Battery or Ground Strap Problem?

-Tried to start, and got a little bit of power. Tried a second time, and no power at all.

- Charged Battery for a little while. Read 11.88

-Put the battery back into the car and installed it, then attached a jumper pack, and attempted to start the car

- Car started up

- Tachometer significantly dropped from 14 to 12

- Revved the engine, but it did not hold well. Sounded as if the car was about to die, after taking foot off the accelerator

- Battery significantly dropped minutes later to 10, on the tachometer.

- Very poor performance and sound. Timing belt not going normal speed, but still managing

- Turned car off

- Started it again, and the electrics worked. Lights popped up good

- Tachometer set at 10

- Attempted another fire up, but not enough power to go. Voltmeter read 11.76

- Alternator belt and Timing belts intact

- Battery seemed to very slowly drop while sitting. 11.76-11.72-11.68 within 10 minutes time.


Ground strap may not be working as it should. I am going to replace it and see if this solves the issue.
I also went ahead and picked up an extra battery yesterday, for a whopping $150 - $12 if I bring the other battery back.

Today is around 25 degrees with some winds. May not be great to go out and work on the car since it is outside. Tomorrow is reading 36 degrees, so I might have some time then. Otherwise, I will have to wait about another week to see what the weather is like.