Nail Penetration

The nail penetration test is very important and is considered to simulate an internal short in a cell. No electronic device can protect against an internal short, so the cell 

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Na-S system/battery 571,574 nail penetration, safety tests 354 naphthalene 428... 

Generally in a nail penetration test, an instantaneous internal short would result the moment the nail is tucked into the battery. Enormous heat is produced from current flow (double layer discharge and electrochemical reactions) in the circuit by the metal nail and electrodes. Contact area varies according to depth of penetration. The shallower the depth, the smaller the contact area and therefore the greater the local current density and heat pro-... 

Figure 12. Typical nail penetration behavior of a 18650 lithium-ion cell with shutdown separator. This test simulates internal short circuit of a cell. Key (a) cell passed nail penetration test (b) cell failed nail penetration test.

Figure 5.15. Impact fracture surface of C/SiC composites [24] (a) after Charpy impact test and (b) after the nails penetrated test...

An abnormal increase in cell temperatnre can occur from internal heating caused by either electrical abuse - overcharge or short circuit - or mechanical abuse - nail penetration or crush. Higher cell temperature also could be a result of external heating. For this reason, battery packs containing Li-Ion cells are designed with safety control circuits that have redundant safety features (PTC, CID, vent, thermal fuse, etc.). Shutdown separators are one of the safety devices inside the cell and act as a last line of defense. The separator shut down is irreversible, which is fine for poly-ethylene-based separators, which melt around 130°C. 

Figure 20.16 shows the typical nail penetration behavior of a Li-Ion cell with shutdown separator. Clearly, there was a voltage drop from 4.2 to 0.0 V, instantaneously, as the nail penetrates through (when internal short circuit occur) and temperature rose. When the heating rate is low, the cell stops heating when the temperature is close to separator shutdown temperature as shown in Fig. 20.16a. If the heating rate is very high, then the cell continues to heat and fails the nail penetration test as shown in Fig. 20.16b. In this case, the separator shutdown is not fast enough to stop the cell from thermal runaway. Thus a separator only helps to avoid delayed failures in case of internal short circnit as simulated by nail and bar crush tests. Separators with high-temperature melt integrity and good shutdown feature (to avoid delayed failures) are needed to pass internal short-circuit tests.

Where PVC insulated and sheathed cables are concealed in walls, floors or partitions, they must be provided with a box incorporating an earth terminal at each outlet position. PVC cables do not react chemically with plaster, as do some cables, and consequently PVC cables may be buried under plaster. Further protection by channel or conduit is only necessary if mechanical protection from nails or screws is required or to protect them from the plasterer s trowel. However, lET Regulation 522.6.101 now tells us that where PVC cables are to be embedded in a wall or partition at a depth of less than 50 mm they should be run along one of the permitted routes shown in Fig. 3.45. Figure 3.44 shows a typical PVC installation. To identify the most probable cable routes, lET Regulation 522.6.201 tells us that outside a zone formed by a 150mm border all around a wall edge, cables can only be run horizontally or vertically to a point or accessory if they are contained in a substantial earthed enclosure, such as a conduit, which can withstand nail penetration, as shown in Fig. 3.45. 

Holes drilled in floor joists through which cables are run should be 50 mm below the top or 50 mm above the bottom of the joist to prevent damage to the cable by nail penetration (Regulation 522.6.200), as shown in Fig. 4.11. PVC cables should not be installed when the surrounding temperature is below 0°C or when the cable temperature has been below 0°C for the previous 24 hours because the insulation becomes brittle at low temperatures and may be damaged during installation. 

Ichimura M (2007) The safety characteristics of lithium-ion batteries for mobile phones and the nail penetration test. In Proceedings of the 29th international telecommimications energy... 

The battery development process included a large number of cell tests. For instance, in order to characterize the cells, the usable power was measured over SOC and temperature. Furthermore, early in the process, cell abuse tests were performed to qualiiy their usage in a vehicle production program. Such abuse tests included overcharge, overdischarge, short-circuit, nail-penetration, hot-box and crush procedures. 

Mechanical abuse through nail penetration is generally performed by forcing a nail through the cell at a specific speed thereby creating an internal short and as a result heat is being... 

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