I just had this same exact problem. Went out with my wife and when we got into the car to go home, nothing. I got out and checked the battery terminals and they were loose. Tightened them and off I went. Next day I went to have BATT tested and it read 12.41 volts, checked out ok. I replaced the terminal connectors since they were no longer holding. Went with all lead terminals instead of the stock thin metal ones. Worked great for about 80 miles.
The second day I was on the highway and started to experience a battery light, headlights went dim, and poor engine performance. Got off at an exit, went to a fast food parking lot and the thing died, nothing - no lights - just a dead car. Waited a little while, about 30 minutes, and the car no miraculously had some power to the interior lights, but not enough to turn the engine. Checked to terminals again, they were tight. Looked for signs of leaking and percolation but nothing. Finally noticed the battery case itself was a slight bit "bulbous" in that the case was starting to distort and bulge indicative to a bad battery - the same one that tested fine 2 days earlier. The additional ambient heat (over 90 the past few days here in Phoenix) combined with the highway driving instigated possible shorting of plate in the battery itself. Common here in Arizona. When the battery and engine compartment cool down, the plates are no longer touching and thus a sort of tease thinking you have your electrical power back. Freaky huh?
I replaced the battery right there in the parking lot with a new one - and problem solved, runs like a champ. Now, I will have the charging system tested to ensure the alternator, etc. are working properly....but my guess the culprit as the old battery that did in fact show signs of age. The plates were crusted over when I looked into the vent tubes, along with not even the slightest spark when applying the terminal connectors as you would experience with a fully charged battery. I would guess that the lack of a tight connection (not charging well) and the age of the battery in combination with the heat wave this week killed the thing. These cars are electric everything, and a fresh battery is a must along with a frequent user check of the charging system and battery. Lesson learned tonight! :)
Reference the picture of my engine compartment on my album page under my profile. Check out the side panel on that battery, not looking too straight.
Few months ago, it was very cold here in Ohio below 0 degree. My battery 1 year old was also killed, thanks for AUTOZONE replaced brand new one without charge. I thought my Alternator was gone, but it was not after I removed the battery and brought to AUTOZONE for checkup. They told me it was shot, everytime, he added the load to it. Otherwise, it seems good condition, +12v without load.