Battery Issues

[Xavier]

When I first built my car to a driving state I noticed the battery could not hold a charge and for a while I could not figure out why. I replaced the battery free of charge (Optima); the battery could still not hold a charge; me thinking it was just a battery I let get bad, becuase it was in storage for such a long time not getting charged. Not surprisingly, the battery still did not hold a charge and a friend assured me I had the alternator wiring incomplete and nothing was getting back to charge the battery. He told me the voltage would jump up 2 to 3 volts when the engine was running, this is the easiest way of determing the alternator is charging the battery. Long story short, we took off the alternator, noted it was wired improperly, put it back together and it seemed to be fine.

The problem is now, when the car sits for more than 2 weeks, the battery will not start the car - needs jumped. Before, you can clearly tell the battery was not getting charged as the lights would be dim, and every function was very weak from start up. This time around, everything is strong, everything lights up bright when driving; however if the car sits for more than 2 weeks, it needs jumped. Everything is dead.

So after not driving the Cobra for a month, due to an upgrade I was performing, the battery is dead. I jump it and it runs fine.

Before I jumped it however, I ran a volt meter to the battery terminals when the car is off; it only read 10v, when the engine is running 13.4v. I compared it to the daily drivers and they sit at 12.5v and climb up to 14 or 15v when running.

I do not want to run out to the store and buy another battery. But I can not trouble shoot this on my own and I have no idea why its initial voltage would start at only 10v DC?

I have a Optima Redtop Battery, I have a solid ground to the frame with very heavy gauge wire, another ground from the block to the frame with a batery cable. I have a keyed battery disconnect that is alway turned off when the car is sitting - always. When I do have power, it starts up on a dime - no problem. 60a Alternator.

How can a battery lose power if not in use? How can I see something is using the battery when it is sitting there? Or, if the battery has a bad ground, and the motor does not, would that physically drain the battery?