Configuring two panels on a boat

I have installed a 450Ah house battery bank with alternator, Iota 90A and backup Xantrex TrueCharge+ 40A chargers run off a genset on my trawler. Now I plan to install two 135W panels to minimize my genset running time while at anchor.

I am considering Kyocera 135W panels that have a junction box and a Morningstar Tristar 45A MPPT controller. Some questions:

1. Should I connect the two panels in serial or parallel? I plan to run one 10AWG marine boat cable approx. 40 feet from the panels to the controller near the house bank. Is it more efficient to send 24V or 12V to the controller?

2. Initially, I want to stack and store the panels below the dinghy's cradle on the upper boat deck while under way then secure them on the cradle while at anchor. I wondered if it would be practical to use a weatherproof polarized quick disconnect near the junction box to remove and stow the cable to the controller. Any thoughts or suggestions?

3. Where should I fuse the connections from panels to controller? What size fuse (135W x 2 / 12V = 22.5A)?

Thanks.

- Rick

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Configuring two panels on a boat

    Welcome to the forum.

    You have a couple of problems. If you wire the panels in parallel and try to run the resulting nominal 12 Volts @ approximately 14 Amps down 40' of 10 AWG you'll lose about 10% of the power due to Voltage drop in the wire. With that length of wire run you'd have to go up to 4 AWG to reduce power loss to acceptable levels.

    That means you're better off wiring the panels in series, producing 24 Volts nominal @ 7 Amps. That will work across 40' of 10 AWG with minimal loss. Unfortunately on a boat having two panels in series can be a problem because of the difficulties keeping panels properly oriented towards the sun and free of shadows. If you can keep them clear and pointed the same way their output will be fairly equal. Just something that needs to be considered in boat mounting.

    With just two panels, no fuses would be required for either parallel or serial configuration.

    Using polarize Anderson connectors would work for disconnecting them when not in use. Just be sure they aren't under load (in sun and trying to charge batteries) when you pull the plug.

    Others will no doubt have further insight on this. :D
  • System2System2 Posts: 6,290 admin
    Re: Configuring two panels on a boat

    Cariboocoot,

    Thanks for the reply. I plan to place the two panels side by side making a 4' x 5' panel on the dinghy cradle where there is little or no shading (I may have to swing the davit away). 40' of cable is an estimate but I doubt it will be less than 30'. I've viewed the voltage drop tables so a serial connection at 24v seems the way to go. Can you elaborate on why no fuse is required with two panels? Thanks.

    - Rick
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Configuring two panels on a boat
    RCollard wrote: »
    Can you elaborate on why no fuse is required with two panels? Thanks.

    - Rick

    This is quite a common question. It's because solar panels can put out the short-circuit current (Isc) indefinitely: full current into a dead short. It will do no harm. If you put two of the same panels in parallel and one of them shorts, the other can feed it full current without any danger. It is only if you put more than two in parallel that issues arise. Essentially two of them can "gang up" on the third, if it becomes shorted, feeding it 2X its rated maximum current.
  • System2System2 Posts: 6,290 admin
    Re: Configuring two panels on a boat

    I have found the Anderson SB50 quick connect plugs for 10AWG, which can handle 50A. But there is no mention whether these plugs are weatherproof. Anyone have experience with the Anderson quick connects outdoors?

    What is the proper way to calculate voltage drop from the panels to the charge controller? I'd like to determine the maximum length of 10AWG cable between the panels and controller for a 3% max drop. The Kyocera's are listed at 135W, 17.7V, and 7.63A. With two panels in series, I'll have 270W, 35.4V, and 7.63A. All of the voltage drop calculators I've found use 12, 24, or 48 volts not allowing me to enter 35.4V. Thanks.

    Edit: Since W=V*A, I get a 3% drop with 32 feet of 10AWG for 24V and 11.25A. Does that sound right?

    - Rick
  • BB.BB. Super Moderators, Administrators Posts: 32,013 admin
    Re: Configuring two panels on a boat

    If this is salt water, pretty much any connector should be sealed with self vulcanizing tape and covered with electrical tape.

    Standard anderson connectors are not weather proof.

    For non-standard voltages, just calculate your own limit.

    Say 30 volts x 3% = 0.9 volt drop

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