NiCD cells generally don't recondition successfully after 5 years, whether
you use them or not. If you use them regularly, they simply exhaust their
normal cycle life. If you don't use them at all like your spare pack, or
very little, then they tend to grow metal crystals which eventually puncture
the seperator, and the cell can no longer hold a charge for very long due to
self-discharge through leakage current through the seperator.
If you still wish to attempt reconditioning of the packs, you'll need a
dummy load (a low resistance, high wattage resistor, or a small light bulb),
multimeter, and power supply. A 12v SLA (sealed lead acid) battery will also
come in handy. You'll see why in a moment.
- Open the pack carefully, and determine what the polarity of each cell
should be.
- Measure each cell's voltage with a multimeter. If the polarity has
reversed, you'll more than likely not be able to recondition the pack, and
will have to replace all four cells.
- If any cells read 0.0 volts, measure the resistance of that cell. Metal
crystal growth may have gone into an extreme case and completely shorted the
cell. If it reads 0 ohms, you'll have to replace all the cells. (do not
replace only one cell. It's capacity will certainly be higher than the
remaining cells, and near the end of a normal discharge cycle, cell polarity
reversal will occur in one or more of the three weaker cells)
- Assuming all cells are not polarity-reversed, discharge each cell
*seperately* with the dummy load. Do not discharge the entire pack as a
single source, because this may lead to cell polarity reversal. Discharge
each cell to 0 volts with the load attached. Disconnecting the load from the
cell will more than likely allow it to recover some voltage. Leave the load
connected overnight.
- Once all the cells are completely discharged, you now have to destroy the
metal crystals that have grown inside the cell. This is where the 12v SLA
battery comes in handy. Connect a couple of heavy gauge wires to the 12v
battery, and BRIEFLY use the battery to pulse-charge each cell individually,
connecting the negative terminal on the SLA battery to the negative terminal
on the NiCD cell, and positive to positive. When I say briefly, I mean half
a second. The wires and cell will get hot. Doing this will (hopefully)
vaporize the crystals in the NiCD cells and destroy them (the cells will
draw anywhere upwards of 20 to 30 amps). Pulse each cell three times, with
30 seconds between each pulse, then let them cool off for 10 minutes. Pulse
each cell a second group of three times as before, and let cool again.
- Once the cells have cooled, use the power supply to *slowly* charge them,
using a constant current. Slow means 1/20C, that is, if the cells are rated
at 500 mAh, then charge them at 25 mA for 24 hours. If they're 800 mAh,
charge at 40 mA for 24 hours. Charge each cell individually, not the entire
pack at once.
- After charging all four cells, let them sit for half a day for the voltage
to stabilize. Measure the cell voltage with the multimeter. If all goes
well, each cell should be within a few hundred millivolts of each other
(e.g., cellA=1.32v, cellB=1.40v, cellC=1.25v, cellD=1.38v). If any one
cell's voltage is way lower than the others (e.g. 0.8v), you'll have to
replace the whole pack.
- If all cells have similar voltages, then it's time to re-discharge each
cell and do it all over again, EXCEPT the part with the SLA 12v battery.
Three to five discharge/charge cycles should be plenty.
- Once you've reconditioned the pack, try it in the Newton, but don't put
any critical data on it. The pack might die suddenly if the cells didn't
take to the recondition. Do not recharge the pack until it is drained. NiCD
cells hate being recharged in the middle of a discharge cycle. It's the
primary cause of crystal growth.
-Raphael
----- Original Message -----
From: "Prabhakar Ragde" <plragde_at_plg2.math.uwaterloo.ca>
To: <newtontalk_at_newtontalk.net>
Sent: Monday, May 06, 2002 11:44 AM
Subject: [NTLK] battery reconditioning
>
> I have two battery packs for my Newton. One has been in the machine
> since I bought it in 1997, and it seems to have become loose or
> something -- I take the Newton somewhere, and take it out, and it is
> dead. If I jiggle it, or squeeze, or pop the battery out and put it in
> again, sometimes it resets, and tells me all power was removed.
>
> The other pack does not have this problem, but it will not hold a
> charge for very long, since I bought it as a spare when I bought the
> Newton and stupidly never used it. Can someone explain to me exactly
> how to recondition it? I have searched the archive without success,
> though I know this topic has been discussed before. I am just not
> choosing the right keywords.
>
> I also have the original battery cage. If I buy four NiMH batteries,
> charge them in an external charger, and put them in the cage, will
> this work (as long as I don't apply AC), or do I have to do something
> else as well?
>
> Many thanks. --PR
>
> --
> Read the List FAQ/Etiquette: http://www.newtontalk.net/faq.html
> Read the Newton FAQ: http://www.guns-media.com/mirrors/newton/faq/
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>
>
-- Read the List FAQ/Etiquette: http://www.newtontalk.net/faq.html Read the Newton FAQ: http://www.guns-media.com/mirrors/newton/faq/ This is the NewtonTalk mailing list - http://www.newtontalk.net
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