If your charging system is working correctly and you don't have excessive drain on it one could be sufficient, I prefer two in my '71.

Mount it in the driver side box.

The model is DF75
http://www.batterysales.com/sonnenschein_prevailerger.cfm

One might work...the factory set up the '73RS lightweights with one battery to save weight. I ran two gel cell Sonnenscheins in my old car. BTW, I kept a "battery tender" hooked to the passenger side battery when not driving. Multimeter testing by myself confirmed that both batteries were charged to the same voltage by charging only one. Nevertheless, I would switch them from side to side from time to time.

Why the dual batteries? Two reasons...to increase the cranking amps, but also a weight distribution thing.

I passed on all my records, books, etc. when I sold my car to my Istanbul friend, so don't have numbers to give you as a reference. A search here might help you...there have been many threads on this topic.

I was a bit of a purist...but I was also quite happy with the dual sonnenschein gel cells. Soooo much better than the liquid batteries of old...no more worries of spilled acid eating out the battery boxes, battery fumes eating the trunk carpet, etc. Before gel-cells it seemed I was constantly cleaning the batteries & the battery boxes...

I'm going to give it a shot with one battery. My car has no power accessories at all... not even a radio. I'd think my '82 alternator could handle it.

Lee,

If you are doing just one battery, and do not mind a bit of simple fabrication, you might want to go with a Optima Red Top Battery (the one at Costco fits just fine). More power and when laid on it's side, fits in the battery box. As a plus, since it is a gel cell, no more worries about acid eating your car.

You will need to disconnect the lead from the other (unused) battery and fabricate a hold down. I used a D ring to catch the clip at the back of the battery box and some metal strap plumbers use to hang pipe. The strap wrapped the battery from the rear hold down point (inside the box) to the factory bolt point in front of the battery.

The battery I ordered (see previous page) is an AGM battery, similar to the Optima (absorbent glass mat), but different in that it the plates are flat and not wound. Seemed like a good deal for $86 delivered.

You are taking a huge cut in battery capacity.

The Red top Optima model 75/25 is rated at CCA=650A, CA=870A.

The Miata Battery is rated at: CCA=320 CCA, CA=400A.

Hopefully you will have enough oooomph when you need it. Let us know how it works out.

Lee, search here for posts on how to remove the "extra" battery cable. You do NOT want the free end of the cable to come loose and contact the fuel tank.

The battery I ordered is rated at 475 CCA. Since the car has no heat, it's never going to be driven in the severe cold. Wait a minute... this is Phoenix. The closest we get to severe cold is 45 degrees!

Yes, I'm going to remove the cable that runs to the passenger side. There's actually a post on how to do that? Is there something to it beyond, "Remove the cable?" ;)