I'm trying to design a circuit for reconditioning NiCd power tool battery packs. I've successfully revived packs by zapping individual sub-C cells with ~18v DC, but it requires disassembling the pack. My redneck solution for a full pack is using a 3A rectifier off 120V ac giving 170v peak. This is obviously too dangerous to suggest to anyone else, and I want something more controlled.
I want to switch 170v on in controlled bursts. A mosfet rated for 1A continuous should be enough, but the cheapest option I've found so far is the IRF630 9A TO-220 (~30c ea). I also should go with a P-channel so I can switch the hot rather than ground to avoid accidental shorting, but a p-channel IRF9610 is almost double the price of the IRF630. If I can't find a cheap MOSFET, I'll probably try a 2n5401 PNP transistor. They're ~5c in small quantity, and rated for 150v/0.6a. The voltage drop across a even a 14.4v NiCd power pack should close to 20V, so it should be OK with 170v peak.
I have already looked through Vishay's mosfet catalog, but didn't find any better options.
MarkT:
This all sounds like an explosion hazard to me - if you don't want metal shrapnel embedded
in your body don't attempt this.
As I said I've already tried zapping individual cells with ~18V with nothing worse than a small burn on my finger from the spark that formed pressing the wire to the battery terminal.
There is a real hazard of getting hit by a falling meteor if I go outside, but that doesn't stop me from getting fresh air...
120V is not 18V - you've got about 40 times more power to vaporize electrolyte in
the weakest cell. Vaporizing any liquid at high power levels is an explosion. At the
very least you need to limit current to prevent dangerous power levels - but seriously
this will not restore a cell to health.
Sometimes NiCd cells get short-circuited by metal whiskers, all you need then is an
electrolytic capacitor charged up to rather more than the cell voltage to melt the whisker,
you should do this cell-by-cell anyway as you have to have access to measure each cell
voltage - otherwise you have no idea which one is doing what.
That article explains how to deal with crystalization, and I've no reason to doubt
their advice, it sounds better than risking an exploding cell.
cjdelphi:
But yeah, you don't seem to be getting the hint.
I just disagree with the degree of danger. I'm sure neither of you are speaking from experience, and I've seen videos of people zapping battery packs with high voltage welders and they didn't blow up, so I doubt there is a serious risk.
And the purpose of the forums is not for people to try and tell others they shouldn't try to do do things because there is a risk. As I said, walking outside poses a risk and nobody is telling me to stay inside...
cjdelphi:
And let's suppose someone here told you what to do...
and if just if.... the battery explodes? who's to blame if someone here told you what to do?.
You must be an american... only thinking about potential liability lawsuits.
I'm not even asking anyone about battery reconditioning. I'm looking for a cheap high-current mosfet, and included the current project I'm intending to use it on for context.
Let's suppose I didn't say what I'm planning to use it for, can anyone recommend something cheaper than the IRF630 that will handle at least 1 amp?
ralphd:
I've seen videos of people zapping battery packs with high voltage welders and they didn't blow up, so I doubt there is a serious risk.
What's a high voltage welder then? Seen lots of high-current welders though.
Seriously you are contemplating something very stupid, we are all telling you
how stupid it is, and you prefer to trust some nutter on youtube? I've seen
people on youtube use a truck axle as lathe but I wouldn't copy them!
MarkT:
Seriously you are contemplating something very stupid, we are all telling you how stupid it is, and you prefer to trust some nutter on youtube?
No, I think you guys are the "nutters". You are asserting that it is dangerous and is likely to cause the battery pack to explode, but provided no credible proof of that assertion.
My mother told me god is real, but I wasn't stupid enough to believe something without proof to back it up. You seem like the same kind of unscientific person that would believe in god despite the absence of any credible proof.
Quote from Einstein: "Unthinking respect for authority is the greatest enemy of truth."
MarkT:
This all sounds like an explosion hazard to me - if you don't want metal shrapnel embedded
in your body don't attempt this.
As I said I've already tried zapping individual cells with ~18V with nothing worse than a small burn on my finger from the spark that formed pressing the wire to the battery terminal.
There is a real hazard of getting hit by a falling meteor if I go outside, but that doesn't stop me from getting fresh air...
as MarkT already told you, 120V is not 18V , and you should be aware than touching the 120V will not only burn your finger, 120V is more than enough to kill you very quickly . It is not a theory, it is just truth.
alnath:
as MarkT already told you, 120V is not 18V , and you should be aware than touching the 120V will not only burn your finger, 120V is more than enough to kill you very quickly . It is not a theory, it is just truth.
Another person who can't differentiate between possibility and likelihood. The chances of electrocution from 120v are actually VERY low. Ask any electrician how many times they've been shocked by 120v, and whether they know any electrician that has been killed by it, and you'll find it's extremely rare. I know one retired electrician who would test for live wires in a box with is bare fingers.
One of my first experiments with 120v was when I was 8 years old, during construction of a room in my parent's basement. A life wall switch had no plate on it, and when my finger slipped and touched the hot screw and the grounded box I got a shock. I tried touching the other screw (neutral) and got no shock. I wondered if all the boxes were connected in a circuit, so I carefully touched my finger to the hot screw without touching the switch box, and then touched my toe to the electrical box of a nearby outlet. I got a good jolt but was unharmed and had a rather memorable start to learning electronics.
One of my first experiments with 120v was when I was 8 years old, during construction of a room in my parent's basement. A life wall switch had no plate on it, and when my finger slipped and touched the hot screw and the grounded box I got a shock. I tried touching the other screw (neutral) and got no shock. I wondered if all the boxes were connected in a circuit, so I carefully touched my finger to the hot screw without touching the switch box, and then touched my toe to the electrical box of a nearby outlet. I got a good jolt but was unharmed and had a rather memorable start to learning electronics.
