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Reconditioning batteries - Page 2

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Posted by Johnny B Good on July 5, 2008, 3:25 am
 



====snip====


 That's exactly the process used to form the plates on the very first
commercially made lead acid rechargable wet cells. Pure lead plates were
used for both electrodes only becoming an anode and a cathode by
applying a charging current to form a porous red lead coating on the
plate connected to the positive side of the charger with the negative
remaining as pure lead.

 The initial capacity was quite low due to the negative allowing only a
small part of the lead plate to interact with the electrolyte. By
discharging and then reverse charging to convert the sulphated positive
plate back to pure lead whilst forming a porous red lead coating on what
was originally the negative plate, more of the lead was able to interact
with the electrolyte and the capacity increased.

 The process was repeated several times until pretty well both plates
had become sufficiently porous enough to approach a maximum practical
capacity.

 Nowadays, the cathodes are manufactured in a seperate process which
effectively creates a "dry charged" cell which, when filled with
electrolyte, is in an already charged state. Unfortunately, the
mechanical robustness of preformed anodes and cathodes is no match for
the older electrically converted 'Plante' formed lead acid cell.

--
Regards, John.

 Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.


Posted by Piccolo Pete on July 5, 2008, 1:25 pm
 


So I'm not sure what you're saying here.  Sounds like the really old
batteries could deal with their polarities being reversed but the new ones
can't.  By the way, I already blew up one charger by accidently reversing
polarity.  I don't want to blow another one.

Bart



Posted by Balanced View on July 5, 2008, 2:08 pm
 Piccolo Pete wrote:

If the battery is "dead", as in no charge, how can it blow up?

Posted by Piccolo Pete on July 5, 2008, 8:12 pm
 

I didn't say the battery blew up in this post.  I also didn't say the
battery was dead at the time.  The battery in question was a good battery
that just needed to be recharged.  I said the charger blew up.  To be more
precise, I believe it was an electrolytic capacitor.  They don't like
reverse current.  Fun as electronic smoke bomb and fire crackers, but not so
fun when they are in a piece of electronic equipment.



Posted by Johnny B Good on July 5, 2008, 5:02 pm
 
====snip====


 The preformed plates type of battery can be reverse charged but you
need to totally flatten them first and use some sort of current limiting
at the start of the reverse charge cycle since you can't totally flatten
a battery within the 24 hour grace time allowed before the lead sulphate
starts to crystalise into a form that no amount of charging will be able
to completely reverse.

 This sort of treatment should only be resorted to when all else fails.

 In practce, a lead acid battery can tolerate having one or more of its
cells reverse charged provided the whole battery isn't left in the
resultant 'flattened' state for very long (usual advice is to put it on
charge within 24 hours of being fully discharged to avoid significant
permanent damage).

 Batteries assembled with other forms of secondary cell such as NiCads
and NiMH types cannot tolerate such abuse where any such reverse
charging would permanently reduce their AH capacity.

 Equipment designed to be powered by such batteries have battery voltage
monitoring circuitry to disconnect/switch off once the per cell voltage
gets down to around 1 volt in order to avoid this harmful condition
(lowest capacity cell being reverse charged by the discharge current
from the rest of the battery pack).

 The problem of reverse charge protection increases as more and more
cells are used to create higher battery voltages. Equipment powered from
a 2 cell battery pack (eg. a digital camera) often use the battery
voltage monitoring primarily to trigger a controlled shutdown to protect
the equipment rather than for the sake of the battery pack's health. It
just so happens (rather neatly) that such equipment protection happens
to save the battery pack as well.

 The ideal "battery" would consist of just one cell since this avoids
the problem altogether. The problem with this ideal is that very little
equipment can be directly powered from 1.2v and the losses in even the
most optimised of switching converters required to generate the higher
voltages required become unacceptable when more than a watt or two is
required.

 The problem is eased somewhat when a 2 cell pack can be used. At a 2
volt end point, it's virtually impossible to reverse charge a cell and
the two cell battery pack represents an optimum battery cell count size
for a lot of portable equipment which can be designed to work off a 2.4
volt supply.

 Considering the reverse charge issue with NiCad and NiMH batteries,
there's a lot to be said for the good old fashioned lead acid battery,
especially when 12 or more volts at high power levels are required. You
just have to make sure you're using the right type of lead acid battery
for the type of service they need to provide (and the appropriate care
and attention).

HTH

--
Regards, John.

 Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.


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