Help Combining Two Systems.

MexDog
MexDog Solar Expert Posts: 28
More than a year ago I posted and got a ton of help designing a 650w, 24v system for a small ranch house I have in mexico (4x Sharp 165's w/ outback mx60 cc, & 8 trojan t-105's). This forum was by far the best source of info I found and was invaluable to making the installation a great success. Thank you!

But now I have another situation to deal with:

In the house there is a separate, existing 12v system set up by the previous owner. That system was undersized and has always had trouble keeping up with usage. My 24v system on the other hand produces more than 3KW daily, nearly double my very modest usage. I'm trying to figure out the best, most cost effective way to use the new 24v/110 system to run the lighting string which is already installed throughout the house.

Here are some of the options I've been considering:

1) Use a DC to DC converter. My concern with this is that I would lose the smart charging benefit.

2) Use a separate charge controller to act as voltage converter and smart charger for the 12v battery bank. the issue with this is that I doubt that any of the cheaper, non-MPPT cc units could function as a voltage converter due to possible amperage spikes, and not even sure if one of the cheaper MPPT controllers can handle 24v, battery input.

3) Run the 12 lighting string off the 110 system directly, bypassing the 12v battery bank, using a 110AC to 12v DC transformer.

4) Convert the existing 12v light string to 110 by simply connecting the main load leads for that system to a regular old plug, changing out the bulbs from 12v cfls to 110v cfl's, and plugging 'er in. The original 12v system was set up using a standard single wire common in Mexico which I believe is 14AWG, and standard household switches and bare bulb light sockets. The downside to this solution is that it would eliminate the 225 amp/hrs worth of capacity provided by the existing 12v battery bank (2-Trojan t-105's).

A couple other issue that complicates the scenario are:

1) i suspect the existing 12v system may have voltage drop issues due to the 14AWG sizing. unfortunately I didn't know enough to test it or measure the runs when I was last there. I believe that is part of the reason why that system has had trouble keeping up with usage all along. Assuming this is correct, I'm wondering if running it as 110 directly off the main system (option #4 above) would possibly offset the loss of capacity if taking the 12v battery bank out of play? (I realize that without exact wiring run spec's any answer to this will be only a guess).

2) The 12v system is located at the opposite corner of the house so voltage drop will be a factor if pursuing one of the DC to DC options.

I'm leaning toward option #4. Anyone care to take a stab at this rather long winded puzzler?

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Help Combining Two Systems.

    I think your #4 is the way to go...
    1. 12 volt loads require 10x the current vs 120 VAC loads--A 120 VAC circuit can have a 2 volt drop and not even notice an issue. A 12 VDC circuit with a 2 volt drop probably will not work.
    2. Anything you use to convert 24 volts to 12 volts will have a loss. Plus, you have the losses of your 12 volt batteries. No efficiency gains.
    3. From what I have seen, 120 VAC CFL's are cheap and many options. 12 VDC CFL's are expensive and not many choices.
    4. To conserve battery power--match your loads with your inverter. For "flip the switch" convenience, look at a small inverter with low standby losses that matches the wattage requirement of the lighting string (if this is an issue).
    5. Can even get into the issue that 12 VDC is very hard on switches (contacts can arc and burnout with DC--AC is much less of a problem).
    Unless you have other uses for the 12 VDC battery bank--just retire it (or move/piece part the panels, lights, and batteries to another, more useful location, where it can live out its life).

    Any money you spend on the system (down converters, some sort of charger, replacement parts/12 vdc lights/batteries) will just keep the problem around longer and probably keep issues/maintenance costs higher than a single unified system.

    My 2 cents worth...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Help Combining Two Systems.
    MexDog wrote: »
    2) Use a separate charge controller to act as voltage converter and smart charger for the 12v battery bank. the issue with this is that I doubt that any of the cheaper, non-MPPT cc units could function as a voltage converter due to possible amperage spikes, and not even sure if one of the cheaper MPPT controllers can handle 24v, battery input.

    This was just recently covered in depth here in the NAWS forum:

    http://forum.solar-electric.com/showthread.php?t=5770


    The upshot is that no, you cannot do it with a non-MPPT type controller, as any other type will just connect 24v directly to the 12v and toast the battery.

    However, according to Solar Guppy, you can use an MPPT controller to down convert from 24v to 12v *if you use a 12v battery in the system*:

    If your going to have a 12V battery, then the MorningStar Sunsaver Mppt will do exactly what you want. It will down convert 24V to 12V and keep the battery fully charged which your loads can be peak more that the 15amp without any issues

    This is different than the question you asked of them which was having the Mppt unit just be a down converter ( no battery ) and hence what I am saying is different than what Adam suggests.

    The Sunsaver is small, efficient and should be a perfect fit for your need, with the understanding you will have a 12V battery. I would suggest a AGM 100 ah should work well for running the electronics"
  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    Re: Help Combining Two Systems.

    G-d invented the "Current Limiting Resistor" so that PWM charge controllers can be used in this application.

    My concern about the MPPT charge controllers is that batteries are a stout current source. How will they handle a virtually infinite power supply? There isn't a Vmppt to track because Vmppt is Vhigh. Which means they were working like high frequency PWM controllers ...
  • MexDog
    MexDog Solar Expert Posts: 28
    Re: Help Combining Two Systems.
    BB. wrote: »
    I think your #4 is the way to go
    ...

    Thanks all. Guess I'll go with #4 then unless anyone sees any clear reasons not too.

