Also if you have a surface discharge problem believe it or not that is very well capable of discharging your battery. Get your multimeter on volts and place one lead on a terminal and take the other lead and just put it on several places on the battery itself, make sure to observe polarity i.e. negative lead to negative terminal. Any kind of debris or deposits that you may or may not be able to see can actually connect the terminals together draining the battery. If you see anything else besides 0 volts you must clean the battery with baking soda and distilled water.

Also if your cable to terminal connection is corroded or not completely secure your alternator may not be able to fully charge the battery. To check this, take the positive lead and put it on the positive terminal then take the negative lead and place it on the cable terminal while cranking the car. You should see less than 0.2v loss. Anything more and the terminal must be cleaned or tightened.

 

The battery was about 12.5 volts, when I keep it on the trickle charger the battery will go over 13 volts, but seem to drop to 12.5 volts over a couple of days. With the car running, I read about 13.4 v, with the stereo pumping, I'm around 13.3 volts so I don't think its the alternator, mind you, it is the original alt.

 

If there is 20 milliamps on a Parasitic Draw and the alternator is charging at 13.3, there isn't much else. Sounds like your battery may be on its way out.

Autozone typically uses a Midtronics style tester which tests the battery without a carbon-pile set up. It smaller and more portable than a VAT 40 style machine that puts a heavy load on the battery. You may have a cell that is shorting out or your electrolyte may be getting weak.

Toyota once told me it was OK to put tap water into my battery since it was low. I insisted on Distilled Water but they were adamant that tap water was ok. It worked... for about 2 months, then the chlorine, flouride, minerals, whatever decided to coat the battery cells and restrict exposure to the electrolye ultimately killing the battery. Did someone ever top of your battery with water? Or is it Maintenance Free?

Your parasitic draw is low so your stereo will not be an issue at 20 milliamps.

 

Its a maintenance free, dry cell Exide Orbital 890 CA, 690 CCA. It could possibly be the battery, but its only a year old.

 

If there was a warranty I'd use it.

You don't seem to have a parasitic draw and your alternator seems to be charging fine. So, 690 CCA is more than adequate.

 

That is not a dry cell battery. It's an Absorbent Glass Mat just like Optimas. With the engine running you should read a minimum of 14 volts across it. Even with the drain you have, the alternator is not putting out enough. However, don't forget the alternator gets it's ground from the engine and the engine from the ground strap from the frame. Someone eluded to a voltage drop test. That would detect such an issue.

So have you tried removing the loads one at a time(especially the BTA)?