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Recovering shorted battery packs

Posted by Steve (Australia) on 19 June 2000, 2:40 a.m.

WARNING - this technique is not for the faint-hearted, and involves some personal risk. But as long as you're not a few snags short of a barbie (i.e. you have you're wits about you) it can be done safely.

What you require:

1) a dead NiCad battery pack. (Say the one out of the HP-97 calculator). It's important that the nicads be in good condition (not leaking).

2) a high voltage low impedance power supply. Something that can deliver say 200 volts at 20 amps should be fine. Seriously!

3) a multimeter (preferably digital) capable of reading DC voltages between tens of mV and 350 volts.

4) something that will charge the pack.

5) No fear of zaping sounds.

Nicads that have been left for a while tend to internally short. The typical symptoms are a pach that charges, but only gets to a voltage of (say) 2.75 volts instead of (nominally) 4.8 volts.

If these cells are subjected to a high current, but short duration pulse, the internal short can be removed.

I use a disposable camera flash unit. I solder a pair of heavy duty well insulated wires across the capacitor. The other ends are tinned. Note that figure 8 cable of some sort (preferably mains rated) is best.

PRECAUTIONS: Never place yourself in the position where the capacitor can discharge across your chest. In other words, don't hold the leads one in each hand. Similarly, don't measure the voltageby holding one probe in each hand. If you're paranoid, keep one hand in your pocket. The voltage is only 350 at a max, but the cap can deliver in excess of 50 amps!!!! While you'd have to be very unlucky to be killed by it (the total energy is quite low) it would scare the crap out of you. Also be careful that the battery dosn't rupture (unlikely, but remotely possible).

Not scared?

First take the flash unit out of the camera. Be careful, the charge on the cap is enough to kill you if correctly applied! Take out the battery and short the capacitor (the big -- usually black -- cylindrical object with 2 wires coming out of one end) with an insulated screwdriver. BANG! (if charged, it _really_ does).

Some of the capacitors have markings on them. A 330 (or 350) volt 160uF capacitor is common and the right value. These are about 1/2 inch in diameter, and about 1 1/2 inches long. There are some smaller ones in newer cameras that are about half the size, but these should be OK too.

Now solder together whatever contacts are required to make the thing charge. As soon as you insert a battery it will start to build up a voltage. Then solder the wires onto the capacitor. At the same time you can remove the wires that go to the trigger -- or just let them flap around.

At this point, if you insert a battery, you have a cattle prod. I would NOT suggest trying it out in this mode!

Insert the battery for about 2 seconds. You should hear a whine, and hopefully not zap yourself. Take the multimeter and measure the voltage across the wires. It should be between about 30 and 100 volts, but this depends on the state of the battery, and the type of flash unit.

I have found that a charge of around 100 to 150 volts works well (and it's not critical). (Use a lower voltage for smaller batteries)

Charge up to this voltage, and place the +ve wire on the +ve terminal of the battery, and the -ve lead on the -ve terminal. BANG! This will leave a small mark like a spot weld on the terminal -- don't worry.

Now place the battery on charge for 5 minutes, and check the voltage. It's probably OK. If not, open the battery pack and check each cell. A cell with less than 150mV after a short charge is certainly dead. Give it a blast!

It's also a good idea to completely discharge the pack, cell by cell, to ensure that all cells will be charged equally. Another good check is to charge, then use the pack, then check that all cells have the same (or very nearly the same) voltage across them.

This technique is faster and cheaper than rebuilding a pack, but PLEASE be careful.


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