Dec 27, 2006 update to the Mazda STS2 Miata project car

Losing weight for the New Year

A typical New Year’s resolution is to “lose weight” and in this installment we do just that! One of the weight-saving items which we never got around to last year was to replace the battery with a smaller, lightweight version. This is not as critical in the Miata as it is in other cars, since Mazda very wisely chose to locate the stock battery in the right rear corner of the car, effectively balancing out the weight of the driver. If you have to have weight, though, that’s a great place for it to be. Still, weight hurts our overall performance and we want to remove as much as possible without otherwise hurting the car

Yuasa makes a great line of motorcycle batteries called the “YTZ” line, which are of the sealed, lead-acid variety, with the fluid absorbed in glass matting (AGM). This allows the battery to be mounted in any orientation, not just terminals up. Also, this particular line has an extremely high charge and discharge rate, which allows it to provide “cranking amps” many times higher than batteries of similar overall amperage. The battery we’ve chosen is the YTZ7S, which is rated at only 7 amp hours. Still, it puts out 130 cold cranking amps which is plenty to start our Miata in summer weather. We’ve been running one for two years in our STS Civic and it has performed well, finally giving up the ghost only recently. A note of warning, though…don’t ever leave your lights on or even leave the car in the “Accessory” position for very long. If you do, you’ll find the battery drained in no time flat! This is a battery for racing use, and should not be be expected to perform like a normal sized OE battery. It has very little reserve power. Also, don’t expect it to start your car when it’s freezing outside. It won’t.

When installing a battery it is important to properly secure it or Bad Things can happen very quickly, especially when you are subjecting it to high-g transitional movements. In addition, we also want to be able to swap the standard battery back in easily for longer-term street use. We work through a number of possible mounting scenarios on paper before finally settling on one that is very similar to the OE mounting. It requires some bracket fabrication and a couple dabs of welding, but is fairly straightforward.

First we chuck up some 1/4” steel rod that we have laying around from an exhaust hanger project, and use a length of 1/2” square box tubing around it as a “persuader”. We bend it into a “Z” shaped bracket that will mount like the stock one, only lower and shorter. Three bends later, we have the basic piece which is then test-fitted and “adjusted” with vise grips. We then bend a small fender washer to a 90-degree angle and tack-weld it to the Z-bracket to keep the bracket oriented corrrectly over the top of the battery. Finally, we mark and drill a 1/4” hole in the trunk floor to allow for our tie-down bolt to pass through. This is a little tricky since there is not enough space to get a standard drill to line up straight either over or under the trunk floor. As such, we use a shorter, smaller bits to drill a pilot hole which then enable us to drill the final hole at an angle without walking the drill. For our tie-down bolt, we reuse the stock piece flipping it upside down with the nut at the bottom. It works like a charm. With everything now test fitted we disassemble it all, clean the bracket and paint it.

The next step is to attach the battery cables. Motorcycle batteries do not typically come with post connectors like a normal car battery. For some, you can install adapters which will provide a post. Our battery is so small, that this is not an option. Instead, we must effectively change our cable ends to a ring type which can be bolted to the battery post. Taking the easy path of chopping off the existing ends and replacing them with rings would make it very difficult to swap the standard battery back in for longer-term street usage. Instead, we flip the terminal on its side, remove the tightening bolt/nut, and bolt that down into the battery post. To use the special nut that comes with the battery, a 5mm x .8 threaded bolt is required. The ones provided with the battery are too short, so a quick trip to the hardware store yields a new pair that are 16mm in length. Together with a pair of 3/8” nuts used as spacers to keep it all oriented correctly, the assembly goes together securely.

Now, for a couple of tricks that we learned the hard way. As displayed in the picture, it helps to use a small screwdriver inserted into the post underneath the special nut to lift it up to meet the inserted bolt which is hand threaded. Be careful as this is easily cross-threaded! Also, it is best to attach the negative cable first since it is so far back in the wheel well. Attach the positive next, and then install the mounting bracket by tilting the battery and inserting the bracket into the body hole. The tie-down hook is the last to be put into place and securely tightening the nut locks it all down.

A quick turn of the ignition verifies our work is good! Of course, now our corner-balance needs to be re-adjusted….

How much weight did we save? The original vented Miata batteries weigh 20 lbs. The maintance-free Mazda replacements like ours are 21 lbs., while our tiny Yuasa battery weighs in at only 4.5 lbs! And we saved another half a pound in mounting hardware. So our net loss was 17 lbs. What a great way to start a new year! Now, if we can only keep the weight off through a program of regular diet and exercise…

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