Wiring 2 Battery banks to 1 DC Panel/converter

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Magellan44
Magellan44 Registered Users Posts: 10 ✭✭
in Caravan, Recreational Vehicle, and Marine Power Systems #1
I'm planning to go boondocking in my RV so I installed a 160 watt solar panel and 2 100 AH AGM "house batteries". In addition,my RV originally came with one 75 AH flooded cell battery feeding the 12 volt panel and is charged by the converter.  I'd like to use the new house batteries as my main source of AC & DC, but keep the original battery as a DC backup for the furnace, etc. I'm using a MorningStar Sunsaver Duo controller to charge both banks and a 1500 watt pure sine wave inverter with the new house batteries.

Can I feed the house battery bank plus the original battery into the 12 volt panel by using a 4 position (OFF 1,2, ALL) battery switch? If I have the switch in the ALL position, will that fry the panel?  Will the converter charge both banks if the switch is in the ALL position?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Can you tell us a bit about your loads (Amps @ 12 VDC, Watts @ 120 VAC, how many hours of running appliances, etc.)?

    Where (roughly) will you be camping? Summer/Winter?

    A single 160 Watt panel is pretty small for such a system... Especially if you are going to be running a 1,500 Watt AC inverter for any length of time.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Magellan44
    Magellan44 Registered Users Posts: 10 ✭✭
    #3
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    Bill, I'll be camping in AZ, NM, & CO (good sun territory) during the summer. Since I'll be at high altitudes, I'm planning on minimal air conditioning. My goal is to use 36 amps per day.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #4
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    Is that 36 AH @ 12 volts per day? Or something different?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Magellan44
    Magellan44 Registered Users Posts: 10 ✭✭
    #5
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    Right - 36 AH. If I'm using more, I'll add on. Any thoughts about my original question?
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #6
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    Magellan44 said:
    Can I feed the house battery bank plus the original battery into the 12 volt panel by using a 4 position (OFF 1,2, ALL) battery switch? If I have the switch in the ALL position, will that fry the panel?  Will the converter charge both banks if the switch is in the ALL position?
    When the switch is in the ALL position, the two battery banks are in parallel.   The first rule of parallel batteries is try to avoid parallel batteries.  The second rule is make sure that the parallel batteries are the same type and age.  (third rule is about wiring them correctly)

    Your two battery banks have different charging requirements... you can't charge both (properly) with the converter.

    Another consideration is battery voltage which varies with SOC (state of charge)... when you connect the batteries together, one bank may discharge into the other bank. 

    Even if you get a switch without the ALL position, you may have problems.  When you switch from battery 1 to battery 2, some switches break the connection to battery 1 before making the connection to battery 2 (called break before make).  In this type of switch, your loads are momentarily without power.

    If your switch is make before break, then there is continuity of power to the load, but there may be some very high currents flowing from one battery to the other during that moment when both are connected.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Magellan44
    Magellan44 Registered Users Posts: 10 ✭✭
    #7
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    Thanks Maps. I will just go with a 3 position switch (eliminating the ALL position). I was considering a 4 position switch so I could charge both banks when movin' down the road.The original bank is simply for DC backup on those cloudy days and I didn't plan to parallel it with the new bank - jiust wondered if I could when I use the converter when traveling.
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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,044 ✭✭✭✭✭
    #8
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    In the "break before make" switch scenario you must consider that charge controllers want to see the battery before the solar input. This could be detrimental to your CC.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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  • Magellan44
    Magellan44 Registered Users Posts: 10 ✭✭
    #9
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    Littleharbor, what is "cc"? If I do switch to my old bank, it would most likely be at dark.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #10
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    cc = charge controller (I think).

    Make before break---Switching between two battery banks, there should be no interruption in DC power (and should not confuse/damage any charge controllers).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,044 ✭✭✭✭✭
    #11
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    Thanks Bill.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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  • Magellan44
    Magellan44 Registered Users Posts: 10 ✭✭
    #12
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    Ok, seems like I should use a 3 position switch, not switch under load and not switch when the solar panel is outputting to the controller.  The original battery bank and new bank will go to the switch which I'll connect to the 12 volt panel/converter.  Anyone think that's not a good idea?
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  • z0xal
    z0xal Registered Users Posts: 5 ✭
    #13
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    Have you got a breaker or inline fuse near the batteries? Might have to be bigger so it doesn't blow or trip when there is flow between both banks
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