My 67 Mustang has me at a loss. Please help

Hey everyone, I am having a problem and I can not seem to figure it out.  My battery is showing me fully charged one moment and fully discharged the next. The starter will turn the engine then then next time it won't. I put the battery on the charger and it will show discharged and really will not seen to take a charge I disconnect the charging cables and and put them back on and it is showing fully charged. I am really perplexed. With the battery cables connected the lights work but when their on the turn Thanks!!!  
Sentinel64

Poss post prob.(internal connection) Take a test light and hold ground on center of post (not cable) and sharp tip of test light on top of pos.post. Now try the things you mentioned before---if test light stays lite and bright you have a cable problem ---if test light goes dim or off you have a battery  bad(internal short or disconnect---had an 04 cobra a couple months ago with very sim.prob)).My bet is in cable connection--bolt end or post end. I have a voltmeter I plug into cigar lighter that makes it a one man diag.though still have to narrow to bat or connection prob(availible at rv stores for $15-25).You can use a volt meter on above test but by stabbing tops of post you get an accurate reading.LOL Blue  

Yes, definitely either a battery problem or a connection problem. If the battery tests okay, clean all of the connections on the battery, solinoid, and starter and put the anticorrosive cream on them.  

A quick way to figure out if you have a battery or connection problem is to turn on the headlights and crank the engine. If they come on and go dim when cranking, it's pretty much a battery cold-cranking issue although bad cables and connections will contribute. If the lights won't come on but the batt has voltage, its in the cables.  

Definitely load test your battery first before spending money on stuff you don't need. I have a battery voltage and load tester myself, but since I'm so smart I have bought cables and starters and solenoids before finally checking it on my load meter and finding it registering bad but still starting sometimes and sometimes not. I've been around 59 years and still embarrass myself.  

cushman350:
Definitely load test your battery first before spending money on stuff you don't need. I have a battery voltage and load tester myself, but since I'm so smart I have bought cables and starters and solenoids before finally checking it on my load meter and finding it registering bad but still starting sometimes and sometimes not. I've been around 59 years and still embarrass myself.

Cushman,
Good advice as usual.  What make & model load meter do you have? (any comments about it also). thanks jlg  

Thanks for all the good advice everyone. I took the battery back where I bought it and the initial test showed it was ok. As I stood there talking to the battery store owner it showed dead then charged twice. He told me there was either a short or a bad terminal connection in the almost new battery. He replaced it for me with a more expensive battery but only charged me the difference. I do have to admit that I did replace the starter and solenoid though before taking the battery back. It turns out that the original starter was weak anyway and I figured why not replace the soleniod at the same time. The cables were already new and were of the heavy duty variety. The first time I turned the key it almost startled me with the power the starter turns the engine and it fired right up. I now think I am good to go. AT LAST!!! Thanks again. Dave  
Sentinel64