Voltmeter Question

Discussion in 'Power - Site Power/Batteries/Generators/Solar' started by mtndrew, Feb 13, 2018.

  1. mtndrew

    mtndrew Member

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    Hello again wise POP folks -

    OK, so I have my new 6v batteries wired in series (230 AH) and my new voltmeter installed directly on a free wire from the converter. I have been testing a few things out in the driveway before heading out for a cold desert trip this weekend.

    My main question (which has been discussed in many threads I have read - but I am looking for a definitive explanation if possible) is whether or not to interpret the voltmeter under load. Most threads I have read suggest to avoid interpreting your voltmeter under load; however, the 12 Volt Side of Life says, "This chart [the standard Voltage --> State of Charge chart] is designed to be used when monitoring a battery under load. This chart is a little more useful to the average RVer, as we are most interested in monitoring the state of charge of our battery bank while it is actually in use."

    For me, the variance is pretty huge (I have confirmed that the voltmeter is accurate with my multimeter). Even with <25 degree temps, the voltmeter reads 12.5 or 12.6 at rest. When I run the furnace and 4 lights (not yet switched to LEDs - they are on order, the PUP is NTU) during the cold of night the meter drops down to 11.9 or 12.0!

    When I shut things down the meter jumps back up to ~12.5.

    Let it go and enjoy, or be concerned? Thanks!
     
  2. rabird

    rabird Howdy!

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    bully for you for reading the fine print on that chart, too bad he does not suggest batt size and load!! Lots of folks post that chart and ignore that fine print, but again the load size is not mentioned.

    The bigger the load the bigger the voltage drop, also poor connection cause voltage drop.

    your voltage drop for such small load compare to batt size seems high.

    Got a way to hit the battery with a few hrs of 14.8v, or more ?
     
  3. mtndrew

    mtndrew Member

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    Thanks for the quick reply - I do have a B&D 15 amp charger. I should have left it plugged in today, couldn't overnight as there was a chance of snow.

    I need to put the PUP down before the neighbors get grumpy (HOA rules) but maybe I'll leave it up one more night.

    Anything specific to watch for?
     
  4. tombiasi

    tombiasi Well-Known Member

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    Most battery readings that are NOT under load are useless for these purposes.
     
  5. mtndrew

    mtndrew Member

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    OK, I appreciate the comment. From a pragmatic perspective, does the lower reading mean anything of concern? The batteries were fully charged - so out camping, if I get a 11.9 reading while everything is running, does this mean you need to do a charge of some sort ASAP?

    I quasi-understand the science, but it does not make sense with this battery bank (fully charged) would be running at sub-50% after only a few hours of furnace/lights. I am 99% confident that there is not some weird parasitic draw occurring from an external source.

    I also wonder if the temperatures +/- 25 degrees makes an impact of some significant source.

    Thank you! ajn
     
  6. tombiasi

    tombiasi Well-Known Member

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    The battery will hold a top charge after charging which will quickly dissipate with a load. The battery voltage under load is the only number you need. If you get 11.9 with everything running I would say you need a charge. If the voltage rises considerably with no load then the battery condition is suspect. This is caused by a high internal resistance which is a sign the battery is not at it's best. In any case, the loaded voltage is what you will be using.
     
    theseus likes this.
  7. xvz12

    xvz12 Active Member

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    It sounds to me that your main lead-in wiring is a bit small for the power you're putting thru it...I don't know the first thing about Forest River, but I do know that manufacturers cut every corner they can to maximize profits. My admittedly older pup showed exactly the same symptoms when I wired in my voltmeter....I had just hooked it into the existing wiring, & what you & I were seeing was the the voltage drop from the resistance of the wire, which is probably marginal for the job it's doing. To remedy this, I ran dedicated wires from the voltmeter to the battery bank itself, right after my master disconnect switch, & now the voltmeter show a very minimal drop when everything is turned on....it is now registering the battery voltage directly, rather than after it's ran thru several feet of wire, with other devices pulling power thru it. Hope this helps.
     
  8. mtndrew

    mtndrew Member

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    @tombiasi & @xvz12 thank you - I hear you both and thank you. I understand clearly that evaluating battery charge under load is the way to go - thanks for addressing my question. Unless I got faulty batteries (which I guess can happen) I am hoping that the scenario shared by xvz12 is what might be happening with me. I have some extra 10-gauge lying around and when I have a moment I will run it directly to the voltmeter from the battery, shouldn't be too hard. I still need to wire a disconnect switch downstream from the emergency brake away anyway.

    I really appreciate your time. If it was just me, I have happily slept on the ground or in my truck for 30+ years, trying to get young kids out more I just want to be safe and comfy - that is the point of all this, to have fun with the family outside.

    Take care, ajn
     
  9. tenttrailer

    tenttrailer Art & Joyce - Columbus, O

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    Have you tried reading the voltage across the batteries when you have that low reading? I thinking you will see a higher voltage across the battery. If so you are looking at a voltage drop due to wire size and load. Kinda like switching on a large vacuum in a room and seeing the lights dim for a fraction of a second.
     
