NiCd discharge: Trying to restore an old battery. Did I reverse charge a cell?

[lmester]

I have some 6V NiCd batteries that are unused but have been in storage for about 10 years. I've been trying to get them restored and ready for use.

I have a fancy LiPo charger that will charge and charge/discharge cycle several battery types including NiCd batteries. I ran each battery for five charge/discharge cycles. The charger is set at 1A max charge current and 1A max discharge current Three of the batteries quickly reached or exceeded their rated capacity (2.2Ah). So far, the best batttery has a 2.7Ah capacity.

The fourth battery is doing poorly. Only 1.3Ah capacity reported by the charger.

From what I've read online, several low rate charge and dischage cycles can get NiCd's going again. I decided to go "Old school" and manually charge and discharge it using lower rates than my fancy charger would allow. I charged it with my bench power supply at .1C constant current (220mA). I let it charge for about two days.

I then discharged it. I started with a 1.6A discharge. I was planning on dropping the discharge rate once the voltage started to drop off. I'm impatient! Starting with a low discharge current would take forever! Oh Crap! I was watching YouTube instead of the battery voltage. I noticed that the voltage was now down to about 4.6V. I was planning on going down to a low discharge rate at 1V per cell (5V total). I removed the load and watched to see how the battery voltage recovered.

The voltage quickly came back up to about 4.7V. It just stayed there. I thought that I might have shorted a cell because of reverse charge. About three minutes later the voltage jumped up to about 5.2v. The voltage then kept slowly recovering.

I was logging the battery voltage. It looks like I nearly killed a cell in this battery. Look at the graph and let me know what you think.

Does anyone have any better info on restoring an old NiCd? I probably can't complain. After 10 years in storage all of these batteries should be junk!

[Gyro]

Well NiCds can be stored in any state of charge, and wouldn't have suffered any cell reversal in storage as each cell would have discharged independently - the danger has come from your discharge tests.

I'd start by treating them really gently... Give them a constant current charge at C/10 (220mA?) for at least 14 hours - in fact I'd double that, they can stand that long term without gassing. It's the best way of equalizing the cell capaciities and allowing any accidentally reversed ones to catch up with the others (as best they can).

I'd also limit your discharge test to 1.2V per cell to ensure that you don't get any further cell reversals, that's the one thing that will surely kill the packs.

Sorry but it really is a case of patience.

[brainwash]

I've never been able to recover dead NiCd or lead-acid batteries. And I've tried everything, including 'pulser' circuits. It's an exercise in futility, I think. Even if you get them to hold a voltage they will not be able to source any significant current.
What 'looks' like it might be working is to set your smart charger on NiMh mode and some decent current. The charger will then overcharge the cells until it can no longer push any charge into them, or they get really hot. So it has to be monitored. But again, I doubt you'll get a significant usage charge out of them and it will likely destroy them.
For NiCd, I've run experiments on each of the cells (drill battery) to see what brings them back to spec and everything failed. Given the choice, I would convert everything to LiPo, it's smaller, so 2 cells + charging circuitry would fit easily into the same enclosure and provide more bang per buck. You just have to make sure you don't run them completely down, or buy batteries with low-voltage protection.

Again, it's only my experience (>>10 batteries) that says you cannot bring capacity back. I'd love to be proven wrong.

[johansen]

i would drill a hole in the battery and add water, then solder over the hole. question is, how much water to add. I would add enough to saturate the plates and that's it. so fill it with a syringe, and then suck out any excess. keep the batteries upright because the vent is under the positive post. if they are upside down then sodium hydroxide water will come out when it vents.

nicd usually fail from overcharge, water escapes and impedance increases.

[lmester]

Sorry but it really is a case of patience.

I do need more patience! I'll use the smart charger to test the rest of the packs. It won't get distracted and allow them to over discharge.

I've never been able to recover dead NiCd or lead-acid batteries.

I've also had poor luck recovering bad NiCd batteries. When I had cordless tools with NiCd's, a cell in the battery would short out. You could blow open the short with a charged capacitor. A few charge cycles later it would short again. I was hoping for better luck with this battery since it was new. I should say 10 years old but unused.
 

Given the choice, I would convert everything to LiPo, it's smaller, so 2 cells + charging circuitry would fit easily into the same enclosure and provide more bang per buck. You just have to make sure you don't run them completely down, or buy batteries with low-voltage protection.

Again, it's only my experience (>>10 batteries) that says you cannot bring capacity back. I'd love to be proven wrong.

If I were buying new batteries I'd choose NiMH or Li. Can you eve buy NiCd's today? I think the cadmium is now a hazardous waste. These batteries are freebies from my workplace. They were replacement batteries for emergency exit lights. The lights that used them were replaced years ago. The batteries were of no use at work so I decided to bring them home and see if 10 year old batteries were still good. So far only one pack is weak. I can't complain!

[brainwash]

You can still buy NiCd, but here (Germany) the specialized NiCd are more expensive than generic LiPo. The only advantages NiCd have is that they can be run down completely and do not require a fussy charger. But both charging and protection circuits can be bought cheap, at least as long as the trilateral post agreements are in place.

[Gyro]

The only advantages NiCd have is that they can be run down completely and do not require a fussy charger.

I think it is still the case that NiCd have lower internal resistance than NiMH, so can provide higher peak currents, other than that there's no advantage (in fact dendrite growth makes reliability worse). You can run individual NiCd and (I think) NiMH cells flat without damage, In fact large NiCd cells for aviation use used to be supplied by the manufacturer with shorting straps fitted.

The trouble comes as soon as cells are built into series packs, voltage reversal on the weakest cell (there will always be one, no matter how well matched), during deep discharge, is the killer.