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Grounding my battery to frame..Bad or good?

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  • Grounding my battery to frame..Bad or good?

    I have a converted a 5x8 v nose cargo trailer into a tiny off the grid camper. Below is a link to a video of my solar system. its all 12 volt no inverter. When I was installing my lighting I grounded the neg wire to the frame and just ran the positive wire back to the fuse box. My battery is also grounded to the frame. I have three panels 2, 40 watt and 1, 120 watt wired in series, a 30 amp mppt solar eagle charge controller and 1, 100 amp agm battery. For some reason I keep blowing diodes. I know my wiring is correct and that i am not overloading my system. When I bench test my controller it will do fine charging with two amp but as soon as it boost it to 6 amps I loose the magic smoke. The charge controller was new when I installed it. Any advice will be greatly appreciated.


  • #2
    https://www.youtube.com/watch?v=Sf7h2CwVwcs

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    • #3
      Wiring dissimilar panels in series isn't a good thing to do. The 120 watt panel is going to act like a 40 watt panel being restricted by the low amperage. You would be a little better off paralleling the 2 40 watt panels and then series wiring to the 120 watt panel. Even this wont be ideal as now your 120 is acting like an 80 watt panel.

      What MPPT controller are you using? There are many cheap "imported" controllers being sold as MPPT that aren't. If you are using one of these bogus MPPT controllers could be when you hit it with the high voltage of 3 series wired panels the voltage is high enough to cook it and hence, the magic smoke.

      ​ As far as grounding to the frame I don't think that has anything to do with your problem. I have built a couple camping trailers, albeit with inverters as well as 12 volt systems and haven't "grounded" either of them. Whether or not this is necessary or not I don't know, it never has been an issue for me.
      2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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      • Parlusk
        Parlusk commented
        Editing a comment
        Yeah bad internet advices is the reason why I wired them this way. I am in the process of rewiring them. I am going to run two charge controllers for right now. The charge controller is a true mppt controller, its made by solar eagle.

    • #4
      Proper Method.

      MSEE, PE

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      • Parlusk
        Parlusk commented
        Editing a comment
        My system is a full 12 volt system, no inverter, and its in a small camper so there isn't a good way to use a grounding rod.

    • #5
      Who said anything about a Rod? It is 12 volts, There is no requirement to ground the system. Ground does not mean earth.
      MSEE, PE

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      • Parlusk
        Parlusk commented
        Editing a comment
        I am sorry, I am looking at the diagram and saw where there was a grounding rod in it. My question is does grounding the battery to the frame of the trailer cause any problems, and if so what are the problems.

    • #6
      Define Ground:

      Ground is a reference point for voltage. It can be earth or something gin place of earth like the frame of a vehicle. In a Grounded System which means one of the current carrying conductors is bonded to a Ground aids in the operation of Over Current Protection Device. The fault current must have a path back to the source to clear a fault. Earth is not a source of power or has anything to do with it. In your case the battery is the SOURCE. Ground is just a path (wire) capable of safely conducting the fault current likely to be imposed on it.

      If you want to float your system meaning you do not bond the negative battery terminal, you must use two over current protection devices, on eon each polarity of the battery as in the top diagram I gave you. Notice the Floating DC System has four fuses, two on each polarity. The Grounded system or bottom picture only uses two fuse on the Hot ungrounded circuit conductor positive going to the load and controller. .

      So the answer to your question is how do you want to provide over current protection? Do you want to float or ground the system? Above 50 volts rules change and you are required to Ground the System. At 12 volts you have a choice. Which do you choose?

      Hint the Green wires and Rod are your vehicle frame. The drawing I used is showing a home AC panel. Just ignore the AC panel.
      Last edited by Sunking; 08-10-2016, 02:28 PM.
      MSEE, PE

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      • Parlusk
        Parlusk commented
        Editing a comment
        Thanks for the explanation. I am in the process of running two different charge controllers one for the two 80 watt panels and one for the 12 watt panel. I have to run a grounded system because when I wired my 12 volt interior and exterior lights I ran the negative side of the lights to the frame, and there is no way to change this. I have my neg and pos wire from the pv panels going into the charge controller and the neg and pos coming from the charge controller on my battery. My battery is grounded to the frame, and the pos wire from the battery goes into a fuse block and then from there it is dispersed to the lights, pump, fridge, etc. I do have cut off switched on the pos wire from the pv panels and one on the pos wire going to the battery. I am just trying to figure out was it my fault that the charge controller blew a diode of was it a problem with the charge controller.

    • #7
      You are fine using a Grounded System. If done incorrectly is worthless. Will not mess anything up, just will not work or work well when needed.

      So the question is how is it done correctly?

      Well you are on the right track. You want a solid made connection between the battery Negative Term Post and the vehicle Frame. Allow me to define a little deeper.

      Take note of any car made the last 50 years how they bond the Negative Term Post to the Motor Frame or Block. They use a heavy cable with irreversible compression connections. They do this for two reasons.

      1. They use the the frame of the engine block as the Negative High Current Starter Conductor which saves them money and no better method to minimize voltage sag from 300 to 800 amp burst it takes to crank an engine.

      2. To ground the electrical system and provide a planned path to safely handle any likely current imposed on the system to facilitate fast operation of over current devices like a fuse. Take note auto manufactures not so long ago used the frame of a vehicle as battery return or the Negative Conductor. They did that for the exact same reason they use the frame for the starter currents because it, is cheap and the best way to minimize voltage sag. But tit does cause problems for today's electronic vehicles of today. The frame having current pass through it causes voltage potential differences, and those differences interfer with the electronic sensors and radios. Today they now run negative returns and use the frame as a noise free plane free of currents.

      So here is the take away of how to do it correctly. Do what the manufactures do. Strategically install/locate the battery near or on heavy framing so you can bond wire to it. Use a minimum #6 AWG or larger copper stranded wire. It needs to be a large wire and as short as possible. Use Compression terminal rings. Remove paint and make vehicle frame metal nice and shinny. Use some No-OX grease and 1/4 inch hardware with two-hole terminals bonded to frame. Bond Single ring Compression Ring Term to Battery Return Post. Mimic what the manufacture does.

      You want to make sure it is heavy duty hardware and secure by using No-Ox and proper hardware configuration. The wire has to be able to handle hundreds of amps of battery current. You want it to be as much current as possible. That is what makes fuses work and work fast. So now if any hot battery wire gets loose and touches the frame, is just like it touching the battery NEGATIVE TERM POST anywhere in the vehicle. You get a really loud pop and fire if something touches anything metal for a milli-second and scare the crap out of you. Its over before you can react because the fuse operated almost instantly thus only minor burn damage to the wire and frame not amounting to much. Most of the damage is in your underwear. .

      Edit Note:

      Do not use the vehicle frame as the battery negative return conductor if given the choice. It was a bad practice decades ago and still a bad practice today, Even worse today as most vehicles do not have solid frames, (uni-body) so not the best conductor today. Ham Radio operators have a beast of a time with it today. Be sure to make you a Return Buss Circuit like a copper buss bar to run Negative with Positive Fused mate conductor. Everything gets two wires from the battery. Don't forget to take length into consideration as it may require larger conductors than the minimum safe size.

      Take note on the pics. Even a Grounded System requires two fuses. One facing and sized for the Controller, the other for the Battery. You are protecting the wires connected to the SOURCE aka Battery. You have two circuit wires on the battery term post. One from the Controller, and the Battery Loads. One fuse for each circuit.

      Good Luck

      SK
      Last edited by Sunking; 08-10-2016, 03:32 PM.
      MSEE, PE

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