Oh, I forgot about you.
No, worries, hope you had a great trip!
If the surface voltage is 12.5 yo 13v "upon return" (this is good), what is the voltage when you go to connect the battery?
Also, these results were "text book" results of what the draw should be. "I warmed up the car. I thought there might have been a spike in amperage when I first touched the leads, but when I measured properly, it was 132mA, dropped to 71mA, then after several seconds, down to 21mA."
It would be a good idea to confirm those results once again when the vehicle is FULLY warmed up when disconnecting the battery terminals. I want to be sure the alternator is not causing the draw when warm as I previously mentioned.
If the surface voltage is good, and there is no excessive draw, you need to either replace the battery again or swap it with a know good battery for test purposes. If it's not the battery there must be an intermittent draw you are not "catching". In which case I would try disconnecting the radio and seeing if the battery appears to loose it's charge when parked.
To remove the radio you will need some radio removal keys. You could get them from a dealer or eBay.
They are pushed into the slots in the corners of the radio with the points pointing outwards and then the radio is pulled out.
If you are loosing .1v per night, there is either a draw or the battery has failed (see precious answers.
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Check out the radio possibility. I would, it's easy enough to do, but I have a gut feeling it's the battery not being able to take an acceptable rate load (parasitic draw). 21ma parasitic draw is quite acceptable and the battery should sustain that load for quite some time.
The radios I have experienced with an intermittent draw were never consistant enough to "When it sites over night with the battery connected, it loses about 0.1 volts per night. Sometimes a little more."
If the draw consistently remains in the 21ma range, I would try a substitute battery. As a test you could even hook up a battery of a different size/configuration by simply disconnecting the battery posts and using a set of jumper wires and seeing if you still "lost" the 0.1 plus volts per night. I suspect you understand, you seem "electrically savie" based on our correspondence here. Let me know if you do not understand.
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