House Bank and engine start?

Feb 27, 2005
187
Hunter 33.5 Missouri
We have a, new to us, 92' Hunter 33.5 with the 3gm30. The current battery set up is a single group 27 cranking battery and two parallel group 27 deep cycle batteries as the house bank. As recommended I want to use the house bank (#2) as the primary and the cranking battery (#1) as the backup.
The engine fires fine on the cranking battery but when switched to the house bank it barely turns over. The house bank batteries are new this season and are at full charge. Someone suggested this is most likely a wiring issue, possibly a bad ground connection.. Does the house bank have a separate ground? I assumed the ground from the selector switch to the engine applied to both banks. Electrical is not one of my strengths so a little guidance would help.
 
Sep 25, 2008
7,075
Alden 50 Sarasota, Florida
There is no telling what a previous owner might have done so best to trace all wiring and clean every connection as “new” house deep cycle batteries are more than adequate to start an engine.
 
Feb 26, 2004
22,759
Catalina 34 224 Maple Bay, BC, Canada
Does the house bank have a separate ground? I assumed the ground from the selector switch to the engine applied to both banks. Electrical is not one of my strengths so a little guidance would help.
None of us was born an electrician We all had to learn it. You can, too.

The selector switch ONLY connects to + wiring. The grounds from both banks are tied together and go to the engine ground.

These links all come from my
Electrical Systems 101 Electrical Systems 101

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OEM 1-2-B Switch Wiring History Alternator/Batteries & "The Basic" 1-2-B Switch BEST Wiring Diagrams

1-2-B Considerations (New 2020 - Rod finally got around to diagramming what I had done in the above link in 2009 :) )
1/2/BOTH Switch Considerations

Basic Battery Wiring Diagrams This is a very good basic primer for boat system wiring: Basic Battery Wiring Diagrams

This is another very good basic primer for boat system wiring: The 1-2-B Switch by Maine Sail (brings together a lot of what this subject is all about)
1/BOTH/2/OFF Switches Thoughts & Musings

This is a newer primer for boat system wiring design with a thorough diagram: Building a Good Foundation (October 2016)
Building a DC Electrical Foundation

What are ACRs, Combiners & Echo Chargers? (by Maine Sail) [scroll to the top]
Battery Switch Question ? and Battery isolator / voltage regulator / batteries

Making Sense of Automatic Charging Relays (2019)
Making Sense of Automatic Charging Relays - Marine How To
 
May 17, 2004
5,028
Beneteau Oceanis 37 Havre de Grace
The engine fires fine on the cranking battery but when switched to the house bank it barely turns over. The house bank batteries are new this season and are at full charge.
It does sound like there’s some kind of bad connection or maybe insufficient cable sizing between the engine and the house bank. You probably will need to trace the wires from the house bank at least until you get to a place where they merge with the start bank. Check all terminals for cleanliness along the way. You could get a digital volt meter and check various spots when starting too, to see where a voltage drop occurs.
 
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Feb 8, 2014
1,300
Columbia 36 Muskegon
Maybe the house batteries are not grounded to the engine. In that case the only ground connection your getting is through the instrument panel and that won't start the engine. To make this work all the battery grounds should be connected together with the same size cable as the start battery positive. House battery positive also the same size cable. That's the way it came from the builder but who knows what was changed later. You don't need a "starting" battery for that little engine. Big block V8 yes, but not a little sailboat engine.
 
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