Edge IoT backup power: Choosing the optimal strategy

Defining criticality and autonomy requirements for Edge IoT devices

Critical Edge IoT devices are system components whose failure leads to significant financial losses, safety threats, or business process disruption. Examples include medical equipment, industrial controllers, security systems, and critical infrastructure monitoring. Production downtime can cost large enterprises over $5 million per hour. For hospitals, system failures can result in patient evacuation and prolonged facility closures.

Power outage durations can vary: from a few minutes to several hours for minor malfunctions, to days or even weeks in the event of severe weather or infrastructure damage. Typical power outages last 1 to 2 hours. However, if power lines or substations are damaged, restoration can take 24 hours or more. Critical IoT devices must have sufficient runtime to safely complete operations, transmit final data, or maintain functionality until primary power is restored.

Comparative analysis of backup power technologies for Edge IoT

The choice of backup power technology for Edge IoT devices depends on specific requirements for energy, runtime, temperature range, lifespan, and cost. Let's consider the main options:

Supercapacitors (Supercapacitors / EDLC)

• Operating principle: Store energy electrostatically rather than through chemical reactions, ensuring rapid charging and discharging.
• Runtime: Ideal for short-term backup power (seconds to tens of seconds), e.g., for safe shutdown, transmitting final data, or smoothing peak loads.
• Lifespan: Exceptionally long, up to 500,000 – 1,000,000+ charge/discharge cycles with negligible degradation, significantly exceeding battery performance.
• Temperature range: Operate reliably across a wide temperature range, an advantage over most lithium cells. For example, some models work from -20°C to +85°C.
• Cost: Higher initial cost per unit of energy compared to batteries, but lower total cost of ownership (TCO) due to long lifespan and lack of maintenance.
• Disadvantages: Low energy density compared to batteries (e.g., 5–10 Wh/kg vs. 100–300 Wh/kg for Li-ion), limiting their use for extended autonomy. High self-discharge compared to batteries.

Lithium-ion batteries (Li-ion Batteries)

• Operating principle: Store energy through chemical reactions, providing high energy density.
• Runtime: Suitable for medium to long-term autonomy (hours to days/weeks), depending on capacity and device consumption.
• Lifespan: Typically 300–1000 full charge/discharge cycles before capacity degrades to 80%.
• Temperature range: Sensitive to extreme temperatures, especially low temperatures, where their performance significantly decreases. However, some Li-ion batteries operate in a wide range, e.g., from -60°C to +85°C for certain types of lithium primary batteries.
• Cost: Higher initial cost, but optimal energy density for many applications.
• Disadvantages: Degradation over time and cycles, risk of thermal runaway in case of damage or improper use, requiring complex battery management systems (BMS).
• Varieties: Lithium-ion capacitors (LIC) are a hybrid solution combining a supercapacitor cathode and a Li-ion battery anode, offering 2-3 times higher energy density than standard supercapacitors and longer backup time (tens of seconds).

Lead-acid batteries

• Operating principle: Use sulfuric acid electrolyte and lead plates to store energy.
• Runtime: Provide long-term autonomy, often used for large backup power systems and UPS.
• Lifespan: Fewer charge/discharge cycles compared to Li-ion (less than 350 cycles).
• Temperature range: Optimal operating temperature around 22°C +/- 5°C; high temperatures significantly shorten lifespan.
• Cost: Lowest initial cost per unit of energy, making them cost-effective for large-scale systems.
• Disadvantages: Heavy weight and large dimensions, lower energy density compared to Li-ion, require maintenance (for some types) and ventilation due to gas emission during charging.

Architectural approaches to UPS integration in Edge IoT systems

Integrating uninterruptible power supplies into Edge IoT systems can be achieved through several architectural approaches, each with its advantages and limitations:

• Embedded solutions: Miniature supercapacitors or compact Li-ion batteries can be integrated directly into the IoT device's printed circuit board. This is ideal for space-constrained devices such as sensors or wearables. Supercapacitors can provide power for data saving or transmitting short data bursts in case of power failure.
• External UPS: For Edge gateways or more powerful controllers, external UPS units are often used. These can be compact industrial units or standard UPS systems that power multiple devices. The advantage lies in easier replacement, scalability, and centralized management.
• Hybrid systems: A combination of different power technologies, for example, supercapacitors for immediate response to micro-outages and Li-ion batteries for longer backup power. This architecture leverages the benefits of both technologies: rapid power delivery by supercapacitors and high energy density of batteries.

