Power
Implementing circuit protection in battery-powered systems
By Mark Patrick, Mouser Electronics B
atteries allow technology to go mobile and lithium-ion (Li-ion) is the chemistry of choice in many applications, including tablets and mobiles, cordless power
tools, e-bikes and e-scooters. Its properties enable end users to enjoy the performance they expect from their tech. However, there is a known risk that the battery may ignite if allowed to overheat or if damage causes a short-circuit between electrodes inside the device. While proper design of the battery and its enclosure can mitigate the risk of fire, electrical circuit protection can effectively prevent causes of excessive heating such as voltage and current surges, and ESD events. An understanding of the primary and secondary safety risks, and the protection components that are available, can help designers ensure the safety and reliability of their applications.
Strengths and threats of Li-ion batteries
Outstanding characteristics of Li-ion batteries include high energy density, lightweight design, fast charging, high power output, long cycle life, and low self-discharge. These ensure great performance, long runtime between charges, fast turnaround time, low weight, manoeuvrability, and low maintenance in applications like cordless power tools and home appliances, e-scooters, and e-bikes. On the other hand, Li-ion batteries can overheat and release flammable gases that can ignite or even explode under certain circumstances. The main cause of overheating is thermal runaway, which usually results from a short circuit when the thin dielectric separator layer between the positive and negative electrodes becomes damaged or compromised. This can happen due to manufacturing defects, physical damage such as punctures or impact, electrical overstress, or overvoltage
14 June 2024
Figure 1: Li-ion battery safety threats and protection hierarchy. (Source: Littelfuse)
when charging the battery. In addition, overheating through exposure to high temperatures can cause internal damage, leading to thermal runaway. To maintain battery safety, it’s crucial to prevent these conditions from causing thermal runaway.
Causes of overheating
If the separator becomes ineffective, the short circuit causes a large current to flow through the damaged area and a localised “hot spot” arises within the battery. If the temperature reaches a critical threshold, typically around 150–200°C, the battery enters a state of thermal runaway. At this point, the heat generated exceeds the battery’s ability to dissipate it, leading to a rapid increase in temperature. This can cause the battery’s electrolyte to decompose, releasing flammable gases. In addition to the risk of fire, the release of gas can cause a rise in internal pressure that may cause further internal damage and raises the risk of rupture or explosion. Li-ion batteries
Components in Electronics
typically feature safety venting, which allows the gases to escape and avoid excessive pressure. However, if an ignition source is present, such as a nearby flame or spark, the flammable gases can ignite.
Other factors that can cause potentially dangerous overheating include exposure to high temperatures by leaving the battery in direct sunlight or subjecting it to heat sources as well as the use of low-quality or counterfeit batteries with inadequate safety features or inferior construction, which may be more prone to failure.
Safety standards and applicable measures
Various safety features are incorporated to mitigate these risks. They include the use of flame-retardant materials, as well as electronic protection circuits and thermal fuses. Adhering to proper usage guidelines is also extremely important.
The standards and regulations applicable to Li-ion batteries specify required safety
and risk-reduction measures. They include IEC 62133 in Europe, for portable sealed secondary lithium cells and batteries, which covers general safety considerations in relation to mechanical design, resistance to abuse, and electrical hazards; including those that can result in overheating or thermal runaway. As well as IEC 61000- 4-2 for ESD protection and IEC 61000-4-4 immunity test standard for Electric Fast Transients (EFT). For the US, there is UL 2054, which covers household and commercial batteries, and UL 1642, which applies specifically to Li-ion cells. These standards refer specifically to reducing the risk of injury due to fire or explosion. Also, subsection 38.3 of the United Nations Manual of Tests and Criteria (UN/DOT 38.3) outlines the requirements for the safe transportation of Li-ion batteries, including packaging, labelling, and testing procedures.
Primary and secondary safety Various safety and risk-mitigation features are typically implemented as part of the
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