For some time now my ' 72 Cutlass rag top battery has constantly drained after a week of sitting - I do have an aftermarket radio (Custom Autosound) with station/settings memory and the Parts Place tic toc tach. These are the only constant draw items I have. All wiring except for the taillight harness is fairly new (all M&H harnesses). I'm running a Powermaster internally regulated alternator (100 amp) (coupled to MSD electronic ignition) and an Optima Red battery. Battery cables also newer M&H and all grounding throughout the car appears good. Starter is a 4yo Powermaster (the OEM-style replacement) - I do have a McRobb mini starter on the shelf for a someday swap whenever I get around to it.

This alternator replaced a Bosch 63 amp unit which also tested good even has I had the same drain problem with that one installed. Alt and battery are only about 3 years old and I've had this issue for most of that time frame. Didn't notice much at first since I drive the car fairly often in nice weather (so the battery essentially stayed charged) but with things as they are currently, it sits more often now, which has reminded me again of this lingering issue

At first I thought maybe the under-hood light was the issue - possibly being triggered even with hood closed, but after removing it no change.

My battery charger allows the car to fire right back up and within a few miles of driving I'm fully charged again and holding steady between 13-14v with engine running. Battery volts with engine off are in the appropriate 12v range after the car's been run. By the 3rd day or so of sitting, volts are dropping measurably and by day 6-7 i'm at the low to no crank point.

I've not yet disconnected the tach (i.e. clock) power lead for a week but contemplating that barring any other ideas to pinpoint

Both the battery and alternator were new local part store purchases at time of install and appear to be functioning correctly in and of themselves (the issue preceded them both). Horn relay unit on the firewall also replaced at same time. There is no more firewall-mounted voltage regulator due to the ignition/alternator conversion.

I also replaced the steering column and ignition switch some time ago after I first got the car (have had her 4+ years at this point) and all seems good with that as well.

Any other thoughts on what I should check would be much appreciated!

 

Well, the first thing I would do is remove the positive battery able and use an ammeter to check the current drain. Then start disconnecting things one by one to see how much the current decreases. A systematic approach to this is mandatory to isolate the fault.

 

Sounds like only a few items with constant 12v draw on the battery. Was the stereo installed at the same time as the alternator and battery? It is inconvenient to reprogram the radio but that is an easy place to start. Try disconnecting the constant power lead to the stereo and see what happens after a week. The stereo should only draw in the miliamp range, but they do sometimes have issues. If the problem persists, disconnect the main power lead of the MSD as well. Then the clock. If you have a suitable ammeter you can test them quickly, but if not, this is just as accurate and easy. Just takes a lot longer.
I had a Jeep that would last for 3 days and everything was factory on it. Replaced the alternator, battery, stereo, fuel pump, coil, ultimately the entire engine and harness. It would also randomly die. Had to disconnect the battery and reconnect it to get it to re-start sometimes. Never did find the problem before I sold it.

 

many thanks for the replies - will be working on this as time allows over the next week - will probably start with the least invasive disconnect (tic toc tach) and work my way forward of that. You are so right about the radio being a pain to reset - not difficult but not my favorite thing to do either, so I'll save the full electrical disconnects for the next effort after checking the t-t-t.

I do have a digital ammeter so suspect I will put that to good use over the next week as well

 

Disconnect the negative battery cable and connect a test light between the negative post and the negative cable end. If the light lights up there is a draw, if not then you have a bad battery. If the light does light, start disconnecting things until it goes out.

 

Removing fuses from the fuse block, one-by-one, will also give you additional disconnects to try.