I'm a little confused here but that's typical! I would change one thing at a time, Optima first and not the 180 thermostat and see it that helps your low voltage situation, after that not sure what your goal is.
The battery is not supposed to supply power when the engine is running... The battery is only supposed to supply power to start the car and that's it... The alternator is supposed to take over after the engine is started and then recharge the battery to replace the energy needed to start the car...
He needs to increase the power that the alternator is putting out at idle... Bumping up the idle RPM, put it in neutral when stopped at a light will help it idle a little higher, or putting a smaller pulley on the alternator so it spins faster at lower RPM will help...