[G.A.S.]

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.