In reply to Stephen A. Lawrence's message of Fri, 27 Mar 2009 22:49:13 -0400: Hi, [snip] >[email protected] wrote: >> In reply to Harry Veeder's message of Fri, 27 Mar 2009 11:15:39 -0400: >> Hi, >> [snip] >> >> The conventional explanation is an internal short, however I wonder about >> that. >> An internal short can't be any worse than an external short, so has anyone >> managed to cause a fully charged battery to explode simply by shorting it >> (Vorts >> with an experimental bent, and a love of fireworks, this is your chance! :) > >Dunno about that -- but the explanation I have run across for the fires >in early Mac powerbooks, and maybe some other Li batteries, is that the >steel rollers used to flatten the plates which are used in the batteries >suffered wear, and tiny steel splinters were -- uh -- "spalted" from the >rollers (I think that's the right word). The splinters, which were >*really* tiny, went unnoticed and didn't interfere with battery >operation ... initially. > >However, the plates in Li batteries are extremely close together. Under >heavy charging current the splinters would "stand up", lining up with >the applied E field, and occasionally bridge the gap between plates, >shorting them together. We're talking about a situation where the >"wire" involved (the splinter) is really thin but also extremely short, >and supposedly they'd get hot enough to set fire to or burst the battery. > >Whatever, take it or leave it, I got this orally from an electric car >conversion guy. Don't have a reference to back it up. [snip] That's the same explanation I've seen elsewhere on the web. It would concentrate all the energy of the battery in one tiny spot, resulting in a high pressure plasma. Also, the heat would locally boil the electrolyte, creating additional pressure. Furthermore, once a plasma formed, the short would increase. Perhaps that's the difference between an internal and an external short, where the heat can escape.
If such a battery contains say 1 Amp.hr @ 1.3 V, then that's the explosive power of 1 gm of TNT, which my gut feeling says would be enough to explain the results. Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/Project.html
