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Battery Charging with Dual Circuit Battery Switch

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I'm accustomed to the OFF / 1 / 2 / 1+2 battery selector switches in boats.  I have a (new to me) 2017 22VLX and the manual describes operation with an OFF / 1 / 2 / 1+2 battery switch.  However, my boat came with this switch: https://www.bluesea.com/products/6011/m-Series_Mini_Dual_Circuit_Plus_Battery_Switch_-_Red

When turned "on" this switch connects one battery to engine / ignition and the other battery to all the other electronics for "house power".  It's a good idea because that way you can still start your boat if you've been running stuff like the stereo without running the engine for long periods.  It also has a combine position that's for "emergencies" when you need both batteries to start the engine.

Anyways, I'm wondering how the batteries charge with this setup?  Of course you can put it in combine whenever running the engine to charge the batteries but that seems super inconvenient.  Is there an automatic charger that pushes charge to both batteries when the engine is running?  I do notice that there's an extra 8 AWG wire going to each battery on the back of the selector switch so I'm wondering if that's coming from some sort of diode box or something off the alternator?  The reason I ask is I notice that one of my batteries is at 13V and the other is at 12.3V.  This is after two days on the lake with the both fully charged externally before those two days, so wondering if they should charge automatically, and if so if something is wrong.  Also, wondering what battery voltage the screen is indicating, because it says 12.6V which is neither of the batteries (it may just be off like my temperature gauge which is off by 15 degrees....).   Thanks!


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There should be an auto combining relay/voltage sensing relay that closes and passes alternator charge to the house bank when engine is running. Opens when engine is off. If the relay is not working, the usual cause is the control ground wire is either left off the battery or unplugged from the relay. 

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@MLA Thank you, I'll check it out!  I found the relay you're referring to and I see a red LED on it indicating charge sharing when the engine is running.  That tells me it should be working correctly.  This puts me on the correct path, I'll take my voltmeter with me next time to see what's going on.  Thanks!

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