I just got my 2001 Mazda Miata out for the spring and my battery was dead. We jumped it. It started. I drove it for 30 minutes. When I got home and turned the key off, the whole care went dead..like the radio antenna didn't even go down. My brother thought it must have been a bad battery since it was relatively new last fall. Auto store replaced the battery. He installed for me. The car started right up and we moved the car 50 feet back into my garage...same thing happened. Car went totally dead. When I test the battery...it's reading like point 4. Any ideas?

 

Great first post... Plenty of ways to troubleshoot this can be found in the Google box. Easy mode is hook up an ammeter in series with battery, and start pulling fuses until the draw disappears. If it doesn't then its probably some hack-job wiring, like your car stereo, or light bar, or mini fridge, or that seat "massager" you installed.

 

Since you said it has ".4" volts I assume you have a voltmeter. Jump start the car, attach the voltmeter to the battery. You should see ~12-14 volts at the battery with the engine running, jumper cables disconnected. Anything under 12= likely bad alternator. It's unlikely you bought a defective brand new battery. The only other things to consider are bad connections, not just at the battery but at the other ends of those big cables too.

 

Your funny. I wish I had all those things! I'll try this when I get home.