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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 10th Jun 2021, 6:49 pm   #1
Jon_G4MDC
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Default Recovering capacity in NiCad Cells?

I have looked around the forum using the search tool for any posts on this subject. None found so far.

I want to recondition some NiCad cells for a 1990s hand portable transceiver.
I have tried fitting it with replacement NiMhs and they were hopeless. Even when freshly charged the internal resistance is so high that the current required for TX drops the battery Voltage to the point the set signals the battery alarm and will not TX.

I used to have a source of replacement NiCad cells, sadly no longer, so I am left wondering how recover capacity of the cells that I have. They have varying off load Voltage and similarly varying short circuit current delivery. Some had developed shorts due to internal crystal growth but it has been possible to clear these.

I have read a paper which recommends slow discharging to about 0.4V per cell to clear crystals. Has anyone tried this? Does it work?

Please no NiMh/LiIon other chemistry suggestions. This is about NiCads.
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Old 10th Jun 2021, 7:28 pm   #2
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Default Re: Recovering capacity in NiCad Cells?

In times-past we shot a brief burst of high current through failing NiCd cells; a H4 car-headlamp bulb and a 12V car-battery were involved [the bulb acting as a current-limiter in the event the cell was totally short-circuit].

30 seconds of bulb-limited current into a single "D"-cell was the usual treatment. This was for "Portapack" Pye Westminsters on 86MHz AM.

Sometimes it worked; usually it didn't, and I would never assign a 'zapped' battery-pack to one of my better-paying clients!
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Old 10th Jun 2021, 7:33 pm   #3
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Default Re: Recovering capacity in NiCad Cells?

I have done a similar thing in the past using a 30V bench power supply and a very large capacitor to provide a brief, but heavy, pulse of current. It did get a dead 4Ah SAFT cell working, but it was not a permanent cure. In my case the fault was the type caused by dendrites making short circuits between the plates, but they do tend to grow back again.

I understand a different type of fault is caused by electrolyte loss. A work colleague said he got the rechargeable nicad in his Russian multimeter going by drilling a hole in the case, introducing fresh electrolyte, and sealing the hole with a steel self-tapping screw.

Last edited by emeritus; 10th Jun 2021 at 7:41 pm.
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Old 10th Jun 2021, 7:45 pm   #4
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Default Re: Recovering capacity in NiCad Cells?

The original NiCd charging circuit will have problems with NiMh. Are they a standard size (AA, AAA) if so cobble up a pack and use "pre charged" (low self discharge) NiMh cells and their charger.
 
Old 10th Jun 2021, 8:02 pm   #5
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Default Re: Recovering capacity in NiCad Cells?

As said, even when cleared the crystals/dendrites grow back in the same place. I have used a couple of big splats from a car battery to recover a rogue cell in a cordless drill with short term success. Not sure about medium term success as shortly after i converted the drill to alternative chemistry.

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Old 10th Jun 2021, 8:29 pm   #6
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Default Re: Recovering capacity in NiCad Cells?

I read a paper many years ago which recommended that NiCd's were charged using a "9 units in, one unit out" regime. In other words, with rectified AC, deliberately introduce reverese leakage into the bridge rectifier. This was claimed to solve the problem of dentrite formation.

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Old 10th Jun 2021, 9:05 pm   #7
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Default Re: Recovering capacity in NiCad Cells?

A former work colleague who had done battery research at GEC gave similar advice. An excellent way to kill a set of NiCads was to charge them from a regulated bench power supply.
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Old 11th Jun 2021, 9:54 am   #8
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Default Re: Recovering capacity in NiCad Cells?

The zap of heavy current to clear shorts caused by crystal growth definitely works.
It's how to prevent regrowth and how to regain capacity that is my interest.

This is said to be the benefit of a discharge to 0.4V per cell but no lower or it risks polarity reversal. It is also said that the crystals grow when the cell is left charged for long periods but not doing any real work.

I don't think any electrolyte has been lost since that can always be detected by the dreaded green crystals. No sign of that.

The cells are tagged and soldered in a custom battery pack (Motorola). The charger is for NiCad chemistry. It's recover these cells or find new ones (unobtainium most likely).
Pity since the radio is rather good but useless without a decent battery.
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Old 19th Jun 2021, 11:12 pm   #9
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Default Re: Recovering capacity in NiCad Cells?

PM sent.

Ian.
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