Secondary batteries generate electric energy through electrochemical oxidation and reduction reactions and are widely used for various purposes. The use range of the secondary batteries gradually expands. For example, the secondary batteries are used for apparatuses carried by a human hand such as mobile phones, lap-top computers, digital cameras, video cameras, tablet computers, and motor-driven tools, various electrically-driven power apparatuses such as electric bicycles, electric motorcycles, electric automobiles, hybrid automobiles, electric ships, and electric airplanes, electric power storage apparatuses used for storing power generated through new regeneration energy or surplus generated power, and uninterruptible power systems for stably supplying power to various kinds of information communication apparatuses including server computers and base stations for communication.
A secondary battery has a structure in which an electrode assembly is sealed with electrolyte inside a packaging material and two electrode terminals respectively having different polarities are exposed to the outside of the packaging material. The electrode assembly includes a plurality of unit cells, and the unit cell at least includes a negative electrode plate and a positive electrode plate with a porous separation layer therebetween. The negative electrode plate and the positive electrode plate are coated with an active material, and the secondary battery is charged or discharged by an electrochemical reaction between the active material and the electrolyte.
Meanwhile, in the case where a large impact from a pointed object of a metallic material is applied to the secondary battery, the relevant object may penetrate into the packaging material and thus penetrate into even the electrode plates respectively having the different polarities included in the electrode assembly. In this case, the electrode plates having the different polarities are electrically connected to each other by the metallic object and a short-circuit is formed and a very large short-circuit current flows between the metallic object and the electrode plates penetrated by the metallic object within several seconds. When the short-circuit current flows, a large amount of heat is generated from the electrode plates, and the electrolyte is rapidly decomposed by this heat and thus a large amount of gases is generated. Since a decomposition reaction of the electrolyte corresponds to an exothermic reaction, the temperature of the secondary battery rapidly rises locally around the point through which the pointed object has penetrated, and consequently, the secondary battery ignites and burns.
Therefore, when a new secondary battery is developed, a penetration safety of the secondary battery is verified through a nail penetration test before proceeding with commercialization. The nail penetration test is a test of loading the secondary battery on a test apparatus that may measure the temperature and the voltage of the secondary battery, intentionally inducing a short-circuit inside the secondary battery by allowing a pointed metallic nail having various diameters prepared in advance to penetrate into the secondary battery, measuring a change in the temperature and the voltage of the secondary battery depending on the diameter and the penetration speed of the nail, and determining whether the secondary battery ignites with natural eyes.
However, a related art penetration test apparatus has a problem of having to destroy unnecessarily a considerable number of secondary batteries in order to determine under what penetration condition the secondary battery generates ignition.
Also, to accurately investigate an ignition mechanism of the secondary battery, a change in heat generated by a short-circuit current should be quantitatively calculated by measuring a change in the magnitude of the short-circuit current flowing through a nail penetration point depending on time.
Also, it is required to thermodynamically analyze whether the temperature of the nail penetration point may rapidly rise to an ignition temperature by taking into account heat generated from the penetration point and a heat conduction characteristic of the secondary battery.
However, since the short-circuit current locally flows through the inside of the secondary battery for a very short time through the nail penetration point, it is substantially impossible to directly measure the short-circuit current.
Therefore, a related art nail penetration test apparatus has a limit in accurately investigating an ignition mechanism when a metallic object has penetrated into the secondary battery.