I have a 115 optimax with a 60 amp alternater. I am running a 12V trolling motor with 2 batteries connected in parallel.Beside running down the cranking battery,is there any other downside to also connecting the cranking in parallel to my trolling batteries.  Thanks for your input.

You could hook up 3 batteries in parallel but I for one wouldn't do it. 

I wouldn't do it. You could stuck with a dead starting battery.

I was just wondering if it would fry any thing in the motors charging system

or wreck anything else.  Please reply

thanks for you input

To my knowledge cranking motors *this is from car knowledge* are not deep cycle and therefor multiple ups and downs will really destroy these batteries ability to reach full charge.

That is the main reason I can see to not want to run the cranking motor in parallel.

as far as the charging system on the motor, no it wouldn't hurt it at all.  as a previous post said, the cranking battery will not last IF hooked directly to the trolling motor batterys with out some sort of isolater. deep cycle batterys can handle the discharge, cranking batterys can't. if you really want to charge off the motors alt. I would recomend using a stay-n-charge system.  it isolates the cranking battery from the discharge of the trolling batterys, and only connects when the alternater is charging.

If the sole purpose is to charge the trolling motor batteries then by all means purchase the isolation system. That starting battery is your life ... and in a parallel system, if one battery goes bad, it draws the others down. It would almost be to your advantage to have a a/b switch on your parallel batteries and use one and then swap. That way if one is bad it won't take the other with it.

Only other thing would be an easy cheap fix is when you run put a jumper between the batteries but when you shut the big motor down, take it off.

I also have a 115 optimax,  great running motor. I did have one problem with it. some one had let the battery cable touch and it blew out the 100 amp fusible link from alternator to the starter relay.  In researching this thru multiple conversations with mercury and a couple mechanics, I have been told that it is a very bad idea to parallel your trolling batteries and ur cranking battery. The alternator is designed to quick charge the cranking battery. Trolling batteries take a lower charge for a longer time. They told me the alternator will be overworked trying to charge trolling batteries. That particular alternator runs around $600. In an emergency, you can use your trolling battery to get you home, but not for extended length of time.   I also looked into isolators but you dont need it for that motor. The mercury folks assured me that the alternator could handle several accessories....fishfinders, electric downriggers, stereo, etc.   

Here is another fact about that optimax OB. I have trolled with it all day to verify this. It charges the system at 6 or 700 rpms (low idle), wont load up like regular 2 strokes, and is very good on fuel. I have 2 trolling motor batteries also but i installed a battery selector switch so when one gets low, I just flip the switch and run on the other battery