ACC is key-on power for accesories. If you wire to key-on, you can run with the ignition off, but the key on. With a direct line to the battery (always live circuit) you could run with no key at all, but run a strong risk of draining your battery the day you forget to turn it off. Stick to accesory power. Find a 12 volt line with the key on, then turn off the key and if it drops to 0 volts... You found an ACC line.