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Author Topic: NiCads - Past The Point of No Return?  (Read 2640 times)
dan_c
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« on: December 02, 2014, 09:54:50 PM »

I have a set of twenty 1.2v 40Ah SAFT NiCad cells making up a 24v "pack". I don't know how old they are or how many charging cycles they've been through. What I do know is that the "pack" voltage in only 12.7v  help

I'd like to try to get these babies recharged and see if they will be of any use but I'm wondering if I should do anything else in particular first. I have read online about (snake oil?) ideas about performing massive discharge rates in a very short space of time to break up crystal formations before trying to recharge them. Not sure about that though!

I've got as far as thinking that I should dismantle the pack and just try to charge each 1.2v cell on its own and see how they do. I'd need to have a think about how to actually do the charging though (times and current, plus constant or pulsed current).

Basically, I am open to any feedback or ideas on the subject as I am a long way from being a battery expert.

Thanks
Dan
« Last Edit: December 03, 2014, 01:28:31 PM by dan_c » Logged
dan_c
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« Reply #1 on: December 03, 2014, 01:28:10 PM »

Well I have checked each cell and have the follwoing voltages from the multimeter:

7x 1.1 - 1.2v
3x 0.9 - 1.1v
3x 0.6 - 0.9v
1x 0 - 0.1v
6x reversed polarity (worst case is -0.07v). I'm not sure if these ones are unrecoverable now Sad

I checked all the resitances to the plastic cases and some were as low as 180kOhms which can't have helped anything. I will get them all cleaned up and check again.

The manufacturer's instructions state that electrolyte should only be topped up during the last 15-30mins of a 10hr charge (at 4 Amps) but I'm a little nervous about doing any charging with low electrolyte.
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knighty
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« Reply #2 on: December 03, 2014, 03:13:16 PM »

http://www.powerstream.com/NiCd.htm

NiCads are....  a pain in the arse to charge apparently

every other battery type, you can just use a power supply, set the max voltage to the charged voltage and forget about it....

but NiCads don;t have a 'float' voltage... as you charge them, the voltage rises (very slowly) then stalls... then once they're full they start to gas and the voltage starts to drop instead of rising !


I think your best bet is to get some single cell NiCad chargers and charge the cells individually


you could get a 1v PSU to charge up worst cells a bit faster (can even group them together)... just charging to 1v won't nearly fully charge them, so there's no risk over over charging...  but it'll pull the voltage up ready for your (slow) single cell chargers
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bxman
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« Reply #3 on: December 03, 2014, 06:09:12 PM »

Dan  I am sure these are the wet cells  with a small threaded plug in the case .intended for starting aircraft I believe .

 I do not think this type suffer from crystal formation   

 Do not worry about the reversed polarity this will be cured once you balance them

I had some and I was most impressed with them may be it was wrong but I was advised to  discharge every cell complacently ..

A process that took some time as on a lot of them  the voltage came back again hours after a total discharge and a dead short across the terminals  for some time .

Once I had conditioned by the discharge balancing them they performed brilliantly

They do seem to produce electrolyte from nowhere so be cautious and make sure you not to add to much distilled water  moisten any cells that look bone dry but no more or you may be surprised .

I just used a variac into a rectifier to charge   and keep the charge rate sensible temperature is the best guide as with all batteries . not a lot of sense in continuing charge once the voltage has established around 1.45v  you will hardly increase the capacity at all I was told .
good luck with them . you will not be wanting to buy new ones once you have found the price of them I think .
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dan_c
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« Reply #4 on: December 03, 2014, 10:41:20 PM »

You're right on the money, they are the tall thin vented aircraft batteries and they are getting more interesting the more I read Smiley

Can you tell me any more about the balancing/discharging/shorting you mentioned?
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going green
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« Reply #5 on: December 03, 2014, 10:49:54 PM »

You're right on the money, they are the tall thin vented aircraft batteries and they are getting more interesting the more I read Smiley

Can you tell me any more about the balancing/discharging/shorting you mentioned?
they sounded like my old dead Dewalt batteries
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dan_c
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« Reply #6 on: December 03, 2014, 11:20:44 PM »

Probably a bit bigger  Grin

They look identical to these but not so clean....about 18"x12"x12" altogether.

http://img.frbiz.com/nimg/51/58/5d66c68737a1d44dd94495ae659e-0x0-0/k8_aircraft_battery_20gnc40_24v_40ah_ni_cd_battery.jpg
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going green
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« Reply #7 on: December 03, 2014, 11:31:23 PM »

Probably a bit bigger  Grin

They look identical to these but not so clean....about 18"x12"x12" altogether.

http://img.frbiz.com/nimg/51/58/5d66c68737a1d44dd94495ae659e-0x0-0/k8_aircraft_battery_20gnc40_24v_40ah_ni_cd_battery.jpg

just a tad
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bxman
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« Reply #8 on: December 04, 2014, 12:09:38 AM »

The ones I had were half that size french made   3 rows  of 7 cells with a dummy one that was some kind of regulator  .with the output plug incorporated at end of central row .

Think once I had used a 6v VW headlamp bulb to dissipate most of the voltage I simply wrapped bare copper wire (0.5) around the terminals  some cells glowed for longer than others and others had to have the bulb back on them once it was safe I just left the wire on there a long length wrapped from post to post and left overnight even so the voltage on some cells  crept up again once it was removed .

  http://vehiculeselectriques.free.fr/images/Aprilia%20Eproject%20001.jpg  not identical but similar to the right hand battery approx a 10" cube overall

cheers Patrick
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dan_c
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« Reply #9 on: December 06, 2014, 06:49:50 AM »

Thanks Patrick.

My worst cell (-0.07v) seems to be fine so far, after following the manufacturers overhaul procedures.

It took 6Ah of charge last night and is supposed to be above 0v one hour after that. This one was at 1.31v so looks ok. Smiley

Now I need to discharge it, charge it fully (approx 45Ah) and then do a controlled discharge (pulling 40A) and check how long it takes to get to 1.0v. If that checks out ok then the capacity is fine and it will be "1 down, 19 to go" to make sure they're all ok!

Thanks for the help folks.
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