Of course, it would be better to get a second, deep discharge, battery. A regular car battery is not intended for long discharges, but rather providing a very high current for a short period of time, such as cranking.
Opus will power down once the input voltage drops down to ~11V for more than 10 seconds.
My computer with all of the accessories runs for about 1hr prior to shut-down on my spare 1/2 battery intended for UPS (it is small and was cheap). If I would leave my car battery connected, I estimate the discharge time to go up 3-4 times, maybe even more.
Eventually, I may get Optima Yellow Top to power my PC.