Originally Posted by Erin
I think you've got to decide whether you want to have your second battery dedicated as a 'house' battery or just there in reserve if your primary battery fails. If it is the latter, then I wouldn't get too carried away with complicated circuits and just carry some jump leads in case it is ever required to start the engine. If it is the former, then the start battery should only power the engine starting and not supply any juice to any electornics whatsoever. Your nmea etc should be run from the house batt.
The second battery is to be a dedicated house battery, not just lugged around for the sake of it. The final wiring diagram is attached.
With the new system, NMEA2000 becomes very important that it is powered on the whole time, because it will be necessary for DSC, AIS, GPS, transducer etc all the time whereas in the old system it did not. However, the engine instruments are also powered from NMEA 2000 so cannot be run separately with a set up where NMEA power just comes from the house battery. Ie. you couldnt run the engine with instruments but no other electrics. The multi power input cable sorts that problem.
The NMEA2k will usually be powered from the house battery because that will be turned on first, then all the engine instruments will be working fine no problem. If however you wanted engine + instruments but no electronics, you simply fire up the engine and the NMEA gets power from the engine (not from the starting battery) although that is semantics really.
NMEA2k network diagram also attached, however missing AIS and memory module. Shows how important it will be that it is always powered on.