Hello, I have a '98 S70 with ~100k miles.
I had an odd thing happen to me today. I drove the car on the freeway for around 30 minutes. After I arrived at my destination, I turned the car off for literally 3 minutes. When I hopped back in to go home, everything seemed normal as I started to turn the key (lights came on, etc.), but when I turned the key to "start," the car battery immediately died (no lights, didn't turn over at all, etc). The battery is only 1 year old.
I got a jump, and the car immediately started. At this point, I noticed one of the headlights was dead (it was definitely not dead on the way to my destination). I drove home, again on the freeway for 30 minutes. When I got home, i was able to start the car 3 times with no hesitation. Why did this happen? Is the alternator intermittent failing? Could it have something to do with the headlight (seems like the headlight dying was caused by jumping, and not the cause of the battery dying right?)

Three notes of interest:
1) Immediately prior to leaving for my trip, I topped off my coolant and spilled some off the sides of the resevoir. I'm almost positive none spilled into the alternator but i guess I can't be sure. Could that be a cause?
2) After taking off the jumper cables, the pos terminal seemed a little loose on the battery. Is it possible there wasn't a good enough connection to charge the battery on the way to my destination, even though there had to have been some connection since the lights turned on at first?
3) Around 2 months ago, the same battery was inexplicably found dead one morning after sitting overnight. At this point, the car was in my parents' possesion and they just weren't sure if they left a light on or something. After charging the battery, all seemed to be normal.

Sorry for the novel, but I thought the details might help...and thanks!

 

After the car runs for about 10 minutes or so see if the positive terminal is hot.
A bad connection could have been the problem.Somewhat good connection and when the key was turned the Amp draw was to much and connection was lost.

 

Will do. Thanks!
Seems like the connection is the culprit. When I arrived at the destination though, I think the battery was actually dead. After letting it sit for 30 minutes, before jumping it, when I opened the door, the lights came on for around 10 seconds and slowly died down to nothing. I assume that if the connection was lost due to amp draw the lights wouldn't die down gradually right? So maybe the lousy connection wasn't allowing the alternator to charge the battery on the way to my destination?

 

That sounds about right. It wasn't making a good enough connection for the Alt to charge the battery.
If you have a multi meter make sure it is charging good.
If the terminal gets hot probe the wire before the terminal and see how much of a difference in Voltage there is.

 

All seems to be normal again. Terminals felt pretty cool to the touch, besides burning from the battery acid (just kidding).
I cleaned the battery terminals and made sure they were TIGHT. Battery is staying at 12.8V with engine off, and 14V with engine on.
Also replaced the headlight.
Thanks for the help!

 

Glad all is well now.