I have a deep cycle battery for my Minnkota 50-lb thrust trolling motor. Can I use a cranking battery that was in my car for a back-up trolling motor battery, in case I drain the regular battery?

As a back up battery it would work.

For a trolling motor, no, it will not. A cranking battery is nothing more than an automotive battery. The plates won't hold up and after very few deep discharges, it will be toast.
You should not be using the battery you depend on to start your motor to run the trolling motor. Spend a few bucks and do it the right way.

Bob, all I need is a second battery for my 10� Pelican Bass Raider to get me back to the launch if my deep cycle trolling motor ever runs down before I get back. The boat is powered only by a trolling motor.

A second thing is that deep cycle batteries are so heavy to lift in and out of the boat, which I have to do every time I use this little boat. (I�m no longer young and strong.)

The battery in my car still works, but is a few years old, so I�m going to get a new one before it eventually dies and leaves me stranded. Hope this sheds a little more light on what I�m trying to do.

Yes it will do for a backup not as a full time troller.

Thanks for the reply sae. It will probably never get used, but will give me peace of mind.

Once you get your new battery for the vehicle, sure, why not. You paid for it. May as well "git ALL the goody out of it". Stay safe out there.

Since it is a back and shouldn't be deeply discharged when used only in emergency situations, you will be fine. If you use it to finish out the day and discharge it over 50%, then it will considerably shorten it's life. The difference in Marine and Automotive is plate support and thickness of plates. I doubt you'll be out on a trolling motor only powered craft in waves that will beat the battery up.
John