Source : Mouser Electronics
The Need for Protection Elements in All Li-Ion Battery Operated Circuits Is Clear
The advent of lithium-ion (Li-ion) batteries has changed the world. Power tools that run on Li-ion batteries have excellent performance. Portable consumer items such as digital watches and cell phones require Li-ion rechargeable batteries to perform well. If you look around your home, chances are you will find Li-ion batteries in more devices than you realized.
Ubiquity of Portable Batteries
In my own garage, you’ll find a cordless drill, a cordless hedge trimmer, a cordless sander, and a cordless jigsaw. Battery-operated tools are commonplace and considered an absolute necessity in various trade industries, such as carpentry, as well as for camping and gardening. Li-ion batteries are also in demand for laptops, electric cars, and electric scooters. Residential solar battery systems also use this technology, making them vital in the uptake of renewable energy.
While Li-ion batteries have become the power source of choice, the need for robust protection from problems caused by external shorts, runaway charging conditions, and abusive overcharging is essential. In this blog, we outline the latest safety standards used to mitigate the risks to portable batteries and detail how developers can integrate products designed to protect against these risks.
Safety Standards
To meet best safety practices, UL, IEC, and IEEE have established regulations and testing requirements to demonstrate resilience to short circuit and overcharge events. Lithium batteries have higher energy densities than legacy batteries (up to 100 times higher), meaning, there is a lot more opportunity to cause trouble. They are generally safe and unlikely to fail, but only so long as there are no defects and the batteries are not physically damaged, overcharged, or over-discharged. When lithium batteries fail to operate correctly or are damaged, they may present a fire or explosion hazard. The overcharge, deep discharge, or short circuit conditions create heat that can cause a lithium battery cell to bloat, rupture, or possibly start a fire (Figure 1).
Figure 1: Battery safety includes protection against short circuits, overcharge, over-discharge, and thermal events. (Source: Mouser Electronics)
When designing with Li-ion batteries, there are several key standards to keep in mind. UL Standard 1642, Edition 6, is the Standard for Safety Lithium Batteries, and UL 2595 is the Standard for General Requirements for Battery-Powered Appliances. Also, be aware of UL 2054, which covers portable non-rechargeable and rechargeable batteries used as a power source in products, as well as the packaging of those products. Meanwhile, IEC 62133 focuses on safeguarding for fire or explosion hazards with the battery.
To ship Li-ion battery cells or packs in the US, the cells must pass eight tests in the UN DOT 38.3 regulation. Testing includes external short circuits, abnormal charging, forced discharge, impact shock, vibration, thermal cycling, and altitude simulation. To ship internationally, batteries must pass nine tests in IEC 62281, which adds a drop test.
Mitigating the Risks
From specification requirements to product testing, the safety standards surrounding battery protection help product developers reduce risks in a number of ways to ensure Li-ion battery systems have the protection they need for high performance.
High Discharge System Protection
An unprotected battery cell or pack can deliver an extremely high current if accidentally shorted. The power dissipated in the battery cell’s internal impedance can lead to a fast rise in cell temperature, but there are devices worth looking at for this type of protection.
During a high discharge current battery fault, a Polymeric Positive Temperature Coefficient (PPTC) resettable device, such as the Littelfuse PolySwitch®, rapidly heats up. As it gets to trip temperature, the device increases resistance by several orders of magnitude, limiting the fault current to a low level. When the fault condition is removed and the power is cycled, the device cools and returns to a low-resistance state. If the fault is not cleared and the power is not cycled, the device will remain latched in the high-resistance state. The 0805L Series PTC are surface-mount PolySwitch overcurrent protection devices that feature a small SMT package and trip points from 60mA to 3A (Figure 2). DCR is from 70W to 80mW, and operating voltages are from 63V to 6V at the highest current.
Figure 2: The 0805L PolySwitch PPTC protects batteries from high current discharges. (Source: Mouser Electronics)
Additionally, the metal hybrid PPTC battery mini-breaker, MHP-TAC, offers a 12V rating and small size (Figure 3). This protection device features a bimetal switch parallel to a PPTC device. The PPTC device acts as a heater to help keep a bimetal switch open until the fault is removed—providing resettable overtemperature protection. Unlike traditional bimetal protectors, MHP-TAC will remain in the latched (open) condition until fault is removed or power cycled. Its contacts are rated for 6,000 cycles at 12V and 12A. Versions are available with +72°C to +90°C trip points, and its compact size—4.75mm x 2.8mm x 0.8mm—allows for ultra-thin battery pack designs.
Figure 3: MHP-TAC Mini-Breakers serve as a heater and keep the bimetal latched until the fault is removed. (Source: Mouser Electronics)
The ITV5432 Series is a surface mountable device from Littelfuse that protects against overcurrent and overcharging. It comprises a fuse element to ensure stable operation under normal electrical current and to cut off the current when overcurrent occurs. These battery protectors also include a resistive heating element that can be combined with an external voltage-detecting IC. When overvoltage is detected, the heating element is electrically excited to blow the fuse element and provide protection.
Cordless power tools are among the most susceptible devices for overcharge, but fortunately, Littelfuse offers numerous components to ensure their battery packs are protected for overcharge conditions and more.
Voltage Transients
Figure 4: The SMF4L surface mount TVS diodes come in SOD-123FL packages. (Source: Mouser Electronics)
Another good choice for voltage transient protection is the SC24COM-01ETG 24V bidirectional TVS diodes from Littelfuse. The devices are fabricated using proprietary silicon avalanche technology. They can safely absorb repetitive ESD strikes of ±30kV without any performance degradation, while their capacitance is only 13pF.
Overcurrent
Another fuse choice for overcurrent protection that’s a bit different would be the 881 Series SMD Fuse, part of the Littelfuse Nano2® portfolio, which is a small, square, surface-mount device designed mainly as supplemental overcurrent protection for higher-current applications with sub-divided circuits (Figure 6). Rated for 60A to 100A, the fuse has a nominal cold resistance as low as 0.45mW, and an interrupt rating—the maximum amount of energy the device can safely withstand—of 1500A at 75VDC.
Figure 6: The 881- Nano2® high current fuse comes in a small square package. (Source: Mouser Electronics))
Thermal Protection
When it comes to protecting against thermal events, the KC Series of epoxy-coated miniature leaded thermistors are low-cost solutions for overtemperature detection (Figure 7). These high-reliability devices come with a base resistance of 100W to 100KW and a temperature coefficient of around -4 percent/°C at room temperature. Their resistance follows standard R-T curves, and the diameter of the bead is just 0.095” (Max).
Figure 7: The KC series of thermistors fits the bill perfectly for overtemperature detection. (Source: Mouser Electronics)
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