I post the answer on another website but I am posting here also, maybe it can help somebody else:
As stated, using a kicker when your main engine use an electric power steering pump will discharge your main battery because everytime that the kicker turn the pump is activated and it draw a lot of amp. This is a major problem specially if you want to use an autopilot.
To start you need two batteries a kicker kit from Mercury and a isolator.
The part# for the kit is: 8M0149838 which I understand does NOT include an isolator. For Verado models, an isolator that can handle the FULL OUTPUT of the alternator is recommended, so the recommend part# for the isolator is: 8M6006077 (140 Amp Isolator).
The kit and isolator enables the Verado to charge both batteries ...as needed . Done correctly, it charges the start battery first then the house as needed.
One is the start battery and one is what we call the "house" battery. The "house" battery is needed for a boat with Verado power steering and a kicker. The kicker doesn't put out enough charge to keep up with the amount being used by the power steering pump when the Verado is not running but you are using the Verado power steering on the kicker. A kit installed (sold by Mercury) enables the Verado to charge which ever battery needs it Tournament style boats with huge electronics and lots of them, also incorporate the "house" battery .