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New poster here. My daughter's 2005 ex was 2.5 hours into a drive home about a month ago when she had some dash lights (air bag,abs,battery)come on. She got home and I had her start it a couple times fine. Next morning battery wouldn't start car. I charged it, and it was ok for a week or so. More dash lights flashing, and no start. I had battery and alternator tested fine at parts store and a mechanic who suggested a possible bad diode. One test I read was to test ac voltage when running. With an analog tester, it showed 30v ac. My digital meter shows 2 different sets of readings. One time it was .35v ac steady. I tried it again, and it flashed rapidly between 0 and what looked like 84v ac. I bought her a booster pack in case, but it usually happens at home, and she can borrow our car. I also noticed the positive wire was warm to touch after driving. Last time(now about third time in a month) I charged battery, the charger showed a high rate of charge even the next morning. Battery voltage was about 11.5 that morning. Any good tests I can do with a multimeter.? I was thinking possibly a bad battery, was reading about hydrometer tests for that. Any ideas there?

 
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If you really want to test something......Connect a voltmeter to the battery and drive it around until warning lights come on.
(Heck, you might just watch the voltage with it running and see if it's 15v or higher...that's bad.)

At the moment the airbag warning light comes on, check the voltage. I bet it's showing over 16 volts.
OR the thing quit charging completely and the system has dropped into the 11v range or lower.

Most of the time the alternator is bad (bad voltage regulator for high voltage) to cause this, AND it can ruin the battery (has anyone been smelling rotten eggs?) AND high voltage will blow out light bulbs as well. (check all bulbs)

You said the battery was at 11.5v AFTER charging all night(?), it's either ruined, or your charger is running low voltage.
If you can take the vent caps off of the batttery to see the the acid level inside, it might be super low too.

 
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Thanks ezone, I like the idea of a voltmeter. Now if I can just have my daughter remember to look at it when there's an issue. Too many of her father's genes. With the overnight charge, I was suspecting some kind of power draw present, but I suppose unless it was really bad, it should have charged the battery anyway.