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Electrically Powered Ultralight Aircraft
At 22:48 13 August 2007, Bill Daniels wrote:
'Dana M. Hague' wrote in message .. . On Sat, 11 Aug 2007 15:35:25 GMT, Larry Dighera wrote: I would assume that the source of these Li-ion Polymer battery fires is excessive electrical current flowing through the battery either from too high a charging rate, too high a discharge rate, or a short internal (as in the case of the Sony laptop cells) or external, or being over charged. Perhaps it would be prudent to install a circuit breaker of fuse to prevent too high a current and a timer to disconnect a forgotten charger. I imagine the root of the problem is very low internal resistance which, while making them very efficient, also allows the current to 'run away'. Good circuit design can alleviate many of the issues, but safety if the batteries are damaged in a crash is still an issue. -Dana -- Any battery chemistry, including lead-acid, can overheat with excess charging current - usually to the detriment of the battery and whatever it is in at the time. All can do damage if they are shorted. The problem with the first generation lithium cells was the chemistry released oxygen when overheated which combined with the flammable lithium made an incendiary bomb. The newest lithium-nanophosphate cells do not release oxygen and thus do not burn or explode although they can be damaged by overcharging. Cells made by A123 Systems, Saft, Valence and others are more than safe enough for use in aircraft or cars. They have a little less energy capacity than the old chemistry but they make up for it with fast charging and long life. They can typically manage a 20C discharge rate without harm - that's 200 amps for a 10 AH battery. Admittedly, you don't want to short that. Bill Daniels Here is a link to an article about some new batteries, Silver Zinc rechargeables, with 30% better energy density than Lithium ions'. They are much less volatile and much more eco-friendly, with 95% of the critical materials able to be recovered, check it out. http://www.gizmag.com/go/7743/ Paul Hanson "Do the usual, unusually well"--Len Niemi |
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