|Re: Recovering shorted battery packs|
Message #4 Posted by Steve on 28 June 2000, 9:26 a.m.,
in response to message #3 by Andrés C. Rodríguez
Yes, high current, very short duration.
The capacitor I mentioned (160uF) charged to 200 volts stores a charge of 1/2 * C * V^2, which in this case is about 3.2 Joules.
That is the same as using a 6V 500mA power supply across the battery for a tad over 1 second.
The amount of energy is quite small.
I still urge caution (mostly because the bite you can get is pretty nasty.
Oh, and the capacitor will probably discharge at between 30 and 50 Amps (conservatively) in well under 1000th of a second.
A common fault in NiCad batteries is the formation of small dendrites that bridge the battery. A small high current burst will vaporise them. Lower currents may melt them.
I have had success with voltages as low as 50V (0.2J) but usually use higher voltages. Even 330V (which is about as high as they go) will give you only 9 Joules, but the sound is very dramatic and will scare animals and small children (and wives, neighbours, and the local police :-)
I repeat that the energy is very low, and the battery has so much thermal inertia that nothing large enough will get hot enough to rupture a battery.
Here's the voltage on 4 batteries that were zapped over a week ago, then charged for between 3 and 5 hours... 5.17V, 5.20V, 5.14V, and one with new cells (the old ones had vomited) 5.38V.
Now one that had been on charge overnight 0.55V
One belt at 118V
In the charger for a minute... The cells are .1v, .12, .12, 1.2V. Even though I'd probably give them another belt for good measure (the voltages on some of the cells are not stable), I'll charge these for a whaile and give you the results later.
Oh, and notice that the cells are not in an even state of charge. I'd be tempted to discharge the cells individually later on and see how they charge up again.
(oh, and 118V is 1.1J, or that 6V 500mA power supply for less than 1/3 sec ;-)