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Intrinsic Safety (Glossary) |
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| Cases of burning or even exploding mobile phone or notebook batteries and spectacular recall campaigns, where large numbers of these power units were exchanged (in one single case 1.6 million and since 2006 more than 10 million units altogether), raised public awareness of the safety aspects of rechargeable batteries. Contrary to the often spectacular reporting in the media, the actual damage caused by overheated brand-name rechargeable batteries has remained relatively limited and the total number of incidents involving damage has been low. However, when high-performance rechargeable batteries are used for hybrid electric vehicles the large number of interconnected battery cells required (see: rechargeable battery packs) could have disastrous consequences if they got out of control, and it is critical that safety considerations play a key role in evaluating the different rechargeable battery technologies. Intrinsic Safety Although the term 'intrinsic safety' is used, there is no standard interpretation or definition of this term with regard to batteries and rechargeable batteries. This means that the term is used to describe different circumstances and different expectations as well as different behavioural patterns that have developed, particularly where the final consumer is involved. A technical standardisation of the term is found in the ATEX Directives of the EU (Explosion Protection Directive), which refers to the inflammability of explosive mixtures through electric switches and power sources. The term also appears in DIN specifications for solar installations. However, none of these definitions is applicable or transferrable to the safety of rechargeable batteries and batteries. The intrinsic safety of rechargeable batteries and batteries is understood as a design-oriented protection against thermal runaway (see below). In an ideal case this means that there is no excessive heating up, no development of smoke, poisonous vapours or flames and no explosion in the event of a short-circuiting, exhaustive discharge, overcharging, impact, falling, vibration, destruction and in the case of extreme temperatures and air pressures. However, until the term has been standardised the ruling out of risks that such a term is meant to entail cannot be guaranteed. 'Intrinsic safety' certainly does not mean that a cell is completely safe from possible destruction nor that its functionality can be ensured nor that it is not dangerous under any circumstances. Thermal Runaway The most important danger potential emanating from NiMH and Lithium-ion rechargeable batteries is thermal runaway. Thermal runaway occurs if – for instance because of shortcircuiting – the heat generated within an accumulator accelerates exothermal (heat producing) electrochemical reactions to such an extent that in turn even more heat is generated and the heat energy produced in this way can no longer be dissipated. Such a chain reaction can cause an abrupt and big increase in temperature, as well as the generation of smoke and flames and even an explosion. The individual electrochemical processes that can lead to thermal runaway differ from one type of rechargeable battery to another. For instance, in some lithium-ion cells under unfavourable conditions metallic lithium can be precipitated. As soon as this reaches its melting point (181° C) it becomes extremely aggressive and can release a great amount of heat during chemical reactions. Other factors can be the heat-related release of oxygen or the combustion of inflammable electrolytes. Possible examples of causes of 'thermal runaway' which must be taken into consideration are, for instance, short circuiting caused by external influences or changes within the cell and also extreme exhaustive discharge, overcharging and extreme operating temperatures, etc. © Marc Stenzel |
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Keywords: accumulator accumulators secondary cell cells rechargeable battery batteries intrinsic safety fire explosions li-ion |
