Battery Bank Grounding

[Sequoyah]

Morning all. Didn't find this exact question in older threads:

A dock neighbor asked me to look at his battery wiring setup. Since I'm no expert in twelve volt electrics I think he must have been at the end of his rope, but anyway:

His problem is that both his two battery banks seem to drain about evenly whether his selection switch is set to 1, 2 or Both. Looking at his wiring, I see he has his starter battery grounded not to a bus but to the negative terminal of one of the two house batteries in Bank 2. I think that is why he's drawing current from both regardless of the switch setting.

Is that how it works? If so, why does it do that? Is it a path of least resistance matter, or what? If I'm wrong about what's happening, what do you think may be?

[smac999]

ground wire to other battery is fine and is not the problem.

you will want a clamp on amp meter to trouble shoot. easy to see which batteries are drawing when switch is moved back and forth.

[tkeithlu]

The switch(es) is on the positive side. All the negatives are common, and common to the engine block, propeller shaft, all the electronics, and any other items. The switch controls which battery(ies) connects to the users. If you disconnect the positive side on one battery, and put the ammeter between the battery positive and the loose cable (as smac99 suggests) you can see when that battery is supplying the users. Think of that big red switch as having several available inlets from batteries and one exit for positive voltage to users.

[FPNC]

Look for a connection from the alternator B+ post directly to the house bank. With the alternator B+ also connected to the starter motor, this effectively bypasses the battery selector switch and puts all batteries in parallel. For some reason, people do this a lot and without understanding the implications.

[Sequoyah]

Guys, I appreciate it. I'll go back with this advice and see what I see.

[tkeithlu]

"With the alternator B+ also connected to the starter motor,"

Wow, FPNC, thanks for posting that. I never would have thought it. Any conjecture as to why someone would do this?

[Uncle Bob]

Originally Posted by tkeithlu

"With the alternator B+ also connected to the starter motor,"

Wow, FPNC, thanks for posting that. I never would have thought it. Any conjecture as to why someone would do this?

Hi, my best guess would be ignorance. People read that the alternator must be connected to the house bank, without understanding that usually small engines are supplied with the alternator output connected to the starter enabling one connection for both starting and charging, the start battery!!

[Stu Jackson]

Originally Posted by Uncle Bob

Hi, my best guess would be ignorance. People read that the alternator must be connected to the house bank, without understanding that usually small engines are supplied with the alternator output connected to the starter enabling one connection for both starting and charging, the start battery!!

So true.


OEM 1-2-B Switch Wiring History http://c34.org/bbs/index.php/topic,4....html#msg30101

1-2-B Considerations (New 2020 - Rod finally got around to diagramming what I had done in the above link in 2009 )
https://marinehowto.com/1-2-both-bat...onsiderations/


Maine Sail also covered it, somewhat indirectly, in one of his other alternator presentations. On Catalinas (and he owned a C36 at one point), it was called "The Orange Wire" between the starter and the B+.