I'm stumped. Here's the story : couple of weeks ago I go out to start the car - been sitting for 4-5 days. Turn the key - nothing. I've got a Optima Yellow top in so I figure " 2 years old , got to be the battery". I pull it, charge it, and test it - tests "good". Put the battery back in, car starts up, run the car on and off for the next 3 days- charging voltage at a steady 13.5. Go out of town for 8 days.Yesterday I go out to start the car - dead. Pull the Optima, put on the charger, won't take a charge, tester says "dead". I have a 1 year old heavy duty Auto Zone battery, fully charged and tested, that I put in. Bam, starts right up.Run the car through out the day - no problem.
Today I get in the car it starts right up, I drive 5 minutes to my Doctor's office. Come out 30 minutes later and, NOW GET THIS, I turn the key- the car starts right up - RUNS for maybe 5 seconds and STALLS OUT ( never stalled in it's whole life).Turn the key - NOTHING ! Pull the key, wait, turn it again - NOTHING. No horn, no head lights, no dash light, NO TRUNK RELEASE.
Open the hood - VERY dim under hood light. Hook up a jump.Hood light gets bright, I hear a clicking that I think is the disk player recycleng. Let the jump stay on for 5 minutes - now we have dash lights, horn, trunk realease - turn the key and the starter clicks buy dosn't turn over. I pulled the battery, left the car in the Doc's parking lot, and brought the battery home to charge over night.
I've already checked for any lights that may be staying on or any other obvious electrical leaks. But based on the complete instant battery failure 5 SECONDS after starting the car I don't think this is a slow leak problem, nor do I think it's a battery problem.
Enoughj of what I think, what do you think??
Thanks for any thoughts and or help.

 

You did not mention when you saw the 13.5 volts, cold or hot engine and if anything else was on like lights, A/C, radio . . .

13.5 is low it should be more around 14.5 to keep up with all the current draw C5s have.
It may be 13.5 at battery posts but less then that at the PCM.
Have the charging system tested

Also some relay could be stuck closed so GM says to move anything like seats, mirrors, anything that uses a relay to force it to change state.

Check to see if sunvisor mirrors, under hood or glovebox lights are stuck on

 

If your current battery (the one that "failed" 5 secs after startup) load tests fine, I'm gonna bet on one of two likely suspects.

A bad battery cable end (the connector that screws into the battery). These sidepost clamp terminals have been known to produce some bad/weird/intermittant connections.

The other possibility is a bad (loose or corroded) major ground connection, either from the battery negative-to-frame cable, or the engine-to-frame connection.

Since you have an Optima, it makes things a little easier to check...if you can get the car to reproduce the problem, with the car OFF, use a voltmeter to check the battery voltage at the (unused) top posts of the battery, and then go over to the alternator, and use the voltmeter between the big red wire ternimal connection (covered with a rubber boot) and a good clean ground point (hood hinge nut on the framerail works) and compare the voltage readings.

Any significant difference between those reading indicates it's something other than the battery, it's a connection problem, and most likely one of the two above-mentioned. Isolating which might be a little tougher to nail down, but no sense going further until (and if) you get to this point.

 

I'll try moving things around ( seats, etc ) and I'll check the connections and voltage but, since the second battery went dead also and sooo quickly, I don't think it's a ground problem . But I'll give both suggestions a try. Thanks.

 

Check The End Of The Negative Batt. Cable Where It Bolts Down To A Stud At The Under Hood Fuse Compartment. The Nut Was Loose On Mine & Gave Me The Fits.....jc