Olin, I agree that it sounds suspicious that one would use 300V to restore NiCd's, but I think that Al meant microfarads when he wrote Mfd. You should know that it was once common for capacitors to use the abbreviation Mfd for microfarad, even though it could cause a lot of confusion (potentially - although one should use common sense about the impossibility, given current technology, that Mfd could mean either Megafarad or even millifarad when one is talking about 100 millifarad at 300V in a small package). 100 microfarad would only be 4.5 Joules when charged to 300V. Sean On Sat, Jul 25, 2009 at 3:51 PM, Olin Lathrop wr= ote: > Al Shinn wrote: >> About NiCds - when a NiCd battery dies due to whiskers shorting out >> the cell, a common practice is to discharge a large high voltage cap >> into >> the battery - blows out the whiskers and restores function. >> >> Voltage- =A0 about 300V with =A0about 100Mfd capacitor > > That's obviously absurd! > > 1/2 (300V)**2 * 100MF =3D 4.5TJ =3D about 1 kiloton of TNT equivalent, or= a > large fleet of B-52s fully loaded with conventional bombs, or about 1/15 = the > energy that leveled Hiroshima in 1945. > > Assuming you really meant millifarads instead of megafarads (additionally > assuming "fd" was supposed to mean Farads), that's 4.5KJ unleashed on the > battery in a fraction of a second. =A0That's still rediculous when you > consider that's enough energy to launch a normal 5 pound clay brick 300 f= eet > into the air. > > Don't try this at home folks. =A0This is messed up advice on a number of > fronts and could get you seriously hurt. =A0This guy can't even get basic > units right, so anything else he says is best disregarded. =A0You don't w= ant > the equivalent of a 5 pound brick falling 300 feet onto your battery, and > you don't want to be nearby if you did try it. > > Even with a lower capacitance, there is no need for 300V. =A0A NiCd cell = is > only 1.2V. =A0The point is to produce a short current pulse to blow out t= he > little whiskers, or dendrites, while minimizing any other damage. =A0It > doesn't take 10s of amps to do this. =A0Ideally the damaging high voltage= is > gone just as the dendrites fuse open. =A0That's tricky to arrange since t= he > amount and strength of the dendrites is unknown. =A0It's best to start at > 5-10V on 1mF and work your way to higher energies until the dendrites blo= w. > Increase the energy by 2x each time, but 15-20V is max. =A0Use a higher > capacitance if you get that far and they're still not blown. =A0If 20V and > 10mF doesn't work there is little point trying to salvage the battery. > > > ******************************************************************** > Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products > (978) 742-9014. =A0Gold level PIC consultants since 2000. > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist