Clearly not an engineer

  1. jdc

    jdc 9

    I have a three circuit solar array. Live off the grid. Each is 48 volt rated but two are really 90 volt rated (3 x 250 watt x 24 volt which are really 30 volt). The other is more like 60 volt. (2 strings of 4 80 watt panels x 12 volt -really 17.5 volt - total 640 watts. I have three + armored cable (3 wires plus a bare ground) - Tec cable #6. Why can't I run three circuits - using the three insulated wires - with the bare copper as the common neutral? The armor acts as the ground.
  2. jcsd
  3. jedishrfu

    Staff: Mentor

    Are you trying to hook them together in parallel or series? It seems that in parallel one might burn out the other because of the voltage differences.

    As an example, I can hook two 12V batteries together in parallel and everything is okay but if I hook a 12V and a 3V in parallel then there'd be an unwanted current going thru the 3V battery which wouldn't be good.
  4. berkeman

    Staff: Mentor

    Welcome to the PF.

    What is your electrical background? What is your experience with home wiring and building codes? What kind of inverter system are you running with these solar panels? What kind of loads are you wanting to drive with this power (including any storage systems, batteries, etc.)?

    What kind of terrain do you live in? Rural, urban, etc.? How close are your nearest neighbors?
  5. jdc

    jdc 9

    Electrical background: Zip. Nada. Zilch. Used How-to books to wire my house. Used how to guides to wire up original solar array and OUTBACK system. Outback is 48v. Original system was 640 watts. Keeping that system. New system is also 48 volts (by label) but made from different panels and even has different voltages. So, I was going to wire up another charge controller to handle the second system. Even tho the batteries are three banks (220 ah each), I was gonna 'pulse' in the 'charge' from the old system into the front of the battery bank and MPPT charge the 'back end. Whatever the different 'types of charge were, they would be 'cushioned by the battery bank. I think.

    Mountainous terrain. Solid rock, kinda. Ocean. Remote - beyond rural. Closest summer neighbours are half a mile but closest all-year neighbours are two miles.

    My house is typical off-grid stuff. CF lights. Propane appliances. The odd drain from the washing machine or the monitor/satellite/computers. I can live 'off the sun' in the summer with just the old system (640 watts) but I expect to live off the sun for the spring and fall with 2100 watts.

    Building codes hardly apply. NO one cares. No house insurance available. No roads. No 'grid' of any kind. Having said that, I built to exceed all codes so that part is NOT a problem.

    When I say, "I built....", I mean: my wife and built it all ourselves. No help. Except this kind of thing - a forum.
  6. berkeman

    Staff: Mentor

    Well all of that makes me a bit more comfortable with having a discussion here. It sounds like if you start a fire, you won't be catching your neighbors' houses on fire, or wasting the local fire department's time. Your wife is still at risk, but hopefully she understands the risk since she helped you to build it.

    It's also good that you are trying to build it all to code. Even if inspections are not an issue, the codes are there for safety, so it's good to always follow them anyway.

  7. jdc

    jdc 9

    More than that, B, if you read the codes as well as the How-to books, it is all easier to understand. And we needed 'easy'. Both of us are pretty stupid. How stupid? Well, I used to race motocross when I was young...just for starters (seeing your avatar) ;)

    No problem with wasting anyone's time. We are way, way remote. See:

    Having said all that............are you sayin' that what I am proposing is dangerous? I mean, aside from using the ground wire as a neutral and aside from back-and-front loading of the battery bank? By the way...I pretty much resolved the ground wire-as-neutral thing. I found some wire (we gathered a lot of supplies to be able to do this and sometimes I forget what I have) and so I am running dedicated wires from all three systems now.
  8. NascentOxygen

    Staff: Mentor

    I believe he is wanting to run them independently, but sharing a common earth return. 3 wires, plus an earth return of a bare copper wire.
  9. jdc

    jdc 9

    The original question was really two questions - how can I use my Tec cable for all of what I wanted to do. I was going to run three circuits using the three wires in my cable and use the ground as a common neutral. But then I found some more cable so I have decided to give dedicated wires to each circuit. Sounds silly to those who live 'on the grid' but, for those off, making do with what you have is crucial. So, I collect junk. Fortunately I collected some cable I forgot about.

