Years ago a mechanic friend of mine intentionally wanted to seize a Datsun engine by running it without oil (a long story). Took over 15 minutes, a lot longer than he expected. As a 16 yr old kid I inadverdently backed my Mom's '70 Mercury wagon off the ramps without oil while doing a lube job, probably about 20 seconds or so. No obvious harm at all. And through cumulative dry starts over an engine's life, it essentially runs many minutes without oil.
A deep cycle battery can provide a surge when needed, but nothing like the surge a car battery can. A deep cycle battery is also designed to be deeply discharged over and over again (something that would ruin a car battery very quickly). To accomplish this, a deep cycle battery uses thicker plates.
The difference is in the way that the batteries optimize their design: 1  A car's battery is designed to provide a very large amount of current for a short period of time. 2  A deep cycle battery is designed to provide a steady amount of current over a long period of time.