NiCd Battery Repair?

[krivx]

I have some NiCd cordless drill batteries that won't hold a charge. They are a few years hold but have had very few charge/dis-charge cycles. I have seen videos of people "zapping" batteries with higher DC voltages to revive them, what is the mechanism that makes this work? If it works, what are the voltage and current requirements?

[poot36]

I would start with a transformer based 12V 1A plug pack and see where that gets you.  If you are careful you can try using a camera flash circuit to zap them (use the output of the high voltage cap around 250V to 350V or so).

[IanB]

I have some NiCd cordless drill batteries that won't hold a charge. They are a few years hold but have had very few charge/dis-charge cycles.

This is normal for NiCd batteries. If you leave them sitting around they discharge themselves within a few weeks or months. I have a brand new NiCd cordless drill battery that refused to hold a charge from new. I "cured" the battery by getting a replacement lithium ion battery after I got frustrated that the NiCd battery was always flat when I went to use it. The lithium battery holds its charge just fine for months at a time and is always ready to go when I want it.

I have seen videos of people "zapping" batteries with higher DC voltages to revive them, what is the mechanism that makes this work? If it works, what are the voltage and current requirements?

"Zapping" NiCd batteries only has any effect in the one specific case where the battery has grown metal whiskers that have shorted out the electrodes internally. This symptom is caused by long term trickle charging and is not that common.

In the rare case a battery actually has whiskers, applying a high current source charging voltage to it may melt the whiskers and temporarily revive the battery. But the whiskers will soon grow back again and the battery ultimately will need replacing.

[krivx]

I have some NiCd cordless drill batteries that won't hold a charge. They are a few years hold but have had very few charge/dis-charge cycles.

This is normal for NiCd batteries. If you leave them sitting around they discharge themselves within a few weeks or months. I have a brand new NiCd cordless drill battery that refused to hold a charge from new. I "cured" the battery by getting a replacement lithium ion battery after I got frustrated that the NiCd battery was always flat when I went to use it. The lithium battery holds its charge just fine for months at a time and is always ready to go when I want it.

I don't mean that these batteries won't hold a charge for weeks, they are drained within an hour of charging. They do seem to deliver a normal amount of current for a short while after charging though. Unfortunately a lithium battery would cost more than a new drill...

I have a HP bench supply here, I may just try "zapping" one of the batteries. I don't really have anything to lose.

[macboy]

The best result will come from combining the remaining good cells from two packs to build one good pack.

Start by charging the batteries, preferably a good old overnight trickle charge. If you only have a fast charger, you can use a bench power supply. Set it to about 100 mA and a voltage at least 1.5x the nominal pack voltage (18 V for a 12 V pack), and charge 24 hours. Don't worry about over-charging, NiCd are happy to be overcharged at this low rate. Now that you have all cells fully charged, wait and check which cells are self-discharging. You will need to open the pack to have access to all individual cells. I'd just let it sit overnight and then check which ones are still over 1.2 V when put under a load of ~ 0.5 A. If you have only one or two bad cells in the entire pack, the easy fix is to just short them out (after discharging any residual energy from them) by soldering a wire over them. Then your 18 V pack become 16.8 or 15.6 V, but it will work just fine. If you have several bad cells in each pack, combine the good ones into one pack. You can cut the spot-welded tabs that connect the cells, and solder those tabs together in the new arrangement (never try to solder to the cells).

You can also try to zap life back into the bad cells. These often have a high internal resistance, which can be overcome with a lot of current. You will need to do this per-cell, not on the whole pack. For sub-C cells (most common), I use a 20 A bench supply. The bad cells will show a high voltage at first (over ~10 V), but as the internal resistance drops, the voltage will drop to 2 to 3 V. This usually takes 2 to 5 seconds. Be aware of cell temperature and stop if it becomes more than warm. This zapping can make the pack chargeable and usable for a while, but within weeks, it will return to a bad state.

The best advice to maintain good life of multi-cell NiCd and NiMh packs, is to stop using the device as soon as the voltage starts to drop. By the time you notice that the drill is slowing down a little, one or more cells are completely drained while others still have some charge in them. Continuing to use the device will force current through those drained cells, effectively charging them in reverse and causing damage. Don't try to get that last 5 or 10 % of charge from the pack, just stop and recharge.

[krivx]

That sounds like great advice, thanks

I may take a look at the charger as well, I'm curious if it's damaging the cells somehow. I have probed it before without a battery connected and measured at ~18V but I don't know if that's a constant voltage or if it hit a rail in constant current mode.

I'll do a little reading, can anyone recommend an intro to battery chemistries and charging methods? Also, is it possible to source replacements for dead cells, or do they need to be closely matched to the existing cells?

[johansen]

refill them with water.

[krivx]

Isn't that only for lead-acid batteries?

[IanB]

I'll do a little reading, can anyone recommend an intro to battery chemistries and charging methods?

Battery University is one of the best resources out there.

[SeanB]

http://www.battcon.com/ArchivePapers.htm

Good source of papers, and there are a lot of methods and usage papers there as well.

[johansen]

Isn't that only for lead-acid batteries?

nope, they still make car battery sized ni-cd cells for locomotive starting and other purposes, and they are refillable.

good luck figuring out how to do it with a C or D cell though.. you may be able to rip the positive tab off and under it, pry up the spring loaded vent and inject water in with a syringe. or just drill a hole in the case with a carbide pcb drill, inject water, then solder over the hole.