Piggybacking a Charge controller

ncalbuild
ncalbuild Registered Users Posts: 1 ✭
in Solar Beginners Corner #1
 I have 4 100w 12v panels with MPPT controller charging 6 6v 220ah batteries wired in series parallel to a 12v 3500watt inverter to power my RV.
 
 However I also have 2 6v 110ah batteries wired in series to power the 12v system of my RV, which means the converter has to use inverter power to charge them.

Is there any reason I can't piggypack a 2nd 12v charge controller from either the MPPT controller, or the panel fuse before the MPPT controller, and unplug the 12v RV converter?

I realize there will be a current drop, but I think the drop would be less wasteful than running the converter to charge the RV batteries.

you're thought and comments,. GO..
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
    If I understand what you want to do--It does not appear to be a problem.

    However, there are DC to DC battery chargers too... Here is one:

    https://www.solar-electric.com/xantrex-82-0123-01-echo-charge.html

    Basically, when your main battery bank is charging, it will take the energy (current) it needs to charge the secondary bank.

    With MPPT type charge controllers, you really should not connect one solar array to two different charge controllers (MPPT+MPPT, or MPPT+PWM). MPPT controllers "get confused" when there is one array and multiple controllers connected to that single array.

    Other questions... Any reason that you cannot have (for example) 8x 6 volt batteries for one battery bank (2s * 4p strings)? (I am not a big fan of running >3 parallel battery strings vs choosing to use larger AH batteries and going bank to 1-3 parallel strings--GC batteries being very common and among the cheapest out there).

    Then we get into other "rule of thumb" designs questions... A 660 AH battery bank at 250 Watt of inverter per 100 AH @ 12 volt FLA battery bank would suggest that ~1,650 Watt rated AC inverter would be the maximum I would suggest for that bank (for 12 volts, ~1,800 Watt max AC suggested--Unless using AGM or Li Ion type batteries--Which is still an issue with heavy copper wiring, etc. for 12 volt high wattage systems).

    And the recommended charging of 5% to 13% for solar, with 5% for weekend/sunny weather and 10%+ for full time off grid and minimizing generator use.
    • 660 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 621 Watt array minimum
    • 660 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,243 Watt array nominal
    • 660 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 1,616 Watt array "cost effective" maximum
    So--Adding panels to your existing 400 Watt array and 660 AH @ 12 volt battery bank would not be a bad idea anyway (depends on your energy needs--Suggest that 5% is minimum rate of charge and a 10%+ rate of charge is nice--if you have the room and can justify the extra costs).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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