    The second is a bit more 'physics' oriented. I am going to charge a bank of batteries from the charge controller that manages two fo the circuits and I am going to charge that same bank with a second charge controller that manages the third circuit. The only real difference is that one of the charge controllers goes on to the first (front) set of batteries and the second CC goes to the back set. In theory each CC is delivering the right charge so what difference does it make that one is getting a different input from the other? BUT one CC is PWM and the other is MPPT. That means they pulse 'juice' in differently. I figure the 12 batteries are like a giant electrical cushion and it doesn't matter if the front four get an MPPT charge and the back four get a PWM charge. Let the middle four sort it out.
    Last edited by a moderator: Aug 14, 2014
  10. that doesn't bode well for the lifetime of the batteries. I would suggest incorporating ultra capacitors, but i don't know if you have the capabilities.
  11. jdc

    jdc 9

    My capabilities are pretty limited but if there are capacitors for this, I can do it. IF I KNEW what an ultra capacitor spec for this was. But back to the main question...why does this bode poorly for the batteries? Why would the PWM going in one way and the MPPT going in the other make any difference to a lead acid battery? Aren't they even dumber than me?
  12. jdc

    jdc 9

    An ultracapacitor is, it seems, just a HUGE, more efficient capacitor acting as a battery. MIT is developing them. Which is good. For maybe, Ford Motor Company. But, for those of us living in the forest and 'making do', it is not an option. We Luddites have to use lead acid. Rich Luddites maybe Lithium ion. Maybe. Who knows? OUR rich Luddites still just use more diesel in their gensets.
    So, back to the lead acid....I have the option (I suppose) of 'isolating' four of the twelve batteries and then 'swtiching' them in and out on a rotational basis (like with the BIG RED Marine switch). But, I'd rather plug and play and let the sun do the work. Btw, the inevitable genset charge that comes in the winter comes by way of the charger built in to the OUTBACK.
    Your point is well taken. We OTG'ers (off the gridders) kill batteries like psychopaths kill hitchhikers. It's ugly. I do what I can but this is generation three in the last ten years. Five years is about max. Well, I may get 7 this time because each battery has a personality and I have learned to 'read' them a bit. Don't forget - we may be Luddites but batteries are prehistoric. Just a smidge more sophisticated than a big club. Battery development is the largest great leap forward that has yet to be accomplished.
  13. dlgoff

    dlgoff 3,158
    Science Advisor
    Gold Member
    2014 Award

    bold by me

    If you haven't seen this site yet, you'll probably enjoy it and it may help you get a longer battery life.

  14. jdc

    jdc 9

    Thanks, Don. No, I hadn't seen it and it will provide me with a lot of info. Appreciated. Mind you, I have come to know that everyone has at least two opinions on the nature of batteries but, as I have about wtenty, this might help narrow it down a bit.
    But what about the PWM up the back end and the MPPT in the front?
  15. jdc

    jdc 9

    Good ol' Battery U seems a bit dated. I have read some of their stuff but it is 'past era' stuff. They have solar panels at $5.00 a watt and that sort of thing. Doesn't make him wrong, of course, but it also seems NOT to be current. And, sadly, there is no reference to combining charger types - my main question.
  16. You should be using a MPPT charge controller between the panels and the batteries. The controller manual and/or the manufacturer's web site will probably instruct you on how to connect multiple panels of different voltages. They probably will even give a circuit diagram that you could follow.

    I believe that may be a better route to your answers than questions posted to this forum.
  17. jdc

    jdc 9

    You are probably right. But this is a theoretical question as well as a practical one. I really want to know what would happen doing it the way I proposed and why.
    Last edited by a moderator: Aug 14, 2014
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