2006 Miata - ran the battery down this morning with the radio. AAA jumped it - started right up. Started up at the end of the day, bought new battery on the way home. Dropped new battery in and I get zero electrical - no lights no nothing. Put old battery back in - same thing. Didn't reverse polarity. Connected positive first then negative. What on earth is going on? Don't see any obviously blown fuses but will re-inspect.

 

anything you recently added? car starter, possibly? Are you leaving in a car phone charger that is constantly on?
There is an open circut someplace that is draining the battery.

 

Unless you've got a battery cable that's on its last legs, or a bad cable to body ground that doesn't make sense. Be sure to check the connections within the battery cables as well. Remember, they are only a dry press fit and are susceptible to moisture and corrosion.
You might also check the main fuse, usually located under the hood in a fuse block there.
If you live in a high humidity or sea scape climate, corrosion and bad grounds are a definite possibility. Moving the cable around to change the battery may have just brought a weak link out in the open faster.

 

Can't imagine a brand-new battery could have drained so fast as to not even power the dome light. I'm leaning towards the main fuse as there is absolutely no power to anything. Can't tell from if it is blown given it's location. Sounds pretty tricky to get out from what I've been reading.

 

This is an example of why ALL cars should have at least an ampmeter. You would have seen this comming a long time ago. Cars w/fuel injection and/or a slush-box make their owners vulverable to being stranded. You can't push start an A/T vehicle. You CAN (not reccomended) push start your stick shifter, but you had better have a can of starting ether (also hazardous) in the trunk. . . Or install an ampmeter. . .

 

Ammeter Many gauges I like to see on cars are being replaced by idiot lights or nothing at all because cars have evolved to the point where failures are VERY rare and when they do happen IF there was a gauge for it I don't think the general population of today would know what to do with that information. I also don't think many of them even bother reading the manual or even checking their oil and tires. I will say though that I have been stranded more often with a carbed car than an injected one, carbs are just not as reliable as an injected car, yes it CAN run in more detrimental situations because the parts are not built to as tight a tolerance but only to a point. I personally wish all cars would come with a Volt, Ammeter, Oil Pressure, Coolant Temp, Fuel Level, EGT(Turbo Gasser/Diesel) and Oil Temperature (can be used as a crude oil level gauge too, low oil->higher temps) as gauges and Coolant Level and Oil Level for lights. But that is just me, I like having information about my car, I even modified the printed circuit of my F250 to get the oil pressure gauge to work as a gauge instead of an idiot light with a needle. Maybe we should have aircraft engineers design the dash layouts of cars instead of automotive engineers? THAT I would like

 

Digital voltmeters are good. The one i have is useless. It lights up to corresponding voltage in 1.5 volt increments with LED's. It does let me know the car is charging because it goes up, however, when it's low it does not do a good job.

Proper gauges would be a welcome addition. Exhaust temp may be a good with to help solve pre-cat issues.

 

A voltmeter is the way to go. It not only indicates if your charging system is working, but also the condition of your battery. Two for the price of one. And it's much easier to install as well.

 

If you literally put-in the new -known good- battery and had nothing, it's not your alternator. (It) could be completely dead and you will be able to at least start the vehicle. It might be one of the cables/wires, but not the alternator itself.

You may need to consider a problem with a relay vs. a fuse. A new Main relay may be worth a shot; they're like $9-20.