Combine PV and dc from shore

opampca
opampca Registered Users Posts: 10 ✭✭
in General Solar Power Topics #1
Hi,
I have a PV system on a sailboat.
I use AC 117v when I am at dock.
I do not want to switch power source from CC to ac charger, either manually or with relay.
Can I combine a DC 24v power supply with my PV (2x 120w in series) at the PV input of CC (Tracer 3215BN MPPT) if I isolate them with diodes ?
That would make a system with only the Tracer charging the battery (stable system)
Thank you
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #2
    You would combine the dc outputs of the charge controller and ac charge at the battery.

    You would NOT want to feed the pv input to the cc with ac charger output. The cc wants higher voltage pv (~35vcd) input for one thing.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • opampca
    opampca Registered Users Posts: 10 ✭✭
    #3
    I have been doing what you propose without luck in the last year.
    Probably because the CC and the charger do not have the same way to direct the charge.
    Sometimes the charger would be in a fault mode because of the load that is always present on the boat at different intervals and with different load current also. When the charger got unplugged, sometimes the CC would stay in float mode even if the battery required charge,
    If the CC needs more voltage, it is not a problem for me to supply more, ie 36vdc if necessary.
    Also I am planning th replace the batteries with LiFePO4, so I really want a unique charging system to avoid trouble.
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  • mvas
    mvas Registered Users Posts: 395 ✭✭✭
    #4
    opampca said:
    Hi,
    I have a PV system on a sailboat.
    I use AC 117v when I am at dock.
    I do not want to switch power source from CC to ac charger, either manually or with relay.
    Can I combine a DC 24v power supply with my PV (2x 120w in series) at the PV input of CC (Tracer 3215BN MPPT) if I isolate them with diodes ?
    That would make a system with only the Tracer charging the battery (stable system)
    Thank you
    You can use diodes to isolate the Solar Panels and the DC Power Supply at the input terminals of the Charge Controller.

    BUT ...
    You will lose a small amount of power across each diode.
    You will need to use Low Forward Voltage ( ~0.25 v ) Schotky Diodes to minimize the power lost.
    10 amps x 0.25 volts = 2.5 Watts of heat that must be dissipated appropriately

    Do the two PV Panels add up to 36 Volts, suggest maybe a 36 Volt DC P/S?
    If you already own a 24 DC P/S, then maybe OK.

    One more important issue ...
    Which device ( DC P/S or Tracer ) is going to limit the amps during Bulk Charge?
    a) If the DC P/S is rated < 30 amps then the P/S must be able to run at max power continuously, thanks to MPPT
    b) If the DC P/S is rated > 30 amps then the Tracer must limit at 30 amps max
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  • opampca
    opampca Registered Users Posts: 10 ✭✭
    edited February 2017 #5
    Thank you mvas,
    Any suggestion for the diode, I cannot seem to find lower than .45v ....
    Or I put few in parallel:  they have low Vf if current is lower and if junction is hotter, so not too much heat sink... and room around...

    I will be making tests with different supply, and buy the appropriate one. The Mean Well are well built and pretty cheap.
    The panels are 36 cells each in series (2x120w), 72 cells (36v), so matching the panels is a good idea.

    The Tracer CC is going to control the max current, and the psu will be delivering approx 10 amps if 36v.
    So, yess, the psu has to be able to run at high duty cycle for a while, but thanks to mppt, it could charge the battery at more than 10 amps, if I understand mppt correctly....

    I still have to lean more about the LiFePO4 batteries, and how they behave in charging cycles. At least the Tracer is fully programmable. I am expecting delivery of 3215BN next week from Sure Electronics....

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