    That options leads to a new "part B" question though:

    The existing 12v lighting system is ungrounded (the previous owner set it up this way, not me!). If I switch the bulbs over to 120v and run the two leads to the string off the 110, do I need to worry about polarity or grounding? The 110 system is not a hard wired system. I am running the 110 loads (basically just one small 7ft frig and a few miscellaneous items) off a Go Power 1000 inverter which has a GFI outlet. I was planning to simply add a plug to the two leads from the lighting string and plug it in.

    Again, the question is; does the 110 converted 2 lead lighting string need some sort of grounding added, or will the GFI outlet take care of that. If it does need additional grounding, is there a way to accomplish it without having to re-wire the entire lighting string? I assume that there are no polarity issues, but would like a reality check on that as well.

    Thanks!
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Help Combining Two Systems.

    Grounding... That is a big old pile of worms.

    First "polarity"... In North America, we have a Hot and Neutral (really grounded) wire for 110 VAC...

    However, for many/most MSW (Modified Square/Sine Wave) Inverters--you cannot/should not ground reference one AC output. If one AC output is grounded, and the battery bank negative terminal is grounded--usually will release the "magic smoke" from inverter.

    For TSW (True Sine Wave) inverters, many/most can be ground referenced (as always, read the manual for your product).

    Ground referencing for light fixtures is common--prevents people from getting shocked if the touch the metal base of the bulb while screwing it in.

    Also, for many florescent fixtures... They need a true earth grounded metal frame/reflector near the bulb to help them start.

    Next--Ground Fault Breaker. The GFI measures the current going out the "hot wire" and coming back on the "neutral wire". If the current is not the same (i.e., current flowing through the safety ground)--the breaker will trip/fault.

    In and of itself, a GFI does not need the AC to be ground referenced to work. However, the self test circuit connects the Hot to the Ground to test itself... If the Neutral (and the GFI body) are not ground referenced--the self test will probably fail. If you have a MSW inverter and a GFI grounded to the same battery ground--the self test should work (never tested myself--but I think it will be fine because the MSW is does not isolate the AC output from the battery--no isolation transformer of most MSW inverters).

    Lastly, grounding is intended to prevent metal parts of appliances from being energized with respect to ground and having a person accidentally touching the device. You can certainly create a local ground with a ground rod (battery negative grounded, GFI outlet)--if you need to ground something. But, this will not exactly copy a grounded neutral (which you probably cannot do with an MSW inverter) where a hard ground fault by the hot lead would trip the protective fuse/breaker. You are sort of limited what you can do for grounding with MSW inverters (sorry--being repetitive--but trying to be clear).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MexDog
    MexDog Solar Expert Posts: 28
    Re: Help Combining Two Systems.

    Thanks Bill. Unfortunately much of that was over my head. The 110 inverter is pure sine wave and is properly grounded already. The lighting string I'll be converting from 12v use to 110 use by simply replacing the bulbs with 110 cfl's and pluggin it into the inverter's built in GFI receptacle, has no ground connection anywhere along its length. Will the inverter already be providing the ground referencing you talk about, even though there isn't a third ground wire in the lighting string that will be plugged into it? I am assuming that since many appliances, lamps, etc have only two wire leads these days, that the two lead lighting string will be OK, i.e. protected by the properly grounded inverter, but I am looking for a reality check on that.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Help Combining Two Systems.

    Sorry MD,

    I probably read MPPT as MSW--dyslexic.

    Anyway, if you have a properly grounded TSW / PSW inverter (neutral is ground referenced. And in install a GFI breaker in a grounded metal box (or GFI breaker is connected to your system "green wire")--there is not a lot of reason to run a second green wire unless you have exposed metal boxes (such as where your lamp socket mounts in a wall/ceiling.

    It is possible with ungrounded metal boxes that you get a hot to box short--and it waits to bite somebody (the GFI can reduce the shock hazard). Using plastic boxes could make things safer (if the box is exposed and where somebody may touch (say a light switch box next to the kitchen sink).

    Using two outlet appliances (typically, double isolated) is perfectly safe and sane not to run a separate ground wire (that does not connect to anything anyway).

    The "old" wiring in homes 50 years ago did not really care (most outlets only had two prong outlets and GFI was not even invented yet).

    Just pretend you are a person who is not attentive--or possibly a little kind who puts their hands on anything--and imagine there is a "loose" hot wire touching an ungrounded piece of metal near a sink or tub, etc.). If it worry's you--do something about it (insulate or bring a local green wire to box). The GFI will help too (any shocks should be small).

    I would probably not rewire a cabin to fix the issue (I only fixed my home "issues" (GFI/Three Prong Plugs) when I remodeled or if I needed a three prong outlet for tool).

    There is SAFE, Safe, safe, and safe. When is it overboard to be safe... I really cannot tell you anything more specific. We have all been in old homes/apartments and survived nicely without GFI and 14 awg green wire grounds...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MexDog
    MexDog Solar Expert Posts: 28
    Re: Help Combining Two Systems.

    "Anyway, if you have a properly grounded TSW / PSW inverter (neutral is ground referenced. And in install a GFI breaker in a grounded metal box (or GFI breaker is connected to your system "green wire")--there is not a lot of reason to run a second green wire unless you have exposed metal boxes (such as where your lamp socket mounts in a wall/ceiling."


    OK. The GFI is "built in" to the inverter. My little receptacle tester indicates all good there. I'm running my appliances off that one double receptacle - not hard wired - but I'm well below the 15Amp limit anyway . The new, ungrounded lighting string I'm adding to it will be a half dozen cfl lights only - no appliances. All lights are ceiling mounted and switches are in plastic boxes. Relative to a lot of other wiring in mexico this will probably be extremely safe. Your replies nails it home and will keep me sleeping soundly though. Thanks!