External Short

We experienced no safety problems during the external short tests because of the Polyswitch inside the cell. We confirmed that even if the Polyswitch fails to operate, the short-circuit current stops flowing before thermal runaway occurs because the micropores are closed by the polyethylene separator, which melts at 125 °C ( separator shutdown ). 

In the overcharge tests we carried out, there was no fire or explosion. The cell impedance increased suddenly in every test. This was due to the oxidation of the electrolyte with a low charging current, or to the separator melting with a high charging current. In practical applications, an electronic device should be used to provide overcharge protection and ensure complete safety. 

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In the previous ease, it was a question of a type of bimetallie eorrosion without eleetrieal eontaet between the metals involved. The proeess initially is diffieult to understand it is, however, easy to understand if one eonsiders it as a battery with an external short eireuit eaused by an eleetrolyte behaving in the same way. The results of measurements on a model and the aetion of eathodie proteetion are deseribed in Ref. 23. 

Underwriters Laboratories (UL) requires that consumer batteries pass a number of safety tests [3]. UL requires that a battery withstand a short circuit without fire or explosion. A positive temperature coefficient (PTC) device [4] is used for external short-circuit protection. The resistance of a PTC placed in series with the cell increases by orders of magnitude at high currents and resulting high temperatures. However, in the case of an internal short, e.g., if the positive tab comes lose and contacts the interior of the negative metal can, the separator could act as a fuse. That is, the impedance of the separator increases by two to three orders of magnitude due to an increase in cell temperature. 

Short-circuit tests with lithium-ion batteries have been reported recently [35]. This work shows that the separator provides shutdown when the battery is subjected to an external short circuit with the PTC bypassed. The large increase in impedance of the separator is attributed to the temperature rise in the battery. 

Figure 10. Typical short-circuit behavior of a 18650 lithium-ion cell with shutdown separator and without PTC (positive temperature coefficient) and CID (current interrupt device). This test simulates an external short circuit of a cell.

Thermalite Ignitacord. It is mentioned in Ref 57, p 207 but not described. The cord burns with an external, short, very hot flame and comes in two types 0.75 1.5 sec/in BT. 

Apart from this important feature of a corroding system, there is another characteristic that arises from the short-circuit condition of the corrosion cell and of the equivalent cell. (It will be recalled that the electron sources and sinks in the corroding metal are internally short-circuited the two electrodes in the equivalent cell are externally short-circuited.) The total potential difference V across the equivalent cell is zero. But this cell potential is composed of the absolute potential differences across the interfaces at the two electrodes and the potential drop IR in the electrolyte ... 

When the sedimentation current is measured, top and bottom of the cell are externally short-circuited. Then = 0. but now a current flows through the external circuit. Under stationaiy state conditions, at high xa. most of the counterions move with the particle, a small fraction lags behind and is removed from the top side. Just as the (small) negative excess is removed from the bottom side of the tube. 

External short tube, vertical exchanger, natural circulation OK for foaming but not for fouling, crystal formation, or suspended solids. Viscosities <20 mPa-s. Relative to agitated film retention time of 16 1 and volume 10 1. Overall heat transfer coefficient 1 to 1.5 kW/m °C. HDHI = 10 to 10 Pa s. Area 3 to 1000... 

Thyristor bridges are used where the DC output voltage needs to be varied. For example for control purposes such as varying the speed of motors or for protective purposes such as limiting the maximum DC output current that can flow when an external short circuit occurs. 

Figure 1 Externally Short Circuited Cells and Dual Phase Membranes...

Most of the primary batteries have high internal resistance, so that brief external short-circuiting is unlikely to produce extensive damage. 

External short circuit Shutdown behavior Separator shut down stops the cells from overheating... 

External short-circuit protection at the cell level in cylindrical cell designs, such as the 18650, 18700, 26650, etc. is provided by the PTC, and in some cases by a shutdown... 

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