Battery Zapper Project

[jaxart]

I am looking for some help in planning a NiMh battery zapper circuit. I have the fundamentals down but am looking for a little help to get the project tweaked and ironed out.

I would like to charge a 47,000uf cap to approx 80 or 90v DC and then discharge the cap through the battery. The batteries are approx 1.2v and 3300mAh rated with an internal resistance of approx .002 to .003 Ohms. The circuit should discharge the cap through the battery in less than a few milliseconds and should be able to withstand the current associated with the discharge.

The other problem would be the circuit to actually charge the cap since I want to be able to operate the unit off of 12v DC.

Any help would be greatly appreciated.

Thanks

 

[evandude]

someone correct me if i'm wrong, but I thought 'zappers' were only for NiCd batteries? to reverse the memory effect? but... NiMh batteries don't really have a memory effect to speak of, do they?

if you're sure that this circuit is appropriate for NiMh batteries... you can get the 80 or 90 volts using some charge pump circuitry... I'm sure you can find some circuits around the net for that.

As for switching, you'll probably want a powerful FET of some sort, as 90 volts across a near-zero load like that would produce immense currents. at the very least you're going to want a high-power resistor in there to calm things down a little. Any idea how much current you're going to need? Also, I do hope you're going to be using capacitors that can handle over 90 volts.

in any case, it sounds like a fun project but probably something you should try out in an explosion-proof chamber the first couple of test runs and if it doesn't work out as a zapper, it will probably at least work out as a battery exploder!

 

[jaxart]

What about the discharge circuit?

Should I use some sort of SCR or Solid State Relay the high surge current or do you have some other suggestions?

 

[evandude]

I'm not really an expert of high-power switching like that, by any means. i'm sure someone else could offer a lot more help there. I have used MOSFETs for switching solenoids, but that hardly compares to switching the kind of load you're talking about.

 

[Johnson777717]

NiMh batteries don't really have a memory effect to speak of, do they?

I've read that the NiMh batteries don't have a memory effect to speak of.
**broken link removed**
(see chart in section/page 5.3, also good info on discharge rates)

NiMh battery info:
**broken link removed**

NiCd discharge and revival info:
**broken link removed**

 

[jaxart]

Thanks for the links.

I still want to go forward with this since I have seen some results other people have had with zapping NiMh batteries. It seems to lower the internal resistance enough to raise the output voltage over the discharge cycle.

I would only be doing this on new cells as well.

 

[ante]

The reason for zapping the cells is usually to burn off internal shorts and has nothing to do with memory effect. I would get a big SCR and use it to “fire away” the charged caps.

Ante :roll:

 

[zevon8]

I agree with ante, a really big SCR would be best. I'm talking 3/4" stud mount size, like a Powerex T600. You will want to parallel up a bunch of capacitors also, to lower the ESR, and use good "power supply grade" caps, with high ripple current ratings. I have seen similar circuits in action, and when things go wrong , they tend not to go quietly.

BANG!