and consumer applications such as cameras.

On the market, most Lithium-Manganese dioxide cells have a spiral design, but prismatic and button form factors also exist.

 In its spiral design, allowing more electrodes surface, Li-MnO₂ technology is compatible with high continuous or pulsed currents consumption profiles. Compliant to a wide range of temperatures (from -40°C up to + 80°C for some cells), the Lithium-Manganese dioxide technology differentiates by the absence of significant passivation effect, which reduces greatly the voltage drop that may occur during pulsed discharges with other primary cells technologies.

This chemistry is already widely adopted for high power applications, but its lower nominal voltage (3.0 V against 3.6 V for Li-SOCl₂) had always presented a barrier as it was close to the cut-off voltage (normally 2.5 V to 2.8 V) for IoT devices electronics. This situation has changed, and Lithium-Manganese dioxide cells can be successfully selected for IoT devices communicating with LPWA technologies, if the cut-off voltage of the components and the operating temperature range are compatible with the operating voltage of this technology.

These batteries can be re-charged during their lifetime, restoring the original, depleted composition by applying a reverse current.

For small devices, the most commonly employed battery type is the lithium-ion battery; Saft is designing and manufacturing several technologies which can be classified as lithium-ion, which is a generic term.

Most IoT devices are designed to be fully autonomous, thus have no connection to a power source to recharge, and are as a consequence using primary batteries. Nevertheless, the Saft xtd secondary battery range is a very good backup power source for IoT device using renewable energy sources such as solar. Consult our application engineers to ensure that your battery solution will match your application and specifications.

There are different ways to design and build battery cells, each of which has a direct impact on the performance of the cell. Proprietary production techniques are used in conjunction to further enhance aspects of the design. Typically, primary cells employed in the IoT Industry are often produced in a sealed cylindrical casing. For these, we can basically distinguish two main construction types:

• Bobbin architecture
• Spiral architecture

Bobbin design consists of a straightforward cylindrical construction consisting of a central porous carbon mass, electrically insulated from the can by a separator material, and connected to the positive battery terminal placed at the center (cathode). A lithium-metal layer plated on the can forms the negative battery terminal (anode). The design is then completed by a liquid electrolyte filling available volume inside the can.

Bobbin cells provide higher energy density and lower self-discharge than spirally designed cells, because of the relatively smaller contact surface between the electrodes. That said, the drawback is in the cell limited current and pulse current capability, which is often required in Low Power Wide Area applications (23dBm transmit power in Power Class 3 devices). Thus, these cells might be used together in parallel with a pulse sustaining device, such as a capacitor, EDLC or Hybrid Layer Capacitor, to achieve higher pulse currents profiles.

Spiral architectures usually leverage a construction consisting of anode and cathode sheets with a separation layer in between, rolled together and fitted into a can with electrolyte.

As the need for higher currents in IoT devices increases with Low Power Wide Area (LPWA) communication networks – take a Power Class 3 (23dBm) LPWA device – battery technologies need to deliver the necessary peak currents during active time. A spiral construction offers significantly more electrodes per surface area, enhancing the cell’s current capability, while showing a lower energy density compared to bobbin cells. The greater contact surface between electrodes however may lead to an increased self-discharge rate.

This question is not an easy one! To make sure you are using the best battery technology, you have to consider several parameters from your use case:

• The nominal and cut-off voltage of your electronics: indeed, Saft has technologies with different voltages, you  have to select the one that will ensure to remain above the cut-off voltage throughout the device’s lifetime
• The environment’s temperature: some technologies perform better than other in hot or cold environments. Therefore, you have to consider where your IoT device will be deployed to ensure an optimal and continuous supply power to your object.
• The consumption’s profile and the maximum pulse current and frequency: Li-SOCl₂ bobbin technology is more relevant to use for limited pulse values and for a long lifetime, whereas Li-SOCl₂ spiral, Li-SOCl₂ bobbin + pulse support device and Li-MnO₂ are particularly suited for high pulse applications

You can submit your use case consumption profile for a personalized recommendation from our application engineers.

For a Li-SOCl₂ battery, accelerated life tests may be used to evaluate passivation or self-discharge behavior. Yet, it must be stressed out that the implementation of relevant accelerated aging tests and their interpretation are not straightforward and require a good knowledge of the technology. There are 2 ways to “accelerate” aging:

• Increase the discharge rate or current pulse occurrences: Saft does not recommend this method, as a cell’s behavior at high discharge rate and low discharge rate are not comparable. Therefore, the results will not be relevant in regard to an IoT typical application (low current + pulse).
• Store the cells at medium temperature before performing tests in pulse at + 20°C. This method is fine to accelerate aging phenomena such as passivation. Based on long-term quality survey testing on Saft’s Li-SOCl₂ cells, level of passivation due to 1 week storage at + 70°C is roughly equivalent to 2 to 2.5 years of storage at ambient temperature Nevertheless, such tests are revealing trends in the behavior of a cell, but values such as voltage drop are not to be taken literally.

From our experience, Li-SOCl₂ electrochemistry does not behave as simple as that and this rule can not be strictly enforced.

The self-discharge phenomenon is intrinsic to any electrochemical system, which leads to a loss of the battery's capacity.

A battery’s cell self-discharge is an important factor to consider for IoT applications as the IoT devices must operate several years with a given battery.

One should distinguish between the following two self-discharge phenomena:

• Self-discharge in storage: The storage period of a battery could be significant, from:
  • the manufacturing date of the battery,
  • the lead time until its integration into the IoT device,
  • its delivery and storage up to the beginning of the deployment of  the device,

Thus, it is crucial to take into account the self-discharge occuring during all the steps of the storage in the lifetime calculation.

• Self-discharge in use, while the IoT device is in normal operating mode.

