Ac Delco Battery

I have had the chevy express for just under 6 months and the battery has given me problems. I have a feeling it was completely discharged on the lot as it was sitting for many months. Since I took delevery I had to jump start it if I let it sit for over two weeks. Considering there was some parasitic draw I installed a battery disconnect and it has been doing the same thing.
I took it to a chevy dealer - it wouldn't read on the handheld tester so they charged it overnight and then tested it and it came out good. I checked it with my fluke at 12.87 when I got home at 11 am, and by 8 pm it is down to 12.5
Should a "good" battery drop to 75 percent capacity within a 12 hour period?

the proper method for testing a battery is to fully charge and then do a load test.
testing the voltage alone after a charging wont tell you a thing.
After purchesing this batterey did you fully charge it before instalation? Alot of folks will buy a batt and just throw it in the vehical and go.
Not really recomended. Shelf life for a battery is good but they need to be ( re-juvenated )charged, given a kick in the ars to make the electrolite work.
Start from scratch, fully charge the battery for a good 24 hours and then have a load test.
this will assure you that is one possible problem out of the way.

I did try at one point, putting the battery minder charger on for a week but it did not help at all.

your comment about leaving the vehical sit for weeks/days at a time concern me as your battery hasnt been conditioned.
charge it good and load test it.
even if you did charge it once and stuck it in a car but let the car sit, your not completeing the cycle of charging.
One thing you can do is this.
charge your battery, hook up your test voltage meter to your battery, unhook the coil wire/ignition wire so it wont start and roll the engine over for 15 sec's. and watch your guage on the meter.
This should not drop below 9.5 volts (repsectiviley + - .5 v.) while cranking the engine for that time.
then it should re stabileize at about 11 volts and still slowly climb to level off at a good 12.5 .
If not take it back.
Just make sure you follow the right procedure for charging a store bought battery.
When I was a kid, they wouldnt sell you a battery if they didnt charge it first.

Thanks, I'll start test it tomorrow.
The battery os the orig. that came with the vehicle so maybe the dealer didn't prep it properly. It's down to 12.44 now in voltage. Thing I don't understand is my deep cycle and another starting battery I have in the workshop have been sitting for a month charged and they are still 12.7 and 12.62 respectively, but this has gone down from 12.87 to 12.44 in 24 hours -- but the tester says it's good so that would rule out an internal short? I know the alternator is good as the charge rate is just over 14 volts.

The true test on a battery is to charger it up and let it sit a few days not hooked up.
Then see if it will start the vehicle.
If not it is time to replace it and be done with the problem.
MT

I purchased a delco battery new for my van. It lasted 4 months and then died. I got a refund and purchased an optima and so far so good.