FEATURE BATTERIES & CHARGERS
INTEGRITY TESTING IN BATTERIES L
ightweight and long lasting, lithium- ion (Li-ion) batteries have proven
invaluable in product development, but they carry with them potential safety hazards, and must therefore undergo thorough testing. Such tests include: • Life cycle testing to verify how long a battery lasts and demonstrates the quality of the battery. These tests include cycle life testing, environmental cycle testing and calendar life testing. • Abuse testing to simulate extreme
environmental conditions and scenarios. • Performance testing to demonstrate
the efficiency of batteries, such as performance testing under various climatic conditions. • Environmental and durability testing to demonstrate the quality and reliability of a batter, such as vibration, shock, EMC, thermal cycling, corrosion, dust, salt and humidity testing.
TRANSPORTING BATTERIES There are also safety concerns where batteries are transported via aircraft, with manufacturers having to meet stringent requirements. The United Nations (UN) Manual of Tests and Criteria contains test methods and procedures. The sixth revised edition of the Manual was published in 2015, with Part 3, Section 38.3 addressing the requirements that apply to lithium cells and batteries. The provisions are used as the basis for transportation companies to accept batteries, and customs officials will check that the battery manufacturer’s self-declaration meets the requirement of UN38.3. However, with such a complex set of requirements, manufacturers often call on third-party specialists to verify compliance. This is because UN38.3 requires that lithium cells and batteries are subject to as many as eight separate tests. Five tests are mandatory and apply to all cells and batteries, with three being dependent on whether a cell, battery or a rechargeable battery is being tested. Tests must be completed in a defined order, primarily as this
Batteries carry with them potential safety hazards and must undergo rigorous testing
creates a method for ageing the battery to replicate its real lifetime usage. The eight tests evaluate samples for risks from electrical, mechanical and environmental conditions, and the sample must not leak, vent, disassemble, rupture or ignite. Unlike most other standards and
regulations that relate to product testing, UN38 requires that the test sequence must be repeated from the start if any of the five mandatory test phases are non-compliant. If a product fails any of the five mandatory test stages, the entire test process must go back to square one. The manufacturer must then provide new samples, as well as pay for tests to be re-conducted.
SAFETY IN DESIGN Manufacturers of Li-ion and Li-ion polymer batteries include internal protection devices in addition to the protection circuits within the overall battery pack to guard against excessive heat and pressure. Typical protection devices include: • Vent plate/vent tear away tab. Excessive build-up of pressure within battery cells is caused primarily from excessive abnormal heat generation or over-charging. The vent allows the safe release of gas. • Positive temperature coefficient (PTC) PTCs act as both a current fuse and a thermal fuse so that, when excessive current is drawn, the resistance of the PTC increases, resulting in increased heat generation. The resistance of the PTC is
selected so that it trips at the pre- determined current. • Separator. When the separator reaches its defined temperature (typically 130ºC), the pores are blocked by the melting of the material, preventing electrical current to flow between the electrodes. The separator is also known as a ‘shut-down separator’. • Thermal Fuse. Some prismatic batteries have an additional feature, a thermal fuse that limits the current under fault conditions. A protection circuit (PCM) is usually
fitted within the battery pack consisting of a custom designed integrated circuit that monitors the cell and prevents over-charge, over-discharge, and over- current. This in combination with two Field Effect Transistor (FET) devices to control the charge and discharging. Also present is a temperature sensing device (thermistor) designed to invoke protective action via the control IC in the event of an over-temperature scenario.
TECHNOLOGY EVOLUTION The development of Li-ion technology has played a significant role in the pace of technology evolution and today’s user demands that mobile devices and other technologies give them increased functionality with portability. Li-ion has helped manufacturers deliver on that. While Li-ion batteries still have some
disadvantages, an improvement of manufacturing processes through the introduction of more robust standards, as well as increasing consumer understanding of how to respect the batteries, means that the safety of Li-ion has dramatically improved. However, safety concerns still exist, particularly in connection with their transport in aircraft. UN/DOT 38.3 details testing requirements that all lithium cells and batteries, and manufacturers of lithium batteries and products using lithium batteries, must account for these testing requirements in the design, manufacture and distribution of their products.
www.tuv-sud.co.uk
30 JULY/AUGUST 2018 | ELECTRICAL ENGINEERING
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