Please note that the self-discharge, under typical operating conditions can be very complex to model and depends on several parameters, such as peak currents and consumption profile, temperature, cell’s age, etc.

It is highly recommended to contact your battery supplier for a more detailed modeling and consulting.

The nominal battery capacity is the rated capacity value in Ampere-hours (Ah) measured in defined operating conditions such as:

• discharge rate,
• environment temperature
• cut-off voltage

The capacity is calculated by multiplying the discharge current times the time until the defined cut-off voltage threshold is met.

Likewise, a cell or battery energy can be represented in terms of Watt-hours (Wh), calculated by multiplying the discharge current times the time until the defined cut-off voltage threshold, times the nominal voltage of the battery.

For IoT applications the available battery capacity varies based on real life conditions. There are many parameters affecting the available capacity such as:

• temperature,
• peak currents,
• consumption profile,
• minimal application voltage (cut-off voltage)

Therefore, an accurate calculation of the expected battery capacity for your IoT application can be quite complex, as it must consider both intrinsic properties of the battery cell and typical parameters of use cases and environmental conditions.

For many battery-powered applications, the peak currents are an important aspect of the design. Particularly in LPWA (Low Power Wide Area) applications, the current peaks of the communication module (Modem) should not be underestimated. Additionally, MCU’s, Sensors and actuators often need peak currents simultaneously with the LPWA communication module.

Generically when reading battery data sheets, it is important to understand that we cannot simply compare the peak currents of an LPWA Communication Module and other components with the peak current (also called Pulse capability) of a given battery.

The maximum peak currents specified in batteries datasheets are determined according to very specific tests conditions. For example, they are often specified for a certain maximum time duration (e.g. 100 ms) and a certain frequency (e.g. every 2 minutes) at a certain temperature (usually room temperature, e.g. + 20°c). These conditions are not fulfilled in most real field conditions. Moreover, aging and temperature also have a significant impact on battery pulse capabilities.

This is why this is highly recommended to contact your battery supplier for further questions and accurate lifetime calculation.

Passivation is the main “observable” effect of a surface reaction that occurs spontaneously onto lithium metal surface in all primary Lithium batteries based on a liquid cathode like Li-SO₂, Li-SOCl₂ and Li-SO2Cl₂ systems. This electrochemical reaction corresponds to the corrosion of lithium metal by the solvent SOCl₂ into lithium ions and leads to the formation of a solid protecting layer preventing from further corrosion, but also most importantly from internal short-circuit of the battery! This surface layer is called “passivation layer”: it acts in a similar way like paint protecting against metal corrosion, i.e. it protects the cell from discharging on its own and enables primary lithium cells to enable a long shelf life.

The passivation layer is electronically insulating, which may have some detrimental consequence for battery operation. Therefore, its structure, morphology and buildup over time must be properly managed. Indeed, internal resistance of the cell is enhanced due to the presence of the passivation layer, and this shall cause low voltage readings at initial times (inferior to ms range) upon connection of a resistive load or current to the cell.

This rapid transient minimum voltage (TMV) is also called “voltage delay”. After the TMV stage, diffusion of lithium ions through the passivation layer enables cell voltage to recover to nominal values. This second stage is called “depassivation” and is very important for efficient operation of the battery.

Several factors are known to enhance passivation effect, affecting the length and depth of voltage delay:

• The lithium cell electrochemistry, construction and manufacturer (i.e. proprietary internal design): Chemistries based on liquid cathodes are by far more prone to passivation than others. Nevertheless, within a given type of technology, some battery brands may display lighter/heavier passivation than others. This is a big part of the know-how of every lithium primary battery maker!
• The storage duration: The longer the storage time before use, the more the passivation layer will grow (like rust on iron).
• The temperature during storage and/or operation: The higher the temperature, the faster the passivation layer will grow, and the more compact crystals will buildup. Conversely, at cold temperatures, the passivation will grow more slowly, but as both electrochemical and diffusion reactions are slowed down and electrolyte viscosity is higher than at ambient, the effect of passivation could be more likely visible especially under high current draw.

The potential disturbance brought by lithium passivation depends in some part on the application to be served by the batteries:

• Applications featuring low-to-moderate current draw (a few mA), voltage cut off below 2.5 V, coupled with a few seconds allowable response time, and for which brief voltage excursions below cut off could be “forgiven”, will remain in practice “passivation-tolerant”.
• Other applications, with high current pulses and voltage cut off, frequent “high” temperature excursions (i.e. above + 40°C), and for which any voltage recording below cut off will trigger a "low battery" warning signal, have more chance to be disturbed by passivation.

Having this in mind, we recommend that you evaluate the effect of passivation very carefully when selecting lithium batteries and that you speak to one of our experts in order to receive recommendations for the best solution for your application. 

Lithium batteries are considered as dangerous goods in the United Nations classification. Thus, they need to conform to specific rules and pass standards tests compiled in the UN 38.3 manual of tests and criteria, prior to any shipment. Please check the text of the UN38.3 for more details.

At any time during transport, you may need a Test Summary Report (TSR). What does that mean exactly?

Any battery shipper has to provide a Test Summary Report, that must be available on its website, for every reference of lithium-based battery it is shipping, and to conform to safety rules as per packaging and handling of the batteries.

Saft has put all its TSRs online. You just have to type the battery part number to get the requested document. Have a look at our Customer Portal or directly scan the following QR code

What if I send devices containing lithium batteries?

Our customers, as they have to ship their device containing lithium batteries, and / or to ship replacement batteries to their own customers, must also comply to the UN38.3 requirements, particularly regarding prior testing of the battery (if they build a pack from our cells), but also packaging, safety, handling rules of batteries. Employees must be properly trained and certified. Check Intertek, IATA for more details