T&T: Batteries

[Candy Chapman and Gary Bell]

<snip>
l
Greast post by Bob Austin..Thanks for the effort!

REPLY:  Hear! Hear!  Thanks Bob!

<snip>
The only point I'd add that is plate thickness is intentionally different for
start versus deep cycle batteries. The logic is that for slow (low current)
discharge deep cycle applications  ion migration can replenish the chemical
reactions deep inside thick plates; for high discharge (big currents, like
starting) thin plates are preferred to power repeated starting attempts...

REPLY:  Yes, that's right.  Thick plates for deep cycle type batteries, where
small currents are drawn for extended times and the battery will regularly be
drawn down to a very low state of charge.  Another reason for the thicker
plate (beyond ion migration from the interior of the thick lead plate to
replenish the surface where the electrochemical reaction that produces the
electric current happens) is that when the battery is regularly discharged to
a very low state, the plates actually begin to warp and also to spall off
chunks of lead, ultimately to short out against their oppositely charged
partner.  End of battery.  Start batteries provide a huge current for a very
short time, and are seldom discharged very deeply.  To produce lots of current
requires a lot of plate surface area, and since they are seldom deeply
discharged -- a larger number of thinner plates is in order (more surface
area).  They are of course more sensitive to deep discharge breakdown.
Regular automotive batteries are start type, and thus provide poorer service
when used deep discharge mode.

<snip>
 ...deep cycle I've used (pair of 8D's in parallel) start my
8V71TI's just fine...Powermaster, Interstate, Exide...but they all last only
about five years. VERY disappointing because I do NOT abuse them. And I can't
find your latest invoice, Candy thinks it was for $7.00 or soOf
course I have no idea if they are REALLY deep cycle designs. But I use mine
steadily six months full time annually.

REPLY:  Did you just say you were using deep cycle batteries to start your DD?
Might that explain your poor performance?  If they were only used as start
batteries, regular automotive type, or marinized start type batteries would be
my choice.  Also, isn't the starter on most DD's a 24 volt beast?  Did you
mean parallel or series?

Robin also said <no snipping here>
I also have read that some manufacturers use different mixes of acids and
acid
concentrations so specific gravity can vary among brands....

REPLY:  Interesting.  Could you suggest where you found that?  My
understanding is that automotive and marine wet cell batteries all share
exactly the same chemistry, which has evolved over a very long time to produce
the same specific gravity vs. charge state curve for everyone.  Refinememts
various manufacturers may tout make insignificant differences in the actual
electrochemistry, and no significant difference in the specific gravity
regime.  Intense efforts to improve battery performance recently (for electric
cars and such), which long ago abandoned lead acid wet cell types as being
about as good as they will every get.  There are newer chemistry batteries:
nickle cadmium, nickle metal hydride (cell phones to hybrid cars),lithium ion
(computers, newer cell phones and soon hybrid cars), and a slew of emerging
new and exotic chemical brews, but I don't think any of them are configured as
open wet cell batteries where one could stick a hydrometer in and check the
charge, and certainly not something offered at competetive pricing for the
marine market.  I've been wrong before, but I haven't come across significant
departures from the modern standard specific gravity regime, at least for
batteries that we could use and afford.  The Gell Cell and Absorbed Glass Mat
batteries all share the same basic chemistry as the good old flooded wet cell,
however their different handling of the wet electrolyte permits different
charge rates and mounting options.  AGM batteries the gas produced has lots of
room to expand into, since the electrolyte can be thought of as being in a
saturated and then wrung out fiberglass gauze held between the plates, so the
ions can follow the wet paths and the gas can move away from the plates into
the 'air' spaces in the 'fabric'.  The lower internal pressure allowes a
virtually completely sealed case, the trapping of the electrolyte in the glass
mat allows the battery to be run upside down without plates poking out of the
juice, and moving the gas away from the plates allows more electrochemical
reactions unblocked by bubbles on the plate, thus higher charge (and somewhat
higher discharge) rates.  Gel cells hold the electrlyte in 'jello', allowing
the battery to tip over without issues, and they engineer in greater 'air
space' above the gel to allow sealing the case.

That old drone,
Mister Science
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