I copy below the following article about those gel-cell batteries.
I thought it was pretty interesting.
The Miata Battery
By Anthony Wilde - NevadaBob@Miata.net
(Courtesy of the San Diego Miata Club)
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The battery in automobiles today is not far removed from the first lead-acid
storage batteries developed in 1859 by Gaston Plante for use in telegraph
equipment. An electrochemical process between the lead, lead dioxide and an
aqueous solution of sulfuric acid, is constant. What has changed in the
intervening years is the technology applied to better materials and advanced
production methods.
If you knew about battery maintenance in 1947, the same procedures would be
valid on automotive batteries in 1997. The liquid electrolyte, sulfuric acid,
must be maintained by addition of water to replace the liquid that is boiled
off during the charge and recharge cycles of normal use. The inevitable spill
of acid on or around the battery would have to be cleaned to prevent
corrosion of the metal. The battery posts would periodically need to be
wire-brushed and cleaned to remove the corrosion present there as well.
Of course we do have maintenance-free batteries now, but the maintenance
still seems to be required, and if you get 3 years on a car battery that
seems pretty good. If you leave the lights on, or the battery doesn't want to
crank the car over, just throw it on the battery charger for an hour or jump
start the car and drive it around for a while. If that doesn't work, toss in
a new one.
Along comes the Miata, and things change. As retro as the Miata was intended
to be in styling, the electrical problems that were common in the British
roadsters of days past, were not in the plan. Since the Miata was meant to
drive, and not to work on, the battery had to be as reliable as the rest of
the car.
In a normal lead-acid battery, the electrolyte is sulfuric acid in sufficient
quantity to cover the lead plates in each battery cell. This type of battery
is referred to as a wet or flooded cell battery. The main drawback to this
type of battery is the continued maintenance and possibility of spillage of
acid during maintenance or an accident.
About 1958, attempts were made to immobilize or fix the electrolyte in the
cell of the battery using various methods. One of these was to use silica gel
to bind the water present in the electrolyte solution into a jelly. Just
imagine a battery full of cherry JELL-O. These type of batteries are used in
many applications and are referred to as gel cells.
For the record, the Miata battery is NOT A GEL CELL.
In 1985, when the Miata was taking shape in Irvine, a new battery technology
was being used in military aircraft. The paramount considerations of power,
weight, safety, and reliability were the selling points of this new type of
battery. The new technology was AGM or Absorption Glass Mat. This technology
was an evolution of previous battery technologies, including gel cells.
In AGM sealed batteries, the acid is absorbed between the lead plates and
immobilized by a very fine fiberglass mat. This glass mat absorbs and
immobilizes the acid while still keeping the acid available to the plates.
This allows fast reaction between the acid and plate material. The intimate
relationship of the electrolyte-filled mat and the lead plate, allows the
construction of a smaller battery, using less electrolyte and metal plates.
This means a smaller battery with nearly equal power ratings to a wet-cell
battery of larger size and weight.
This Miata battery uses the AGM technology and is manufactured by the
Panasonic battery division of Matsushita Battery Industrial of Japan. The
Panasonic battery can only be purchased through Mazda dealers.
A replacement battery is also offered by a company in Anaheim, virtually in
the shadow of Anaheim stadium. WestCo (1-800-214-8040), offers a battery of
the same AGM technology as the stock battery.
The WestCo battery has a rating of 475 cold cranking amps (CCA) to the stock
batteries' 440 CCA. As far as capacity, the WestCo is 35 amp hours vs the
stock 32.
The Panasonic battery is vented to the outside of the trunk through a small
rubber hose. This vent removes the hydrogen gas that can accumulate inside
any battery during use. The amount of hydrogen developed during the use of an
AGM battery is quite small when compared to a wet-cell battery. The WestCo
battery does not have any vent, as it is sealed. WestCo claims the gas
generation is so small, that venting is not required.
The only real difference in the two batteries is price. The Panasonic battery
can be obtained for about $129 at Mazda dealers. Of course, this does not
take into account any discount for being a San Diego Miata Club member. Check
the newsletter for participating dealers. Due to the original classification
of this battery for shipment, it can not be shipped via UPS.
WestCo sells their battery through many of the vendors of Miata accessories,
such as Dealer Alternative, Moss Motors and Imparts. They also sell the
battery direct through their Anaheim facility. For $84.95, minus the 5%
discount for Miata Club members, the WestCo battery is very enticing.
The only real difficulty with the WestCo battery is the terminal posts. The
posts are reversed, as to polarity, when compared to the Panasonic battery.
This will cause an interference with some aftermarket power antennas, due to
the length of the battery cables, so check with WestCo first. Several club
members use the WestCo battery with no problems.
How will you know if you need a new battery? With most cars, a jump start and
driving around will bring a low battery back. Try that with a Miata and
chances are you will think the battery is history. Not so fast!!
One of the differences with the AGM battery compared to the wet-cell battery
is the recharge rate. That is, how fast you can put energy back into the
battery. With a wet-cell battery you can "quick charge" it at about 20 amps
or more. Usually this is when you would boil the electrolyte right out of the
battery. The AGM batteries require a lower charge rate, for a longer time
period. If your battery will not crank the engine, you should charge the
battery at a rate of 1 amp or less for 18-24 hours. If you try and charge it
like a wet-cell battery, you will destroy the battery.
Those of us who enjoy our Miatas everyday-it's our only car-usually don't
have a problem with the battery going flat, unless we leave the lights on.
Those who only get the opportunity to drive their Miata on weekends sometime
will encounter a low charge on the battery. This is due to the constant
electrical load, known as dark current, on modern automobiles. The memory for
the clock, radio stations, engine computer all take small amounts of power
from the battery. Those who add alarm systems, etc. might have a greater dark
current than stock.
WestCo has a small battery charger for the Miata for those who drive their
car infrequently. The EZ-12R is a ½ amp charger that will automatically
maintain the charge in the battery. The advantage of this charger is you can
leave it plugged in and it will not overcharge the Miata battery.
Some people might think that the easier solution would be to put a
conventional wet-cell battery into the Miata. Indeed there are battery
manufacturers who sell wet batteries for our beloved roadster. Before you go
down this road, consider the dangers involved.
The generation of hydrogen gas in a wet-cell battery is much greater than in
the AGM battery. Even those batteries that are vented still have the problem
with all that liquid in the trunk. The potential for corrosion in this area
is very great. Battery acid on your luggage and clothing would not be too
cool either. In a rear end collision, the potential for the battery to crack
open and spill, or possibly explode, should not be discounted. When you
consider the cost of a good wet-cell battery is nearly that of the WestCo
battery, the justification for putting a wet-cell battery in the Miata really
makes no sense. It is false economy.
Some manufacturers are even pushing wheelchair batteries for replacement in
the Miata! I don't think I like the connection.
Overall, the original Miata battery and the WestCo replacement are quality
products that fit the Miata like a string-back driving glove. This is the
first car I have owned where the only battery maintenance I perform is to
dust the top of the battery every year. Several club members have batteries
that are 6 or more years old that are still performing well. When you factor
this into even the non-discounted price of the Panasonic battery, that is
only $21 a year. About the cost of 1 tank of supreme unleaded.
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Many thanks to the author.
Perhaps, I was right?
Perhaps those Miata folk aren't quite so bad after all...
Léon
Triumph Sports Six Club
International Liaison Secretary
1963 Triumph Vitesse 2-Litre Convertible
Wimbledon, London, England.
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