Weird battery problem (long story so get a drink!)

[Just a Six?]

This is a trunkmount setup with Big Ass Welders Cable in case it would matter? My battery has been on the charger set at 2 amps & auto shutoff for a couple days because the car wouldn't turn over. I just went down to unhook the charger (which reads full charge) & when I turn the key it wont even crank over. I get my meter & it reads 12.8ish
I boost the car & it starts asap & scanmaster reads 13.7-8 at idle. I go to alternator & get 14 volts. In trunk battery reads about 13.7-8 same as scanmaster.
I shut it off & it will not turn over at all & the interior lights get dimmer with key on start.
Battery still reads 12.7ish?
Luckily I got on the lifetime guarantee Canuck Tire had back in the early 90's but they stopped doing the lifetime probably from guys like me! This is about #6-#7? I can get a free replacement no problem but this was just installed last May?? Something is eating my batteries?? They usually last at least 3 years.
I unhook it during winter but in summer it sits for 2 weeks in a row often. This year even more due to my back issues since late Feb. Other than BPG I've been out maybe 5-6 times at most. 
Is that possibly what's killing it?? The store guys are going to think I kill them on purpose if it keeps doing this. I'm ashamed to take it in again since the head mechanic (we have mutual friend in drag racing & he has a few Mopars) does me a lot of favours with cash deals, front of the line service etc. & also the head service guy is friends with Ray my foreman.
Sooo??? Why does it read almost 13 volts yet can't even spin the motor for 1 second??
I do have a kill switch that kills everything but the ecm so maybe I need to use it more?
Only issue is it also kills my car alarm for the little good it does.
No way this thing will pass a load test but if it does I'll really be confused!

[Steve Wood]

if it passes a load test, we shall all be confused (except for Zap who is permanently bemused)

I would guess it was a bad battery that shorted out one of the cells.  I have seen in on two day old batteries as well as three year old batteries.  I quit buying DieHards years ago because I saw it happen so many times.


Usually, the voltage will drop like a rock when you turn the key to the Start position

[Just a Six?]

I'll get my friend to turn the key while I watch the meter but the interior lights almost go out.
I guess I should be happy I'm on the Lifetime Freebie Battery Warranty!     
 

[Steve Wood]

the lights tell the story

[gordyzx9r]

I hate electrical issues...I have a viper two way remote/alarm with a back up power supply...if the remotes go dead, the viper freaks out trying to make contact and drains my battery.  But, the pain generators for the alarm make the frustration worth it.

[SuperSix]

Sounds like it has a dead cell.

From:
http://www.partsavers.com/content.wws?fname=batteries.html
"Another common problem is the "dead cell".  Sometimes, a battery will have just one cell go bad or short out.  When this happens, the battery will seem fully charged, but fail to start the car, or it may start the car right after charging, but then fail to start the car an hour or more later.  If this happens measure the S.G. in each cell.  If one cell has a lower S.G. than the rest by a significant amount, replace the battery - there is no way to fix this problem.  This condition most often happens to batteries subjected to lots of vibration, and sometimes to new and almost new batteries that have a bad cell from the factory."