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    The Miata Battery

    The Miata is equipped with an AGM battery mounted in its trunk. In the course of doing research on - http://www.miata.net - I learned that the vehicle was originally fitted with a Panasonic battery in Japan. At some point, North American Mazda dealers stopped selling replacement AGM batteries and instead substituted a conventional flooded electrolyte battery, not necessarily a good option for a trunk-mounted device. To meet the demand for a replacement AGM type, The WestCo company of California developed an exact-fit replacement, part #12V31M, made to connect to the OEM vent tube utilized to keep the trunk compartment free of hydrogen gas. It seems to be highly regarded by the Miata folks. Here are the specs of the WestCo battery:

    Model 12V31M (R/H-POS terminal layout)
    CCA at 0 deg F (-18 deg C) = 320
    Dimensions, inches (mm): 7.5 x 5 x 7; (191 x 127 x 178)
    Weight = 25 lbs.
    www.westcobattery.com



    Another potential AGM replacement is the Marathon battery, part #MAR-8AM-U1R. Here are the specs of the Marathon battery:

    Model MAR-8AM-U1R (R/H-POS terminal layout)
    Reserve capacity: 45 minutes
    CCA at 0 deg F (-18 deg C) = 320

    Dimensions, inches (mm): 7.75 x 5.125 x 7.25 (197 x 130 x 184)
    Weight = 25 lbs.

    BCI Designation: U1
    Marathon batteries are available from:

    http://www.batterymart.com/p-12v-maz...a-battery.html

    For purposes of comparison, here are the generic dimensions and terminal polarity of the BCI Group 35 battery used in Proteges and 3s:
    L x W x H, inches (mm)_______________ Polarity
    9 1/16 x 6 7/8 x 8 7/8 (230 x 175 x 225)____ R/H-POS

    Why is this important and why include this data on a forum that does not deal with Miatas?

    The question that comes to mind is, assuming that Miatas have ~ the same current draw as Proteges and 3s (and operate in the same climates), why is a battery with only 320 CCAs deemed sufficient by the factory for Miatas whereas the factory specifies BCI Group 35 batteries (or its JIS equivalent) with a CCA of ~ 600 for our cars? Could it be possible that we also could get away with smaller, lighter batteries and still obtain reasonable longevity and cold weather starting performance? Keeping in mind however that the OEM Miata battery has about the one half the CCA rating of the OEM Protégé battery.

    Well, I don't know the answer to that but at least we have a rational basis for experimentation.

    One point to keep in mind, however, is that Miata batteries are located in the trunk. A pervasive theme running throughout Miata battery discussions is the extraordinarily long life that the batteries exhibit, far beyond that obtained in other Mazdas. While some of this enhanced longevity is certainly due to the fact that AGM batteries outlast flooded electrolyte types, part of the reason may be that trunk-mounting removes the battery from its greatest enemy - underhood heat. A Miata-sized battery will experience greater stress and undoubtedly shorter life when mounted in an upfront location.

    Another important concern is that the vehicle manufacturer normally sets the CCA specification for its battery. If the vehicle will be operating in cold climates, it may be wise to exceed the specification; in warm climates, however, there is no need to exceed the manufacturer's requirement.

    Two other problems also crop up when mounting a non-standard size, dimensionally smaller replacement: the need for some kind of non-standard mounting bracket and the fact that the cables may not reach the terminals since they're now incorrectly placed. These problems would have to be addressed if planning to use either the WestCo or the Marathon.

    Happy Motoring!
    Last edited by goldstar; 02-17-2015, 08:30 AM.
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    #2
    good to know, I didn't realize that AGM's were original factory parts.


    --sarge

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      #3
      I use a Miata battery on my GTX. it's been in the car for daily driving for... 3 years now I think. Never one problem. Other than when i didn't close my door all they way and the dome light stayed on.

      1988 White GTX - #1 parts
      1988 Black GTX - #2 reliably broken touge monstar (294,000 miles no rebuilds) - dead
      1988 Blue GTX - #3 in progress (view here)
      1988 Blue GTX - #4 BPT swapped weekend warrior.
      1988 White GTX - #5 Rally car

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