Howdy, folks, First of all, sorry for the "Oops". I accidentally hit the send button before composing my reply. :-( Oh well. At least it wasn't a bug in my production code. :-)
On Thu, 20 Dec 2001 14:50:01 +1000, Rob Studdert wrote: > On 19 Dec 2001 at 20:38, Doug Franklin wrote: > > > > [...] they can give you a 2nd degree burn [...] In retrospect, maybe I should explain the (US) burn scale (as far as I can recall it): 1st degree: Pain with skin reddening but no blistering 2nd degree: Blistering up to skin "ablation" (burning off) 3rd degree: "Ablation" and charring of skin, muscle, etc. Flaking away of exposed skin and muscle. > > [...] they violently start venting gas (don't know what type of gas). Oops. That should have been "start violently venting gas". When I've seen it happen, the venting is at very high pressures and flow rates. At least as high, and probably a good bit higher, than a tea pot "whistling". I'm guessing (without much basis at all) that the exit velocity of the gas approaches 20 m/s through the .75 mm vent holes in the battery's "can". Since I don't know what gas (gasses?) is (are) vented, I can't estimate the mass flow rate. > > The current delivered dropped off to zero over the > > next thirty or so seconds as the internal structure self-destructed > > from the heat and and the chemical system was depleted by the venting. > > :-) It's really quite spectacular if you've never seen it. Since the batteries are full of cadmium, I would only do a controlled test under a fume hood, though, even though I'm not sure what gasses are being vented. :-) I guess I should have also mentioned the specifics: six 1600 mAh sub-C NiCd cells, wired in series. The load was a dead-shorted (unmoving) DC electric motor. One characteristic of a dead-shorted DC electric motor is that it draws current in inverse proportion to the internal resistance of the motor (which is often near the "wire resistance" of the wire in the armature windings). So you're looking at 9.6VDC drawn across a load measured in milli- or microamps. Since V=IR, I=V/R=9.6V/10^-3 or less ohms, that's a lot of current. :-) > Keen, I just checked the specs for the 1600mAh AA NiMH cells that I use in all > my gear, the shirt circuit current is 7.75A for 2 seconds max, the cells > internal resistance when fully charged is 25 mOhm. The manufacturers always seem to quote fully-charged internal resistance. I suspect that means that internal resistance increases in a normal discharge cycle. I'd love to see the graphs their labs have of current/internal resistance during a "catastrophic discharge" like a dead-short. You _know_ they do those tests. My suspicion is that events unfold something like this: 1) The dead short is applied. 2) The battery (multiple cells wired in series and/or parallel) "see" a load near 0 ohms. 3) The battery starts delivering (even more) current by the bushel. 4) The current flow begins heating the cells of the battery. 5) As the individual cell temperatures "catastrophically" increase, their internal structure starts to degrade, causing their internal resistance to decrease. 6) Decreased cell resistance increases the overall current flow. 7) Increased overall currently flow causes more heating and more degradation. Go To Step (3). 8) At come "critical" point, the cell's internal temperature rises to the point that there's enough "ambient energy" to fuel the endothermic chemical processes that produce the gasses. 9) Shortly thereafter, the gas pressure inside the cell overcomes the rating of the valves on the cell body, and gas is vented (at spectacular rates). 10) The gas venting decreases the chemical potential of the cell's contents. 11) At some point, the decreased resistance of the (melting) internal structure is offset by the decreased chemical potential due to vented gasses and, over time (a few seconds) you end up with (effectively) an open circuit across the battery (cell). This train of thought makes me a little skeptical of the 7.75 A dead-short current per cell spec they list. :-) Unless you're only talking about the first handful of seconds of a catastrophic discharge. Just from Mark-One-Mod-Zero-Eyeball measurements, I can guarantee that the physical process "ramps up" for about five to ten seconds before gradually subsiding. > AFAIK NiMH cells have a higher energy density than NiCd cells but NiCd have a > far higher short circuit current in most cases. I believe that some aircraft > use NiCd batteries for starting? I'm not sure about the APUs (Auxiliary Power Units, used to fire jet ignitions, among other things), but NiCd definitely can deliver more current (maybe not power) per unit volume than alkaline or wet-cell. I don't know about NiMH. My understanding is that NiMH is about midpoint between NiCd and alkaline in (almost?) all useful measures. > In school we used to use charged AA NiCd cells to heat up the pocket clips on > old Parker ball point pens before sending across the room via air <vbg> For my crowd, it was a Bic lighter, forceps, a handful of pennies, and a "willing" flock of bats. You _can_ hear a bat squeal if you motivate him enough. :-) TTYL, DougF - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
