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Thread: DPDT Switch?

  1. #1

    Default DPDT Switch?

    If I have a positive and negative line from my panel array that I want to switch from going a (charge controller > batteries (when full)) to a grid tie inverter, can I use some sort of DPDT switch? I can't find any products that.

    I want to use the panels to charge the batteries first, but when they are full, I want to use them to supplement the house power.

  2. #2
    Join Date
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    Default Re: DPDT Switch?

    To answer your question we need to know details of your array, combiner, and the inverters. You may need to have the array in a different configuration for each of the inverters. From your question it is not clear to me if you want to sell to the grid or not. Either way, there are inverters on the market that will do what you want, without the need for an external DPDT switch.

    --vtMaps
    4 x 235w Samsung, Outback fm60 & vfx3524 & mate, Midnite E-panel, four Interstate L16, Trimetric monitor, Honda eu2000

  3. #3
    Join Date
    Oct 2010
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    Nova Scotia canada
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    Default Re: DPDT Switch?

    One of the big issues in my mind is the very real possibility of DC arcing inside that DPDT switch. Depending on the switch design and the DC voltages involved, an arc could easily be initiated between the internal + and - components, destroying the switch and possibly causing a tragic fire.
    1900 watts PV, (1000 watts PV feeding MidNite Classic 150; 900 watts PV + 160 watts micro hydro both feeding into a single shared Morningstar TS-MPPT-60) ; Xantrex Pure Sine 1800/12 for heavy loads; Xantrex Pure Sine 1000/12 on 24/7 for everything else; six Rolls Surrette 2 volt L16 @ 12 volts.
    Domestic hot water totally provided by the sun 8 months out of every year via thermal panel.

  4. #4

    Default Re: DPDT Switch?

    What I want to do is start with 4 128W/24V/5.33Amp panels in serial. I want to possibly double that in the near future. Because I plan on having up to 8 of the same panels, I am going have the main wires between the panels and the DC kill switch be 4 AWG. Overkill on wire for now, but planning for the future.

    I am going to use a morningstar 45 Amp MPPT charge controller for the batteries. When the batteries are full, I want to use the panel output and feed into the home. Not looking to sell back, just supplement what the home used during the day. Almost every inverter I see uses 12, 24 and/or 48 V input so I configured the panels so I could direct the same voltage to either the charge controller or the inverter.

    The piece I don't get is how to switch the panel output to either the charge controller for the batteires or the inverter when the batteries are full.

  5. #5

    Default Re: DPDT Switch?

    You may not need to switch at all. Once the batteries are full, they will not be pulling any more power from the array so you might be able to just feed them in parallel.

    However, it depends on whether or not your grid tie system will leave enough to keep the batteries charged.

    Also not sure why you ran such heavy wire for a rather high voltage DC.
    Northern Arizona Wind & Sun Forum & Website Administrator

  6. #6

    Default Re: DPDT Switch?

    Quote Originally Posted by goingtoghana View Post
    If I have a positive and negative line from my panel array that I want to switch from going a (charge controller > batteries (when full)) to a grid tie inverter, can I use some sort of DPDT switch? I can't find any products that.

    I want to use the panels to charge the batteries first, but when they are full, I want to use them to supplement the house power.
    This is the second time this question has come up in as many days.
    Don't even bother to try is my advice.
    Grid tie inverters are not designed to be fed the output of a charge controller. They have their own MPPT function on the input, whereas the output of a controller is meant to charge batteries. This will be 'electrically incompatible'.
    If you want to switch the array from the controller input to a GTI that is another matter, also with problems. For one thing the Voltage level will not be right; arrays design to charge batteries are typically much lower than those designed to power GTI's. One exception here would be micro-inverters which are meant to be connected to a single panel. Their you get the reverse problem; the array in total will be too high a Voltage. Another exception would be to use the Xantrex 600 Volt 80 Amp MPPT controller with a high Voltage array.
    This is a no-go idea. Not impossible to get it to work, but costly and more trouble than it's worth.

    What we normally do in the off-grid world is use a controller with an AUX function (Outback, MidNite) that can be programmed to operate an "opportunity load" when the battery charge reaches a certain state (Absorb, Float). This is the safest and most sure-fire method of capturing those Watt hours that would otherwise be lost.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  7. #7

    Default Re: DPDT Switch?

    Not understanding why you say I don't need a switch. Are you saying grid tie the batteries to the house circuit? (Panels > MPPT charge controller > Batteries > Grid Tie Inverter)? If I did that, would the batteries not be "used" if the main current to the house stays on from the street? In other words, at night when not generating current from the panels, would the house system try to pull current from the street and the batteries thus draining the batteries?

  8. #8

    Default Re: DPDT Switch?

    Quote Originally Posted by Cariboocoot View Post
    This is the second time this question has come up in as many days.
    What we normally do in the off-grid world is use a controller with an AUX function (Outback, MidNite) that can be programmed to operate an "opportunity load" when the battery charge reaches a certain state (Absorb, Float). This is the safest and most sure-fire method of capturing those Watt hours that would otherwise be lost.
    So, the controller AUX function sends current to a grid tie inverter then?

  9. #9

    Default Re: DPDT Switch?

    Quote Originally Posted by Cariboocoot View Post
    Grid tie inverters are not designed to be fed the output of a charge controller. They have their own MPPT function on the input, whereas the output of a controller is meant to charge batteries. This will be 'electrically incompatible'.
    From what I gather, he does not really have a "grid tie" inverter - he has some inverters that will put out AC that he can somehow tie into the house for power. Apparently he is also not in the US.

    However, details on exactly what he has are lacking.

    Need to know exactly what you mean by "grid tie inverter", and what model/brand.
    Northern Arizona Wind & Sun Forum & Website Administrator

  10. #10

    Default Re: DPDT Switch?

    Quote Originally Posted by goingtoghana View Post
    So, the controller AUX function sends current to a grid tie inverter then?
    No. The AUX function merely controls a relay which can be used to switch on/off any number of things. There is no grid tie inverter involved.
    The batteries power an off-grid inverter, correct? Under normal circumstances the loads on that will vary but remain fairly light. What you want to do is use the AUX function to turn on loads (AC or DC) that will make use of the power that is available from the panels but will not draw from the batteries.

    Example (from my own system): batteries full, panels putting out less than 5 Amps @ 27.6 Volts. Panels are capable of 20+ Amps, meaning there are approximately 360 Watts available from the panels that aren't being 'harvested'. Over two hours this ads up to 720 Watt hours that are unrealized. Solution; turn on the water pump. It runs, fills the pressure tank, and I have water for a day without having 'tapped in to' any of the power stored in the batteries. In fact my whole system is balanced (somewhat precariously) this way; the batteries serve mainly as a power source for over night. During daylight hours the panels 'run everything' including charging up the batteries.

    I'm not sure what you mean by Grid Tie inverter either. Here that would mean one that pushes AC to the household wiring and/or to the utility (if allowed and there is surplus). As per my previous post I know of none that would be truly compatible with a standard off-grid array. Not any that would be recommended for use, anyway. The two types of inverters function quite differently: the off-grid supplies steady Voltage against varying load demands, the grid-tie supplies whatever amount of current it can make from the panels. Or OG's output varies according to loads, GT's output varies according to supply.

    I think Windsun is right in that we could use a few more details about what equipment you are using and how it is wired.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

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