Intermittent dead battery symptoms

[royp4]

I am experiencing a strange intermittent problem. From time to time, when I try to crank my 2003 996 c2 cab, the engine turns over very slowly. It mimics a weak/dead battery. This has happened 3 times. Each time after leaving the car alone for an hour or so, it cranks in a normal fashion. Each time I have had the battery tested, and each time the battery has tested as being fully charged. Any ideas? Thank you in advance

[Loren]

Check the cable from the battery to the starter for corrosion - do the same for the chassis ground.

[royp4]

Than you Loren. I will be sure to check them this weekend. I will post my findings.

[DBJoe996]

Sometimes a 12V battery will test okay, but is not really okay.  The cold cranking amps is what you need in the battery as a reserve for starting.  Like Loren suggested, check everything from the battery back to the starter for corrosion.  Make sure the clamps are tight on the battery.  Check the engine ground and even better, unbolt it and clean up any corrosion or surface junk.  How old is your battery?  Anything over 3 years old I would consider suspect and would replace with a new AGM battery.  Also, although a bit more major of a project - some have had the starter Y-cable replaced as this is a known weak point that leads to starting problems.  If you are undoing the battery cables, always undo the negative lead first! And know your radio code before disconnecting the battery.  After disconnecting the battery for whatever reason, also a good idea to clean the jumper junction box in the engine bay where the 12V cables connect.  The main 12V wires from the battery to the starter connect there.  Like I said, disconnect the negative lead FIRST on the battery before doing any work on the main 12V cables.

[JFP in PA]

Quite often, the actual corrosion in the battery cables occurs on the inside, underneath the crimped ends, so is difficult to impossible to see.  This is why most shops run voltage drop tests across the cables; a drop of more than 0.5V says the cable is bad, regardless of how it looks.

[royp4]

Thank you all. i will investigate .