Connecting panel

Discussion in 'Solar Power' started by treeman, Apr 20, 2012.

  1. treeman

    treeman Read Only Funster

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    Got my 218w panel on the roof now and have a 30A charge regulator attached to it and for test purposes i connected this to the LB with a cable and crocodile clips. Now I'm happy with everything works fine I want a permanent connection to LB. I notice the LB cables run through the MH to the back of the Zig panel.

    Is it acceptable practice to connect the battery outputs from the charge controller to the LB cables coming into the back of the Zig panel or should I route them all the way to the engine bay and connect directly to the LB terminals? I ask as it would be so much easier and tidier to connect to Zig cables but don't want to damage Zig or charge controller or batteries.
     
  2. Gromett

    Gromett Funster

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    Without going into all the details as to why. run the cables directly to the battery. Remember to fuse at each end of the long run and use a cable rated for the current at that length. Mount the charge controller as close to the battery as possible.

    I used 6mm cable for mine and have 240Watts of solar.
     
  3. veevee

    veevee Funster

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    Gromett, what is the purpose of having the charge controller as near to the battery as possible?

    If say there is 8m between the solar panel(s) and the batteries, why will it matter if the charge controller is say 6m or 2m from the batteries, the power will still have to travel 8m?

    Thanks
     
  4. Gromett

    Gromett Funster

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    Roughly speaking and without all the techno babble....

    Because there will be a voltage drop on the cable.
    The closer the charge controller to the battery the less this is at the battery terminals giving you a better charge.

    The voltage from the solar panels tends to be quite a bit higher so this is not [HI]as[/HI] critical...

    If you want all the technicalities let me know and I will write something up..
     
  5. veevee

    veevee Funster

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    Thanks for that.

    So it's less of a loss on the higher voltage side therefore the volt drop is less at the battery terminals.

    Does this make a noticeable difference over a total length of say 8 - 10m if the charge controller were at either end of the cable or even in the middle? That's using 6 mm2 cable.

    You been very helpful and thank you again
     
  6. treeman

    treeman Read Only Funster

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    Thanks Veevee for asking the question and thanks Gromett for the explanation. I wanted to ask the same question.

    It makes sense for me as my PV output is 24v so a loss on the run from the PV to the controller is no problem but the controller to battery is sent at around 14v so loss there would be more noticeable.

    Having said that, in my MH it is so easy to install controller 4m run from PV and 5m run to LB (both in terms of time and materials) but a major effort to install 8m run from PV and then <1m to LB so tempted to have a go at the easy install first and see if things work ok.
    Are there any other factors I should be considering apart from power loss?
     
  7. Gromett

    Gromett Funster

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    It not really that there is less of a voltage, it's that the charge voltage to the battery is more critical.

    Roughly speaking the charge voltage is 14.5V. If you put the charge controller 1mtr away from the battery and have a 1% drop you will lose 0.145V If you put it 5mtr away and lose 5% you will have a 0.725V drop. This directly impacts how much current will go into your battery and how close to a full charge you will get.

    Please note the above is much simplified and the figures are made up for clarity reasons. There are quite a few variables that will affect both the voltage drop and amount of current reaching the battery in addition some charges will compensate for this.
     
  8. treeman

    treeman Read Only Funster

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    Many thanks Gromett, I understand what you are saying and it will help me with my installation.
     
  9. veevee

    veevee Funster

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    Measured the voltage at the battery take off of the charge controller and again at the battery, the difference was 0.16v.
    There is around 4m of cable separating them which I am soon to replace, it's 1.5 mm2, far too small.

    I am surprised at such a small voltage drop over that distance, is it being influenced by something else?

    May I add my thanks too Grommet, it's very kind of you
     
  10. Gromett

    Gromett Funster

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    thats probably correct. Wouldn't you rather the voltage drop was 0.016v though :Wink:

    At 10 amps charge rate you are losing I X V = 1.6Watts of power. With a 218Watt panel you are probably going to get more than 10amps at peak....
     
  11. Reallyretired

    Reallyretired Funster

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    Its possible that the voltage drop is low because the current was also low. The voltage drop will incease proportional to the current so you need to know that too.
     
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  12. veevee

    veevee Funster

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    Hello Reallyretired

    Fortunately wrote the two voltages down, they were -

    13.52v to 13.36v

    It was late afternoon on a bright but overcast day, no load at all on the battery.
     
  13. Gromett

    Gromett Funster

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    Those details don't really cover the whole story to be honest.

    ReallyRetired is correct in what he says and I should have mentioned that but I was trying to keep things simple.

    To continue on the simplification route..... Two scenarios....

    Your battery is fully charged and you are in full summer sunlight your solar panels are not going to put anything into the batteries.

    Second option... You do an overnight and you watch 5 films back to back whilst your wife is using an iron and hoover on the inverter the batteries are going to be fairly low the next morning.


    In the first case the voltage drop is going to be approaching zero as nothing is going in. In the second case the batteries are going to be sucking down every milli amp available and the voltage drop is going to be respectively bigger...

    The amount of current flowing through a cable affects the voltage drop.
     
  14. veevee

    veevee Funster

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    Got it!

    I think very close to the first option, the charge controller doesn't show any amperage output to the battery as none is required.

    In two days time we are due a sunny day. I'll load the LB heavily around mid-day and measure the voltages again.

    Thanks to you Gromett and the other contributors, leave you all in peace on this subject now.
     
  15. veevee

    veevee Funster

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    Update

    Indeed the volt drop increased with a load applied.

    Sunny day with a light haze, mid afternoon 13.41v - 12.50v

    ditto late afternoon 12.97v - 12.35v

    I left the load applied for a good couple of hours between tests therefore the lower voltages.

    Both tests prove the point that was made previously.

    The charge controller will be moved about 1m nearer the battery and the 1.5 mm2 cable will be replaced with 6 mm2 cable.

    I wont be able to do a comparative test as a second panel will be added at the same time as the rewire. But thinking about it I may just move the charge controller first and complete the rewire from there to the battery to make a comparative test.
     
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  16. Gromett

    Gromett Funster

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    It's a shame you didn't measure the current going into the batteries for each test :Sad:
     
  17. veevee

    veevee Funster

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    I sort of did but only from the display on the charge controller, no idea how accurate that is. The voltage measured by the charge controller and independently is only 0.05 to 0.1v different.

    2.3A first and 1.2A second with the lower voltage.

    The timing of the first was around 3:30 pm and the second around 6pm, so quite late in the day.
     
  18. Gromett

    Gromett Funster

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    Just to give you some idea....

    13.41v - 12.50v = 0.91V x 2.3A = 2 Watts loss in cable
    12.97v - 12.35v = 0.62v x 1.2A = 0.74 watts loss in cable

    Please note this is not accurate but just ohms law based on the figures you gave.

    I am guessing your panels were not even putting in their full capacity. And if you add an additional panel the losses will only increase at a rate that is above proportional..

    What I mean by this is that if you were losing .74 watts at 1.2A you would expect if you double the current to 2.4 amps to be losing only 1.48 watts whereas at at 2.3amps you are losing 2 Watts. Doubling this up to 4.8 amps wouldn't lose you 4 watts it would lose you considerably more and possibly get to the stage where your cables were getting quite warm......

    Anyway, your solution of 1mtr and 6mm cable sounds great. Remember to upgrade the cable from your panels as well as the losses still apply there but having your controller closer to the battery is best.

    Good luck with the upgrade :thumb:
     
  19. Gromett

    Gromett Funster

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    Just a quick note. If you know the resistivity of the cable you can roughly calculate wattage using

    I2R = I X I X R

    So if the cable is 1 Ohm per meter and the cable is 1 meter.

    1amp = 1 x 1 x 1 = 1 watt loss
    2amp = 2 x 2 x 1 = 4 watt loss
    5amp = 5 x 5 x 1 = 25 watt loss

    The larger the cross sectional area of the cable the lower the resistivity.

    I will leave you to do further examples with longer cables and lower/higher restivity but this should show that current I (amps) has more effect that you would imagine.
     
  20. treeman

    treeman Read Only Funster

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    Gromett - I see why you kept it simple at the start - clearly a complex and involved subject but having read all the posts I get your first point that simply keeping controller close to LB is most efficient practice.

    Is the point also that larger section cable will be a more efficient transmitter of power?
     
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