Classic/Antique Car Repair: 1973 mustang mach1, Battery goes down in 2 days

Question
hello and thank you for your time

my question is this asi said i have a 73 mustang mach1 with a 351c i just changed the batt. and now if i let the car sit over night it starts right up but if i let it sit for 2days or more all i get is a click i changed the starter relay and still the same thing now i have to say that after 2 days or more of sitting the batt is still as 12.5 and is fully charged and all i get is a click but if a put a jump start unit on it it starts right up i cant think of anything that would make the car need more power to start i had the batt and chargeing system both checked and they are good i am so lost i have never seen anything like this once i jump start the car its fine i can trun it off and itll restart if i let it sit for 24 hrs and start it it starts fine but 2 days our more all i get is a click can you help!!!!!

Answer
Something in your car is draining the battery - to find this you need to have a VOM (Volt-Ohm-Meter).  If you have one or can get one, connect it as follows:  Set it on the highest available current "AMPS" scale, then with one battery cable disconnected, put one test lead on the open battery post, and the other test lead on the disconnected battery cable end.  The meter will read the draining current.  It may read backwards (needle goes negative).  You then switch the two lead connections to get it to read positive current.  When you have it connected, move the meter setting to a current scale where you get a reading that is easy to see - it will probably be on a lower scale, since it takes 2 days to discharge the battery.  Once you have a reading on the meter, start pulling fuses out of the fuse box, one at a time, until the current drain drops to zero.  That last fuse you pulled is for the circuit with the current drain. Investigate everything on that circuit until you find what is staying on when you turn off the key.  Likely culprits are any added electric device (non factory accessory like a stereo system), or anything else that has power when the key is off, like the horn relay, the glove box light, the trunk light, the cigarette lighters, power windows or seat, power antenna - I cannot guess what it is, but something is staying on when it shouldn't.  Find that culprit and disconnect it, your problem should go away.

Dick