[time-nuts] OT Gel Cell question

Chuck Harris cfharris at erols.com
Sun Jul 27 17:41:21 EDT 2014

I too have spent a lot of time charging batteries using different methods on
various chemistries.  Once upon a time, I build a number of suitcase battery
chargers for the US Army that allowed them to charge every portable secondary
battery type that they had in inventory... SLA, AgO, NiCD, NIMH,
LiIon...  I proved a concept, and someone else got to make all of the money
off of it when the ARMY shopped my prototypes around... but I digress.

When you try to charge a sulfated lead acid cell, you can think of the cell
as being a bunch of little parallel lead acid microcells (uCell).  Each is in
some state of charge/discharge.

So imagine this:

+--+--+--+--+--+--+.....+--+---Plus terminal
+--+--+--+--+--+--+.....+--+---Minus terminal

In this case, all of the "S"s represent a microcell (uCell) that is highly
sulfated, and the "[B]" represents a microcell that is in perfect condition,
and is taking a charge normally.

If you try to put current into this lead acid cell, the sulfated uCells will
appear as open circuits (due to the sulfate's insulating properties), and the
good uCell will take all of the current, and will keep the voltage down to a
low enough value that the sulfated uCells will not see any significant
electrolysis action.

So you say, "Pooh, I want the sulfated uCells to charge too!" and up the
voltage across the lead acid cell, and poof!  The good uCell dies from over
heating, revealing a new uCell that wants to charge, and poof, it dies,
revealing another uCell that wants to take a charge, and poof...

You get into a situation where your success causes your failure.

If, instead, you apply high voltage pulses to the lead acid cell, you can
sometimes beat the system.  The good uCells will take a hit, but it is short
enough that they don't have time to burn up, and the sulfated uCells will
get to draw enough current during the pulse that a little electrolysis will
happen, and convert some of their sulfate back into oxide and acid...

Sometimes you can win by using a pulse charge system.

However, there is a little physical problem that has to be understood.  Part
of the way the lead acid batteries get their high current handling densities
is because the lead plates are made to have high surface area.  They are kind
of like sponges on the surface.  And, lead sulfate takes up significantly more
room than lead oxide.  Sorry, that is the way it just is.

So, when a cell gets all sulfated up, the lead sulfate that forms in the deep
nooks in the mossy lead electrodes fills the nook up so full that it breaks
it apart, and damages the cell plate.  This happens, albeit slowly, even when
you treat the battery nicely.

You can't win. Long term everything heads towards entropy.

-Chuck Harris

Brooke Clarke wrote:
> Hi Robert:
> I've spent a lot of time charging batteries using different methods and on various
> chemistries.
> When the charge is in the form of a pulse, ideally including a reverse polarity
> pulse, the charge is more effective.
> This is also a way to sometimes, but not always, will recover a battery that
> otherwise will not take a charge from a DC source.
> I think this works because it takes some time for chemical reactions to work and by
> using a pulse you can force the reaction to a higher level that you can't do using DC
> without causing problems such as boiling the electrolyte.  For more info see Burp
> Charging:
> http://www.prc68.com/I/BatChg.shtml#Burp
> Have Fun,
> Brooke Clarke

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