12 volt vs 24 volt charging for boat 12v system mppt

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edeher721
edeher721 Registered Users Posts: 1 ✭
in General Solar Power Topics #1
Hello, i looking for some assistance wiring my panels for a boat system. Boat will also have alternator charging and a 120 volt changer when close to shore power. But im building an off grid solar charging system for when i go camping. The biggest panels i could fit were (2) 210 watts 12 volt panels. I bought a victron mppt 150 / 35 charge controller. My battery bank is (2) 12 volt 100ah battle born batteries. This 2 battery 12 volt bank feeds alot of 12 volt accessories. My question is do i run the panels to the mppt charger in 12 volt or 24 volt? Im sure 24 volt wil be more efficient but im worried about surpassing my 35 amp max. I doubt that will happen but its a bit confusing for me and i only want to start wiring when i fully understand the system. I did calculate this at some point which lead me to buying the 150/35 victron and i believe i had good amperage to spare..... any insight?
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  • 706jim
    706jim Solar Expert Posts: 514 ✭✭✭✭
    #2
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    Well if you run the two panels in series, the amperage seen by the charge controller is half of what it would be at 12 volts with the two batteries in parallel. So that would be the way to go (two panels in series).
    Island cottage solar system with 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 30th year.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #3
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    Two panels in series is normally Optimum for your MPPT controller (~36 VDC in, ~14.8 volts out).

    Just to be clear--Your panels are Vmp~18 volts each, not 30 or 36 VDC Vmp each?

    With boats... Try to keep any shading off of the panels. Shade on even one cell on on panel can pretty much "kill" the output of that panel.

    If shading is unavoidable (sail boats, one panel on port other on starboard of boat), connecting the two panels in parallel may be a better choice (hard to say--Try in series first and see how they well they work).

    Otherwise, the typical maximum output of the charge controller during charging will be:
    • 420 Watt array * 0.77 panel+controller deratings * 1/14.5 volts charging = 22.3 Amps typical max harvest
    For near solar noon, cool/clear day, batteries accepting full charging current.

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