Thermal Runaway and Solar Street Lights: A Helpful Guide

The performance of battery cells is partly affected by their internal operating temperature. Each battery cell has been put to the test under severe and rigorous conditions. Still, appropriate action is required to control the deteriorations, especially those that might lead to incidents. It can have a significant impact on the safety aspect if it is not maintained. Thermal runaway is one type of degradation that may occur from temperature abnormality, and it can be harmful in extreme cases.

Overview: What is a thermal runaway? What causes it?

Thermal runaway begins when the temperature within the battery reaches a critical level and potentially initiating unwanted chemical reactions. In abusive operation or high temperature, an internal short circuit could arise between electrodes and undergo a chemical reaction with the ignited electrolyte inside the cell. Chemical reactions can run in a continuous chain pattern—raising the temperature even higher, decomposing the battery components, worsening the internal shorts, and producing more heat. In the end, the excessive temperature may eventually damage the battery itself due to overheating, or in the worst case causing catastrophic events such as fire and explosion.

Thermal runaway can be triggered by several factors, including:

• Internal short circuit, which may be caused by induced damage, inadequate maintenance, or aging components
• Overcharging beyond the manufacturers’ limit can initiate an electrochemical reaction which creates more heat
• Rapid charging that results in an overcurrent condition and increased temperature
• Ambient temperature outside the allowable range (above/below the limit) would lead to battery cells degradation and chemical reaction

Does thermal runaway happen in a solar street light?

In a solar street light, the power aspect would need careful consideration because it must fit the requirements and, of course, good performance and cost. Each battery has its benefits and risks. Therefore we should review the specifications before adopting them to our street lighting system.

Most solar street lights in the market use lithium or lead-acid battery. The lead-acid battery uses electrolytes and two electrode plates to provide electricity. The drawback of this design is that the lifespan would be greatly reduced due to degraded lead plates if discharged too much and the cycle increases. Furthermore, the electrolyte needs to be monitored and added at regular intervals as it evaporates while in use. Even though this type has the lowest price, but it is relatively bigger and heavier than the other type for the same capacity offered. With all of its cons, this sort of battery starts to be obsolete, specifically in lighting applications.

Lithium battery is much preferred since it offers greater energy density and better tolerance to fluctuating temperature than the lead-acid battery, so frequent maintenance is unnecessary. There are two types of lithium batteries typically used on solar street light systems: Lithium-Ion (Li-Ion) and Lithium Iron Phosphate (LiFePO₄).

As a negative outcome, applying a Li-Ion battery would require an extra protection circuit to regulate the voltage within cells to prevent any electrical hazards. It is also more susceptible to high temperatures, which might also make it deteriorate more quickly. In the short circuit, a highly flammable solution inside may combust when exposed to the heat. Based on this chemical composition, the Li-Ion battery is more likely to experience thermal runaway due to its instability in thermal and electrochemical aspects.

Meanwhile, the LiFePO₄ battery has improved voltage and thermal chemistry stability, making it less prone to experience thermal runaway. This type has more benefits than the other lithium batteries, including more lifespan (9 to 12 years) and a temperature range that suits better outdoor use. Overall, it has excellent efficiency in power delivery.

To minimize the risk of thermal runaway, what to look for when buying a solar street light?

Several factors must be addressed to extend the operating life of solar street lights and minimize the risk that may arise from thermal runaway:

Environment temperature and climate condition

The battery is highly sensitive to temperature, and each type has an operating temperature range that must be maintained to avoid any damage to the battery performance. Besides, climate conditions directly affect the amount of energy that can be absorbed during daylight, so the required solar panel and battery capacity can be calculated accurately based on this fact.

Battery quality and specification

We should ensure that the battery has a robust design, good quality build, sufficient technical specifications, and excellent integration with other core parts such as solar panels and controllers. Hence, the battery has an important role in determining the entire reliability. It is preferable to get the battery from trustworthy providers with the appropriate industry certifications to meet quality compliance.

Material quality and advanced features

The material used in the design should be able to withstand most weather conditions. The external and internal components, such as the fixture, should also use high-quality and durable material. In case of excessive heat from the bulb/battery discrepancy, the melting parts would generate more heat and potentially start a fire. That is why an excellent ventilation system on the solar-powered street light is essential to dissipate the heat produced by the battery. Moreover, a well-designed circuit is also a plus, as it guarantees a safer and more efficient operation.

As a preventive, most Li-Ion battery usually includes a built-in supplementary system that can be used to keep the proper voltage regulation. A controller or a Battery Management System (BMS) monitors the battery conditions and temperature while also functioning as a fail-safe mechanism. It protects the battery from overcharging and undercharging state while assuring other operating parameters are within acceptable limits. In the meantime, the LiFePO₄ battery has a lower voltage drop, and it is less dependent on these regulator systems.

Battery maintenance

Li-Ion batteries would require routine maintenance due to the higher rates of degradation over time. The capacity should be checked periodically, and if the run time within a cycle falls below 80%, the battery may need to be replaced. An off-grid controller/BMS can be used to monitor critical parameters remotely. The LiFePO₄ battery, on the other hand, does not require much maintenance. However, the installation site must be considered, particularly in terms of temperature, which significantly impacts its performance.

Nowadays, the LiFePO₄ battery is one of the most advanced ones, providing many benefits in respect of its safety, reliability, and efficiency. It also offers a temperature range that is more suited to the modern demands of the outdoor solar street light.

Final Words

• A thermal runaway is a chain reaction within a battery cell that is accelerated by an increased temperature, leading to a further increase in temperature. It’s triggered by various factors such as ambient temperatures, internal short circuits, overcharging, and rapid charging.
• For a solar street light, we must pick a battery that has a lower thermal runawy risk and meets the power requirements and environmental conditions while having fewer maintenance needs. The LiFePO₄ battery could be a great solution offering significant optimized improvements for the best safety and performance possible.
• To maximize the battery utilizations, it is essential to be aware of any anomalies or damages that may have occurred since it affects the performance greatly. Thus, the deterioration may be addressed quickly to avoid causing harm to other elements of the system, as well as humans nearby.