Using the procedures discussed here and here (which I skeptically suspected might be Internet disinformation), I just used a welder to zap the “dead” (meaning no longer chargeable with the charging unit) battery packs for my rechargeable cordless drill by injecting direct current at high amperage.
To my utter amazement, this immediately brought a battery pack from absolutely zero volts on the tester up to around 20 volts!
According to this post at a welders’ bulletin board, here’s why it works:
This use of a welder to recover ailing or dying NiCd batteries is Beautiful!
It seems that the momenatary burst of high amperage burns away the ‘soft’ short circuits inside the cells. It’s these semi-short circuits that drain away the cell’s power. The weak cell charges fast and the battery voltage looks good… until you put a load on it, then he battery voltage quickly drops and your power tool is useless. Left sitting, a fully charged NiCd drops in voltage over a few days and you go to use it – it doesn’t last long! Once the cell has lost its charge, it acts like a resistor to the flow of current from all the other good cells. Your power tool slows down early because it’s not getting full voltage or amperage – because the power is going into heating up the weak cell(s).
Any ideas whether that’s right? It seems to me that if the batteries are capable of taking a charge, the charger ought to be able to do the job, but what do I know?
Comments
8 responses to “Zapping dead batteries back to life?”
THIS IS POTENTIALLY DANGEROUS. Wear safety glasses or a full face shield if you try this. Batteries can EXPLODE. (And don’t try it with anything other than a NiCad, Li ion batteries are even more explosive)
I don’t know about the claimed mechanism for failure. Most NiCad failures I’m familiar with are caused by volatiles boiling out of the cell. If the cause is whiskering than it’s possible that zapping the cell with a high current pulse will clear the whiskers. I don’t know if a welder is the best way to do this. I’d be tempted to try a discharging a big-ass capacitor through the cell. The idea seems to be to dump enough current into the cell to blow the micro short circuits, a conventional charger won’t deliver enough.
I’m well aware of the potential danger, so I wore both glasses and a welding hood, plus jacket and gloves. I think it might be whiskers, but I still don’t understand why the charger wouldn’t work. (These NiCads are 20 years old….)
I thought “whiskers” were a lead-acid thing.
DO NOT TRY THIS with Li-ion batteries!!!! Or Li-anything. NiCd is fairly benign.
Whiskers are universal. Some grow faster than others. Tin for instance.
Back in the “old” days a car battery was used. Or a single cycle half of an AC line hoping you got the polarity right – #36 or so wire as a fuse.
Way back when, the way to do this was to charge up a big capacitor and discharge it into the cell. This would burn away the whiskers, but the short-duration pulse wouldn’t risk blowing up the battery. Its best done blasting each individual cell, not the entire battery. Almost certainly a NiCad and possibly NiMh thing; almost certainly less so with LiIon or LiPoly
The RC community has been dealing with these things for years, although it usually takes the form of dead shorting the cells to get rid of the dendrites. However, that still requires cells that will hold a bit of a charge. And it only works for NiCd and NiMH chemistries.
The charger won’t do it because it only puts out a few hundred mA. You need hundreds of amps to blow the shorts. Which is what makes it so dangerous.
I set my arc welder to 130, and it worked.