Hooking up two battery's to charge off alternator.

So I am looking to buy a second battery for my boat, I just purchased an old electric min kota trolling motor but I want a deep cycle battery to run it not my regular old start battery so heres the situation.

I have a 65 hp 2 stroke Mercury motor, I am actually currently using a standard 12v car battery for my running and starting battery ( I know I need a real marine starter battery, perhaps I should buy two battery’s a starter one and a deep cycle?) Heres what I want to do; I want to hook up two batterys, one for starting and one for the rest of the electronics, but I want them both to charge off the alternator when the engine runs.

If I get a switch that has options 1 , 1+2, 2 and off, is that sufficient? Or could I just hook them in parallel to the motor? Also can I use my standard car battery in parallel with a deep cycle marine battery.

Any help on this would be appreciated.

Thanks.

You do not want to just hook them in parallel.....the first reason is when you are trolling you are running both batteries down..then one day after fishing you will have no ignition....no two batteries are the same...even when not in use one battery is pulling current from the other... sooner or later a partial short in one battery will end up with both flat..

your charging system is designed to maintain a charge...not charge a down battery....if the trolling motor battery is run down you will not get enough charge between fishing spots to do you much good...plus you are putting stress on the charging system that it is not designed for..so if it was me i would forget the switch..

Here is what i would do...forget all the dual charging stuff...have two good batteries...hook one to the ignition..hook one to the trolling motor...no connection between them....have them both charged fully when you put the boat in the water...if you fish enough to run the trolling motor down then carry a 3rd battery...that way you always have a hot ignition battery and will not be pulling excess current through the charging system...

th above is just my opinion but it is based on many many years of experience and observation...

Papyson's recommendation is the simplest .....

If the alternator has enough capacity to handle our needs, you may want to look at an ACR (automatic charging relay).