No electrical power, won't start, dash doesn't turn on, lights work, battery charged
 

I went to start my car and everything turned on, I got about a half second crank and now the dash doesn't turn on, won't crank, but the battery has power. I checked voltage 12.6, battery fuses under the hood on pos terminal and in fuse box are good. I just drove the car a few hours ago with no issues, but now I have no power. Does anyone have any ideas? Its pretty hot outside not sure if that has anything to do with the problem. Any help is appreciated.

Gas? Just kidding. Try checking contact between + and - cable and the battery terminals. Clean them as well.

It's not that, I think it may be the ECU relay, it smells like an electrical short.

Ignition switch

I have an aftermarket alarm that's been on the car since I bought it in 2007. When I go to start the car it beeps like crazy and never used to do that.

Try bypassing it and see if it started

Alright so I hooked up my battery charger and have power to everything now, but it goes dead as soon as I go to start. no cranking at all. This is really strange, I don't think the battery could be fine and then just die in the course of sitting for 3 hours. I'm going to attempt to jump start and will report the results.

Check the battery with a multimeter. If you have power with the key on, but it doesn't turn over it could be ignition switch, and or a ground possibly.

As much as you don't think it is, remove the battery and thoroughly clean the posts and wire terminals before doing anything else. My terminals looked great, and I looked like an idiot when I brought my battery in for warranty because it was doing EXACTLY what you describe.

Ok everyone the verdict is in...turned out to be two things. 1. Bad ground wire. Replaced that and got power, but still would die when trying to start. 2. Bad battery. Replaced the battery started right up. I'm glad it only took my time and not too much of my money, but I was freaking out today. I do think it was weird how the battery abruptly died though only being about 6 months old. I'm running an optima mini-battery. Is there any other mini-batteries that are better?

That is pretty quick for a battery to go bad. What is the static draw on the battery with the car shut off?

My eclipse did this it was the relay under the hood for the main power.

check the wiring harness in the driver side kick panel. there's a boot known to go bad, let water in, and corrode many terminals causing shorts, parasitic drains and even fires.