From: kbernste32@aol.com
Date: Wed Jan 08 2003 - 07:54:07 PST
One more posting on this topic. I queried
Interstate Battery expert on this sulfation thing,
here's the superb explanation I got. I'm not hawking Interstate Batteries, but they sure have great answer service
Kerry
Kerry,
Q) Does sulfation occur from deep cycling, or is it just consumption of the
cathode?
A) When a battery is allowed to remain discharged too long, the accumulated
lead sulfate in the plates hardens and is not able to exit the plates to
reconstitute the electrolyte. The plates are not restored to their normally
active composition. Think of it as pouring concrete.
Q) But when a battery is recharged, doesn't the PbSO4 go back into solution?
A) The chemical reactions that take place within a battery during charge are
basically the reverse of those that occur during discharge. The Sulfate
(PbSO4) in both plates is split into its original form of lead (Pb) and
sulfate (SO4). The water is split into hydrogen (H) and restored into
sulfuric acid (H2SO4). At that time, the oxygen combines chemically with the
lead of the positive plate to form lead dioxide (PbO2). The specific gravity
of the electrolyte increases during charge because sulfuric acid is being
formed and is replacing water in the electrolyte. Remember, this is with a
known good battery. A battery sulfated, will not respond the same.
Q) If a battery is kept charged, will sulfation still occur?
A) No. Only when discharged to below 75% state of charge. At this level,
depending on variables, the potential is there for the battery paste
material to begin the sulfation/crystallization chemical reaction.
Q) Many instructions suggest to charge a battery slowly - is this to make
sure that one does not "boil away" the hydrogen in the electrolyte, and
prevent re-dissolving of the sulfate?
A) Which is better for your vehicle's engine performance and gas mileage? Is
it better to gradually increase to desired speeds? Or, is it better to
"stomp" on the accelerator every time until you reach desired speeds? The
same manor in philosophy is the same when charging. The battery with expel
gasses when charging. That's why when charging the vent caps are to be left
on. Interstate Battery vent-caps are "fire retardant" vent caps. Also, if
there is an overcharge/overheat issue, the electrolyte solution can be lost
potentially decreasing voltage capacity per cell.
Q) If a battery is moved while discharged, the precipitate is said to fall
off the plates to the bottom, killing the life expectancy - is this because
the precipitate shorts the plates? Or, is this because it is no longer at
the voltage of the plate, and cannot react because it is not contacting a
plate anymore?
A) I would presume "precipitate" is referring to aged or sulfated material?
If this is so, then yes, deteriorated material can fall from plates. When
ending at the bottom of the cell, it collected between a positive and
negative plate. When enough deposits have been collected, the sediment build
up can connect the positive and negative plate and short the cell. However,
with Interstate Batteries, the negative plate is sleeved inside what is
known as an envelope separator made of polyethylene. With plates separators
already intact, the envelope separators give added protection from sediment
build-up shorts.
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