90 ST

upgrade grounds from Bat to car, power from Alt to Bat. eng to car

The only real reason to add a second battery is if you plan on doing a lot of listen with the car off, or have a SLP system. For SQ and normal use a cap is the way to go, a cap will charge and discharge way faster then a battery, thus doing away with the dimming lights ect, as long as you've upgraded the rest. A second battery will put additional strain on the Alt, which in the long run with hurt the car more, unless you upgrade the Alt too, which in these cars is not as easy as some old no computer cars.