how to connect 0ne or two batterys to starting battery

Dont know where I saw this but, I want to connect one or two 12 volt batterys to my starting battery,so that the battery(s) can be charged from a single charger,but not be discharged by the starting battery when it is cranking the Outboard.saw this some where that you can put a diode at the correct amps rating in the positive line to acomplish this.It has to be in the right direction but I cant remember in what direction,A diode,in reality is nothing more than a one way switch. the way I want to do this is to connect each battery with a diode between each battery so when I connect my 12 volt battery charger,it will charge each battery,But when your using the boat, the batterys are isoleted from each other and not interfearing with each other because of the one way valves(diodes).I think you would wire them with the anodes pointing toward the other battery.Has any one Done this...thanks

You don't want to use a diode. There will be a 0.7V drop across each diode when charging. Use a Perko-style switch. Better yet, use a charger with a bank for the second battery. Putting a single charger on two batteries results in some overcharging as the charger normally runs until the battery draws less than an amp. With two batteries on the charger, it will charge until the total drawn by both batteries is less than an amp.