For effective power management in Edge IoT systems, standards like PMBus (Power Management Bus) are used. PMBus is a two-wire interface, an extension of the SMBus (System Management Bus) standard, which allows monitoring and programming power components, as well as real-time status monitoring. This enables tracking voltage, current, temperature, and other UPS parameters, which is critical for proactive maintenance and ensuring reliability.

Selecting the optimal solution based on deployment scenarios

The choice of the optimal UPS for Edge IoT devices depends on the specific deployment scenario and its unique requirements:

• Short-term outages (up to several minutes): For scenarios where only safe operation completion, final data transmission, or surviving micro-outages is needed, supercapacitors are the best choice. They provide high power for short bursts, charge quickly, and have an exceptionally long lifespan. Example: sensors that need to send an emergency notification upon power loss.
• Medium runtime (hours to days): For devices requiring operation for several hours or days until power is restored, Li-ion batteries are optimal. They offer high energy density in a compact form factor, which is important for many Edge devices. Example: Edge gateways in smart buildings or video surveillance systems.
• Long runtime (days, weeks) and low cost: If long autonomy is required and size and weight are not critical, lead-acid batteries can be a cost-effective solution, especially for large stationary installations or external UPS. Example: backup power for remote LoRaWAN base stations or SCADA controllers in industry.
• Extreme temperatures: For conditions with extreme temperatures (very low or high), the type of UPS should be carefully chosen. Some Li-ion batteries and supercapacitors are designed for wide temperature ranges. For example, supercapacitors can operate at temperatures that would disable most lithium cells.
• Limited space: Supercapacitors and compact Li-ion batteries in thin form factors are ideal for devices with limited space.
• High safety requirements: In critical applications where the risk of fire or leakage is unacceptable (e.g., in medical devices or explosive environments), supercapacitors should be preferred due to their electrostatic energy storage nature, which eliminates risks associated with chemical reactions.

Adherence to industry standards such as IEC 62443 (for industrial cybersecurity) and UL 508 (for industrial control equipment) is mandatory to ensure the reliability and safety of backup power systems in critical IoT deployments.

Operation and maintenance of backup power systems

Effective operation and maintenance of backup power systems are key to ensuring their reliability and extending their lifespan. This directly impacts the total cost of ownership (TCO).

UPS status monitoring

Battery management systems (BMS) for Li-ion batteries monitor critical parameters such as voltage, state of charge (SoC), and estimated remaining runtime. They can also assess the battery's state of health (SoH). For supercapacitors, monitoring includes tracking voltage and temperature. Integrating IoT sensors and cloud platforms enables real-time battery status monitoring, alerts for low charge or abnormal operating conditions, and prediction of maintenance needs. Predictive maintenance using IoT can reduce equipment downtime by up to 50% and extend equipment lifespan by 20-40%.

Replacement and disposal strategies

The lifespan of batteries and supercapacitors is limited. Li-ion batteries have a limited number of charge/discharge cycles (300–1000), while lead-acid batteries have fewer than 350 cycles. Supercapacitors, however, withstand millions of cycles. Regular battery replacement is necessary, and the disposal of Li-ion and lead-acid batteries requires compliance with environmental regulations due to hazardous substances. Supercapacitors are more environmentally friendly as they do not pollute the environment during production, use, and disposal.

Total cost of ownership (TCO)

TCO includes not only the initial cost of the UPS but also installation, monitoring, maintenance, replacement, and disposal costs. The long lifespan of supercapacitors and minimal maintenance requirements can significantly reduce TCO, despite their higher initial cost. For Li-ion batteries, although they require BMS, their high energy density and relatively long lifespan make them competitive. Lead-acid batteries have the lowest initial cost, but their shorter lifespan, maintenance requirements, and fewer cycles can increase TCO in the long run.

The AZIOT platform allows integrating monitoring data for uninterruptible power supplies (voltage, current, temperature, SoC) from Edge devices, providing centralized control and predicting maintenance needs for critical infrastructure, thereby enhancing overall system reliability. Intecracy solutions and inbase.com.ua solutions provide robust infrastructure for such integrations.

Choosing the optimal uninterruptible power supply for critical Edge IoT devices is an architectural decision that requires careful analysis of autonomy requirements, operating conditions, and total cost of ownership. Understanding the trade-offs between technologies such as supercapacitors, Li-ion batteries, and lead-acid batteries enables engineers and architects to create reliable and cost-effective systems that ensure continuous operation even in the absence of primary power. Integrating modern monitoring systems and predictive maintenance is key to maximizing lifespan and minimizing risks.