Adding Solar to an Existing Inverter/Charger system

DanM
DanM Registered Users Posts: 2
in Solar Beginners Corner #1
We currently have a Prosine 3.0 inverter/charger that is driven by 2 8D AGMs for almost 500 AH of power on our boat. It is wired up to a separate AC distribution panel and has an AC input from the original AC panel on the boat. This takes care of supplying power and charging the batteries. We can charge from either shore power or the 8KW Onan generator.
The thing is we have plenty of real estate to mount solar panels and I would like to add about 800W of solar charge capability.

Enter the quagmire.

MPPT Chargers like the Xantrex XW MPPT 60 apparantly don't play well with other chargers (of an unknown type) that are in the configuration and it will shut down due to back flow into itself.

Are there components we can use that will cohabitate with the existing configuration so we can be more independent of generator/shore power usage? I'd like to have the solar drive the inverter bank and charge it as well as the inverter when the sun is adequate, but I'd like the Prosine to either pass through shore power/gen or charge when the sun is down.

Dan
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #2
    Re: Adding Solar to an Existing Inverter/Charger system

    I can think of a 100 ways to do it. If you want Automatic, thats works until it don't and then you end up reverting back to the good old stand by, The On & Off switch. Blue Sea has all kinds on Circuit Breaker Panels you can install to cut the AC to your chargers or the DC. If you are on your Inverter you would not be using them anyway, as they should not be on the same buss as the Inverter output. In a Boat you should have a split Buss that powers everything on the Boat, when you are in Bypass everything has power, when the inverter loses it's AC connection it only powers the circuits on the Inverter buss, that wouldn't be a charger or a hot water heater for instance.

    Sounds like your already good to go. Just cut the chargers off when you don't need them.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Adding Solar to an Existing Inverter/Charger system

    Welcome to the forum Dan.
    DanM wrote: »
    MPPT Chargers like the Xantrex XW MPPT 60 apparantly don't play well with other chargers (of an unknown type) that are in the configuration and it will shut down due to back flow into itself.

    I don't know where you heard that, but it isn't true. You can connect solar panels to your batteries through any charge controller you wish providing it has the capacity to handle the current from the panels. There are absolutely no "back flow" issues. If charging is coming from another source such as gen or shore power and the battery Voltage is up the solar controller will simply read the Voltage at the battery and act accordingly.

    The only potential issues are:

    1). Some controllers may not allow the same Voltage set points as the other sources, resulting in discrepancies over charging state.
    2). During the Bulk stage all sources can feed all of their available current resulting in too high of a charge rate.

    If you set it up so that solar is your primary charge source and you only connect the gen/shore power if needed (i.e. not enough sun) it will be trouble free. I'll also suggest you look at MidNite Classic controllers rather than Xantrex. They're more expensive, but better.
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  • DanM
    DanM Registered Users Posts: 2
    #4
    Re: Adding Solar to an Existing Inverter/Charger system

    I took this statement from the manual to be the exclusive configurations allowed;
    The XW SCC can be installed (in single or multi-unit configurations) with a Xantrex XW Hybrid Inverter/Charger or in a stand-alone installation.

    That is to say, it's this way or the highway.

    and
    Input over-voltage and under-voltage protection, output over-current protection, and backfeed (reverse current) protection. Warning and Fault messages appear on the LCD when the unit shuts down as a protective measure.

    So is backfeed in this context related to driving the panels (no diodes required here), or the MPPT charger being driven by the batteries which are driven by another charger?

    Good point about bullk charge rates as I was wondering how to prioritize the chargers so that only one was driving the batteries. I was thinking a normally closed relay driven by the MPPT aux output could drop the AC to the Prosine. In this case the Prosine would be inverting and the MPPT would be charging at the same time. When the MPPT output drops the MPPT allows that relay to close and the Prosine picks up the slack associated with charging. Pretty much like you said, giving priority to the MPPT charge controller.
    I think if I were to attempt to program thresholds, I would wind up with a "logical race" condition. I have actually seen the voltage to the charger oscillate a bit when the charge rate the Prosine delivers starts to fall off, presumably at the start of absorbtion. The generator cuts back on output and then the inverter raises demand and the cycle repeats. Odd. Rather infrequent but it generally happens at low charge rates.

    As far as (1) goes I can see either charger wondering what the heck is going on when suddenly the active charger switches based on solar output. With random power outages on the shore power side this can happen frequently in the Bahamas (it's why we have the inverter and house bank). Assuming the batteries are in BULK charge from the Prosine, the sun comes up and the MPPT pops the Prosine out via the relay. The batteries are probably sitting at 14.1 when the MPPT takes a sniff and that means (I think) the charge state is going to be all balled up.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Adding Solar to an Existing Inverter/Charger system

    Since we don't actually have charge control via specific gravity sampling, everything is a compromise. :D

    I've no idea what Xantrex is on about, as charge controllers are solid-state devices meaning their throughput is basically one-way (panels to battery) unless something goes horribly wrong. You can use Xantrex controllers with other makes/models of both inverters and charge controllers. They just don't want you to. When you do use all Brand X equipment it can communicate and keep things agreeing fairly well. Same goes for Outback equipment. It is not strictly necessary, though, and doesn't always offer the 'agreement' you might want such as total current to the batteries. You need more complex equipment to handle that.

    I don't think the XW controller even has an AUX output on it. Outback and MidNite do. But there are limits to programming them, and coming up with criteria that would shut off AC IN whenever panels are producing could be tricky. Usually it's Voltage controlled, which is the same less-than-perfect scenario that causes the problem in the first place.

    With the right equipment (like Outback, not sure about the others) you can use some extensive criteria on auto gen start to control AC input based on Voltage and time of day. Detecting actual current from panels is another issue, as you'd want a certain level limit to disconnect AC (as in: "IF PV Iin > Imax OR PVVin > ABSORB THEN AC OFF" - which is a programming style that doesn't exist as far as I know).

    The simplest form is to just use a switch to keep AC IN OFF unless the panels aren't putting out enough, then flip the switch to ON.

    I run both PV and gen power. Normally the gen is not connected. If the panels aren't doing the job (cloudy day) I fire the generator up. Never had the 'too much current' problem but some people have (usually due to running the gen on a sunny day for other purposes and having it remain tied to the inverter-charger).

    With any multiple charging source set-up the usual problem is getting both sources to agree on Voltage. If you set the primary source (PV) Absorb higher and limit the Absorb time on the secondary (gen) it will usually work. Otherwise the secondary can hold the Voltage lower than desired while running through its Absorb stage. Battery temp sensors on both can help keep this in line by adjusting the Voltage level for both according to temp (which is when the discrepancy usually occurs). Using like equipment with a communications net (like all Outback talking through a HUB with MATE) helps too.

    But on the whole, a couple tenths of Volt variation now and then isn't going to destroy anything.
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