  10. mtndrew

    mtndrew Member

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    Yup, so I had to run out there (24 degrees, not too cold) and see...I turned everything on and the voltmeter went straight to 11.9.....Put the multimeter on the batteries and 12.37, which makes more sense.

    So, yet another wiring project in my future....

    I am gonna blow it off for now, charge the batteries, pour a whiskey and go camping in the desert this weekend.
     
    jnc likes this.
  11. tombiasi

    tombiasi Well-Known Member

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    This shows the resistance is in the wire.
     
  12. rabird

    rabird Howdy!

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    This can be from one or more poor connections. The common connection to frame (from battery and/or converter) is a good place to start. All connections need to be clean and tight. Take a ring terminal, not only does the wire need a clean/secure connection to the terminal it also needs a clean/secure connection to what it connect to.
     
  13. tenttrailer

    tenttrailer Art & Joyce - Columbus, O

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    I'm not sure I would no worry about it. You may just have some resistance at the different connection of that circuit the meter is on, out to the battery or on the ground circuit. If it was a concern to me ohm out the wire and ground back to the battery. I would clean all those connections that show resistance. I would think from end to end less than one ohm.
     
  14. tenttrailer

    tenttrailer Art & Joyce - Columbus, O

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    I'm not sure it is wire resistance. I would think you have at least #12 wire. Google wire resistance chart. It's been a few years since I look at those charts, but I would expect you will find #12 has a resistance of maybe 2 ohms for 1,000 feet. you are not even close to 1,000 feet, maybe 40 feet to and returning by the ground. So 40 feet would be 40/1,000 x whatever the chats reads.
     
    Last edited: Feb 14, 2018
  15. SteveP

    SteveP Active Member

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    A few, possibly redundant, thoughts.
    I don't think I've ever bought a battery that was fully charged at delivery, usually 75-80%. I've learned to always make sure a deep cycle battery is fully charged before first use.
    I've read, I think on the Trojan site, that their batteries require 3 to 5 charge cycles before they reach full capacity, but this should not affect the voltage drop you're seeing.
    I expect to see a 0.2 voltage drop on the display of my solar controller when under light load, at 50 to 70 degrees F, which is consistent with the chart on 'The 12volt Side of Life.' You know, based on your testing, to expect a 0.4 drop to the meter so extrapolate from there, but I would also test the voltage at one of the lights to verify.
    I've found that voltage sensitive appliances, my cpap for example, perform best when they are powered directly from the battery, or in my current implementation, separately wired directly at the converter.

    You should be good for a weekend, so go enjoy.
     
    Last edited: Feb 14, 2018
  16. mtndrew

    mtndrew Member

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    OK, thanks to everyone again - It is really interesting to learn this stuff. I had only done one full charge on those batteries, so I will do another over the next couple of days.

    I will also geek out and monitor things over an actual weekend of usage, I see SteveP's point per calculating an ~ 0.4 drop (after confirming at a light).

    Here is a rookie question, let's say I am having disproportionate resistance due to wonky wiring from the battery, using too small of a gauge wire between the switch controlling the voltmeter, etc. will resistance serve to drain the battery faster?

    I would intuit that resistance could potentially represent a fire or fuse burnout hazard, but I don't see how this would strain the battery extra? But perhaps the battery is having to 'work too hard?' This conversation has approached the limits of my pay grade I'm afraid.
     
  17. roybraddy

    roybraddy Well-Known Member

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    I am using 4AWG size wiring on my 12VDC battery bank setup which is giving me 255AHs capacity. I really should be using 2AWG size cabling. 10 gauge wiring you have mentioned is way to small for your 230AHs capacity batteries...

    This is my three 12VDC GP24 batteries in parallel setup. I also use the Blue Sea Switches you can see on the cross member frame...
    [​IMG]

    [​IMG]
    Roy Ken
    [​IMG]
     
    Last edited: Feb 14, 2018
  18. SteveP

    SteveP Active Member

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    12 watts is 12 watts, so as your voltage drops the amps increase to make up the difference. This will have a tiny effect on longevity of the battery but with 220 amp hours you should be ok for a weekend unless you are a real energy hog.

    Your battery can easily handle 15-30 amp loads and if it exceeds that you'll blow a fuse, big deal.

    I doubt that it's a wire size problem, more likely a loose or corroded connection. Like Rabird said. start with the ground connection on the tongue and work through the system checking the connections.

    The 12volt Side of Life
     
    Last edited: Feb 14, 2018
  19. tombiasi

    tombiasi Well-Known Member

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    Steve this is not true. If voltage goes down the current has to go down for the same load. Watts is a rate of doing work.
     
  20. Antipodes

    Antipodes Call me Paul

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    Not quite right in that it may or may not depending on the load.
    Lights which tend to have a resistive load generally dim and the the amperage may go down as tombaisi has indicated.
    Motors are inductive and as voltage goes down current tends to go up. One of the reasons a motor is a risk from brownouts.
    And of course Steve is using Ohms law. (DC)
    V x A =Watts will be less if both voltage and current are down
    12 V X 1 amp =12 watts
    11 V X .9 amps =9.9 watts
     

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