Maximum Charge with limit space?

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System
System Posts: 2,511 admin
in Solar Beginners Corner #1
I’m planning to add solar panels to an existing battery / inverter solution on a boat. I currently have 110v 2.5kw (12volt) inverter/charger, and 8 6volt (in series/ parallel) batteries for ~1000 amp hours @12volt. My problem is, I have a limited amount of space for solar panels to work with. I have enough room for 2 panels. I’m looking to get the maximum charge rate with the least amount of space for solar panels. I’ve gone through the forum, and here’s what I’m looking at;

2 - Solar World s250w panels
1 – Tri-Star MPPT 45

It would be about a 50ft wire run from panels to the MPPT charger, and about 3ft run from the charger to the batteries. My questions are;

1) Do I need to put a battery isolator between the MPPT charger and the batteries or the inverter?
2) I’ve seen a number notes on diodes to deal with shade. If I put the solar panels in series do I diode between panels?
3) If I put the panels in series, the MAX VMP 92v and MIN VMP 54v, will 12 gage wires cover a 50ft run?
4) Is there any other combination of panels or chargers that would give best charge amp with limited room for 2 panels?

thanks in advance
John
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Maximum Charge with limit space?

    Welcome to the forum.
    martinjc777 wrote: »
    1) Do I need to put a battery isolator between the MPPT charger and the batteries or the inverter?
    2) I’ve seen a number notes on diodes to deal with shade. If I put the solar panels in series do I diode between panels?
    3) If I put the panels in series, the MAX VMP 92v and MIN VMP 54v, will 12 gage wires cover a 50ft run?
    4) Is there any other combination of panels or chargers that would give best charge amp with limited room for 2 panels?

    1) Nope. Charge controller can go right to the batteries.
    2) Nope. Although shading can be an issue, especially on boats. Panels in parallel can work better in these circumstances.
    3) As far Maximum Voltage is concerned, Voc can be an issue if there is a potential for exceeding the input limit of the controller. That is unlikely to happen with your proposed set-up. You can run the Voltage drop calculator to get precise numbers for 12 AWG. http://forum.solar-electric.com/showthread.php?t=29
    4) In my opinion the biggest problem is that you'll only get enough charge power for about 270 Amp hours of battery, not 1000. So these panels might add to the charge factor (about 2.7%) they won't be able to handle the whole load. And the 45 Amp MPPT controller might not be the best choice, under the circumstances.

    Just my opinion; others' may differ.
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  • System2
    System2 Posts: 6,290 admin
    #3
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    Re: Maximum Charge with limit space?

    Thanks for the reply,

    I knew about the difference between the amp hours available from the batteries and what would be available from the panels/changer. My thought/assumption is; if we’re out for a few days, being a little conservative, (using about 25-35% of the available battery amp-hours) the solar panels could provide enough of a charge that I can limit the amount of time that I have to run the gen-set. I’m also assuming that if the solar panels are providing ~50-70% of what we’re using, I should be able to go 3x the amount of time before I have to start the gen-set. That of course is hypothetical at this point. Does that make sense?

    I could go with a 60amp MPPT changer, that’s not a problem. I only chose that one because MorningStar’s string calculator for those panels/mppt combination were electrically compatible. But maybe a 60amp MPPT would be safer?

    My other question, are there any other panel combinations, that might give us a higher battery charge rate? The 250w from solar world are about the highest watt single panel I could find.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
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    Re: Maximum Charge with limit space?

    i think what coot is trying to say is that the pvs won't properly handle the full charging of the battery bank, but the power won't be wasted. it can offset some of the usage of power from running your loads and can be a strong float charge when there's no load on fully charged batteries. that is a very large battery bank and batteries should be charged with around 5%-13% of the ah capacity without loads to them. any loads while charging necessitate more current from the charging source to ensure the batteries get a proper charge.
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  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
    #5
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    Re: Maximum Charge with limit space?

    Suntech makes a 280 watt panel STP280-24-vd if you can find them.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
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    Re: Maximum Charge with limit space?

    Thanks for the catch, Niel. :blush:

    I should have put the math in there, but it's travelin' day and I'm a bit stressed.

    To the OP: yes, the problem is that the panels suggested will only produce about:

    500 Watts / 14.2 Volts = 35 Amps @ 77% efficiency = 27 Amps. That would be 5% (minimum recommended current) of 540 Amp hours - slightly more than half your battery bank.
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