I know this may be a very basic question, but I need to know the difference between cranking and trolling batteries.  Typically I use an old cranking battery for the trolling motor, sometimes even a car battery!  So what are the benefits of a trolling battery v/s a cranking battery?

Longer run time , it's just that simple.

TMI answer.

Starting batteries need a lot of amps to start an engine so they have a very large surface area of plates but to fit them in that makes them thinner.  More push, less guts.

Deep cycle batteries don't need a large surface area to provide high CCA (cold cranking amps) so the space is used to provide thicker plates which will last longer before they get used up and need recharging.  More guts, less push.

Since outboard motors are (typically) much smaller than vehicle engines, deep cycle batteries are able to start them without needing the extra amps of a vehicle battery so there is no reason not to use deep cycle batteries everywhere on your boat.

Look for Amp-Hour capacity when selecting a battery, take no notice of the CCA which really only applies to larger vehicles.  Higher CCA means shorter running time for your trolling motor.

Ann Hit it on the nose but if your looking for a TM battery get a Deep Cycle - your starting doesnt have to be but they are ment to be drained than charged drained and charged.

Just a tip Keep an eye on the water lvl of your batteries !

I guess the confusion comes into play when I'm at the marine store and see deep cycle cranking batteries and deep cycle tm batteries,  I have just thought deep cycle is deep cycle.  My current crankin battery is a deep cycle RV battery that actually came in the boat, but I need a new tm battery and couldn't decide if I should get a deep cycle tm battery or a deep cycle crank battery and use the older battery on the tm.  BTW is is appropriate to run your depth finders on the tm battery or the crank battery?

Your best battery should be on the trolling motor.  It is the one that does all of the work.

You will get less interference with the sonar on the cranking battery.

Marine Starting Service

When a battery is called upon to deliver several hundredamps of power to the starting motor, the battery must be able to deliver this power quickly...within a few seconds. This power comes off of the surface of the plates inside the battery. Therefore, a battery with more plate surface area and less resistance will deliver power quicker than one with less plate surface and high resistance. That's why starting batteries are made with thinner plates...because you only use the power off the surface of the plates for starting the engine.

Deep Cycle Service

A deep cycle battery is called upon to deliver a long, slow discharge of fewer amps...for several minutes or hours...in a deep cycle application, such as running a trolling motor or heavy marine accessory load (e.g. depth finders, fish finders, radios, radar, lights, coolers, house power, etc.). In this case, the power comes from deep within the plates, not merely off the surface as in starting. Therefore, deep cycle batteries are specially engineered with heavier, thicker plates with fiberglass reinforcement, special power-producing active material and special heavy-duty separators. With these features, the battery can withstand the potentially damaging effects of continual deep discharge and recharge. The unreinforced, thinner plates of a starting battery cannot handle this constant deep cycling, and will fail in short order.

Dual Purpose Deep Cycle/Starting Service

Dual purpose batteries are ideal for applications that require starting and deep cycle service. They deliver powerful cranking amperage for easy starting, and low amp draw service for reliable auxiliary power.

Dual purpose batteries are a perfect compromise between the unique demands of starting service and deep cycling, low amp draw service.

i use the marine deep cycle for my 36 volt trolling motor and i use a deepcycle as well for cranking ... i like it better