I have a 65 pound thrust MK Maxxum that is currently running off of two Exide deep cycle batteries model number 24MDCST. My last few trips out in the wind I have fished maybe 4 hours and at the end of the trip the things barely moving... battery voltage is low. Does that sound normal? I want to add a third battery that i have..can i do this with my current trolling motor? The third battery i want to add is a combo deep cycle and start...will that make a difference?

You just need bigger batteries---at least a group 27.

3 batteries in series is 36V, your motor is 24V.

Wayne is right. You need to get a larger series pair of batteries. Look for RC (reserve capacity). Get the largest number you can find or afford.

Ok thanks guys