NiCd. The ones I did were mainly AA. Sometimes the de-whiskering lasted for a long time (I had one which was still OK 3 years later), but sometimes it was only for a while.
After a successful splatting, the battery voltage will be around 1.1V. I then connect an ammeter across the battery, and observe the short-circuit discharge current. Typically it is initially around 5A, and rapidly falls to near zero. I then immediately measure the open-circuit voltage. If the battery is fixed, the voltage will initially be around 0.3V, but slowly creeps upwards. If the battery is not fixed, the voltage falls.
If the splatting has been successful, I immediately put the battery in a charger. If, after two or three tries at splatting, you haven't been successful, the battery's a goner.
As for NiMH, I have done the same (although I'm not sure what the failure mechanism is). Again, splatting does sometimes does bring them back to life.
I must repeat that is anyone does this sort of thing, they must be careful not to overheat the battery. They certainly wouldn't want any nasty accidents with it exploding.
The article I read (15 years ago) proposed that a charged cell would blow its own whiskers clear, where a flat one wouldn't - so eventually the whiskers were too many/thick enough to be unremovable.
When I sold an air rifle a couple of years ago, no-one would ship it. (Or advertise it, for that matter.) In the end I sent it by Parcel Force marked "car parts".
The articles on this subject of dendritic crystal growth in NiCd cells (due to trickle charging with pure DC at very low charge rates) some 30 or 40 years ago, recommended the use of a large electrolytic capacitor (15v 100,000 microfarad?) charged to 12 volts being applied across each cell so as to (literally!) blast the dentritic whiskers into isolated globules which, under low duty cycle pulse charging, would dissolve into the electrolyte and be redeposited back onto the cell electrode.
The use of a large value capacitor was recommended over the alternative of applying a 6v lead acid battery to the individual cells for a second or so, totally eliminated the explosion hazard whilst providing the very high current pulse required to clear the dentritic short circuit.
Low duty cycle pulse charging is recommended to reduce the rate of regrowth of dentritic crystals regardless of whether you're recharging such 'rejuvenated' cells or brand new ones. Pure DC is the worst possible way to charge _any_ type of electrochemical cell regardless of its chemistry, particularly when it involves trickle charging regimes.
For modestly low duty cycles of pulsed charging (say 10% at the C/2 charge rate, you can use the same average charging current as for fullwave/halfwave rectified charging current provided by most simple mains chargers.
When you go to more extremely low duty cycle pulse charging (e.g. 1%) you need to derate the average current to compensate for the extra I square R heating losses, depending on the charging rate used (more important when it's a fast 2 hour rate than for a medium 10 hour charging rate).
Whatever the sophistication of the charger used, all elecrochemical 'batteries' have a limited life in regard to charge discharge cycles (eg. typically 1000 to 2000 for NiMh AA cells) and even just simply an expiry date from manufacture.
This has been quite a problem for us aeromodellers recently..
Sending lithium polymer batteries has been almost completely banned by the airlines, who regard the fire risk as too high. As almost all Royal Mail first class post travels by air instead of by train nowadays they prohibit them unless they are an integral part of a machine, such as a laptop or mobile telephone. I understand that RM scans or X-rays all post that travels by air.
The airlines do have a point because Lipos can burst into flame if improperly charged or shorted and the fire can't be put out because all the requirements for fire are contained within chenistry of the lipo cell once it's alight. There's a chap I know of not far from me whose whole house burnt down as a direct result of a lipo fire. Another recent event destroyed the freezer the battery was being charged on and many other things in the garage were written off, including some models.
Most carriers used to have no problem with either nicads or nimh but they seem to be twitchy about these too as well, despite the risk being considerably lower than with Lipos because they tend to just expolde a bit (they go pop), horrible chemicals leak out (but not much in volume) and their temperature rises to levels that might cause surrounding materials to char. I have first hand experience of this and it all ended OK after I removed the pack from the car boot, put it on the wet grass to cool and covered it with a waste bin in case of any more popping. The cause was rain getting into the charger.
Lithium ion are much safer than lipos but still present a formidable fire risk if shorted. All recharchable cells (except the very small ones) can produce thousands of amps in this condition.
When nimh cells were the cell of choice in competition flying it was common practice to 'zap' them individually using a huge bank of capacitors. Current used was about 200A. The leads 'jump' when contact is made with the cell! This considerably reduced the internal resistance of the cell which made the voltage hold up bettter at discharge rates of several hundred amps in the air. Nowadays these sort of competitions use lipo only after some fairly scary nimh explosions in flight. They now run quite safely at about 400A on lipos and have had to introduce watt hour limiters to level the playing field.
Those who sell lipo packs in the UK now have to find a courier service that will deliver them and there would appear to be quite a few, judging by the various firms that have delivered cells to me. I've not sent any to anyone so can't recommend any particular firm.
I recently had to post off a mobile for repair, and I was sent suitable packaging and detail instructions on what to do - leave the battery in, but turn the phone off. Actually the last bit wasn't a problem - it would no longer turn on.
At the Post Office, I was then asked "Have you removed the battery?" I wasn't sure if it was a trick question. They did accept the parcel.
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