Charging system info non mopar

What's up guys, I have a question, well a problem and it's been bugging me a lot lately. One of my work trucks, Isuzu NPR, has a GM 350 vortec motor. I've had a few services calls, guy's truck wont start, cranks but not starting.

I go check it out, it cranks fine, but no spark from the coil, check the voltage on battery it's at 10.56 volts I hook up jumpers and bam it starts. I check the charging system, strait out the alternator 14.5 volts, positive terminal on battery 13.9-14.3 volts. (running) i check the volts with engine off and it sits at 12.5volts, So i figured he left his flashers on, battery voltage dropped below 12volts and would not allow the coil to fire.

My question,
What should the battery voltage be with engine off? higher than 12.5 correct? This is a used battery, I pulled from another truck and with that truck, even if the battery was low if it would crank it would fire. Why would be coil not fire if under 12 volts?

My next step is a new battery, and i'm gonna redo the ground wire from the battery with 2/0, and re run the Charge wire from the Alternator to the battery with a thicker guage. See if that solvs my problem

I have this same problem on my 83 toyota 4x4 It will crank but no spark soon as i jump it it starts.. any ideas?

Thanks

-Chris