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Thread: 12v Problem solving!

  1. #1
    Chilling Out jonathanh1989's Avatar
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    12v Problem solving!

    Last year I bought 4 used 6v Rolls batteries which are connected to a mppt controller (tracer). I noticed the batteries (according to the monitor attached to the mppt) seemed to be draining far too fast so I came to the conclusion I'd bought duff batteries that weren't holding charge.
    This year my 2 fan vents will not run, the error message on both of them tells me it's a problem with under voltage yet my led lights work ok.

    I figured my batteries are on their last legs so bought a multimeter to check them.
    After disconnecting them and allowing them to rest I read 6.5v on each of them (which I understand indicates fully charged), over the last few days I've kept them disconnected and read the volatge multiple times and it's remaining constant so they aren't loosing charge rapidly.
    One of the batteries terminals had begun to corrode so I cleaned them up wondering if that would cause a volatge drop. Once reconnected to the mppt I measure 6.4v on each battery. I checked the connections from the batteries to the mppt and and am getting 12.79v, I checked the leads running to the fans and get 12.79v at both the fuse box and the fan end.

    I feel like I keep banging on about my batteries but I'm running out of ideas. Why would the mppt show them as depleted if a multimeter reads them as full? Why would the fans indicate low voltage and fail to start if full voltage is reaching them?

    I was prepared to buy new batteries but it seems like the batteries might be ok. Can anyone help? Any advise would be sorely appreciated. Is anyone fairly local and good with batteries who could possibly take a look for me?

    Thanks in advance
    Jonathan


  2. #2
    Comfortably Numb Rick69's Avatar
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    Have you set the battery type and the rating correctly on the tracer?
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  3. #3
    Chilling Out jonathanh1989's Avatar
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    Quote Originally Posted by Rick69 View Post
    Have you set the battery type and the rating correctly on the tracer?
    Yes sorry missed that part out. I didn't realise you could edit it when I first bought it but yes it's now set to 440ah flooded. Unsure about the other settings available although I've looked through them and can't see anything else relevant.

  4. #4
    Comfortably Numb Rick69's Avatar
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    Are you sure they are flooded, i may be wrong but i thought they were gel

    If you are using the mt50 the manual is on the interweb

    - - - Updated - - -

    Also what is theah rating of each battery?

  5. #5
    Non of this matters NomadicRT's Avatar
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    So youve got 4x6 volt batteries (24volts) so have you split the output from two of them to the mppt? for 12v systems ?
    Hebridean at heart..everywhere else is just somewhere on the way back there...

  6. #6
    Comfortably Numb Rick69's Avatar
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    Or to add to the above are they seriesed in two and then paralleled to give 12v
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  7. #7
    Comfortably Numb Rick69's Avatar
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    If they are all wired for 24v the ah will be what it says on one battery, not all added.
    If the are paralleled for 12v the ah will be twice what it says on one battery.

  8. #8
    Heavenly Creature realnutter's Avatar
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    Measuring voltage doesn't always tell the whole story....

    A bit of basic science.... The voltage dropped across a resistance depends on the current flowing through it... expressed by the equation V=IR... where v is volts, I is current in Amps, and R is resistance in Ohms...

    Your DVM is designed to have a VERY high input impedance, so as to have minimal effect on the circuit under test... this means is draws virtually zero current when it takes a measurement... thus, if there is any resistance between the measurement point, and the battery, the voltage drop across that resistance will also be VIRTUALLY ZERO, and you won't realise it's there...

    Get yourself a 10 Ohm, 20 watt resistor to simulate a load.. it'll draw 1.2 amps from a 12 volt source.... with the resistor acting as a load, measure the voltage at the batteries, and at the load.... if the voltages are very close, then you have negligible resistance in the circuit between them... If the voltage across the load is significantly lower than the voltage across the batteries, then you have some resistance between them...

    Remember also that cells have internal resistance which rises with age... a new battery (stack of cells) has low internal resistance, so the voltage falls off slowly as the current rises... In older cells, the internal resistance is higher, so the voltage drops quickly with increasing load....

    A good car parts place (emphatically not Halfords!!) should have a battery tester, which consists of a flat copper bar between two probes, with a voltmeter in parallel.... This measures the battery's behaviour under very high loads - 100+ amps - and is the best way to assess the internal resistance of the battery....

    In short... check your wiring between the batteries for good connections, corrosion, and suitably fat cables... make sure the wires to your loads are fat enough for the expected current... and take your 6v batteries in pairs to someone with a proper old school battery load tester... (they're designed for 12V)....

    Hope this helps....
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  9. #9
    Chilling Out jonathanh1989's Avatar
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    Definitely flooded. Yes they're wired in 2's to provide 12v, 220ah rating each so 440h at 12v.

  10. #10
    Chilling Out jonathanh1989's Avatar
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    Thanks for the replies folks, keep them coming. Realnutter I'm still trying to digest your post, I don't fully understand the resistor part. I can confirm I've checked all the battery cables/connections and they look fine. I also worked out the voltage drop on cabelling and opted for thicker cable than necessary. Battery cables are 40mm, mppt/fuse box is 16mm.

  11. #11
    Chilling Out jonathanh1989's Avatar
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    Just read through your post again and it make sense! (Its been a long tiring day).
    I really don't think the problem lies with voltage drop though, as I said I deliberately oversized the wiring and was careful not to overload each circuit. It also wouldn't explain the mppt info.

  12. #12
    Heavenly Creature realnutter's Avatar
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    Sadly that does point to your cells being tired.....

  13. #13
    Chilling Out jonathanh1989's Avatar
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    Quote Originally Posted by realnutter View Post
    Sadly that does point to your cells being tired.....
    My cells are definetly tired today!

    As for my battery cells... If they are tired why is my multimeter lieing?
    I was all ready to buy 2 Trojans to replace them but the multimeter has thrown me into uncertainty.

  14. #14
    Comfortably Numb Rick69's Avatar
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    As realnutter explained above - basically ignore what the multimeter says, you cannot properly test a battery with one.
    Go to a battery place and ask them to test them for you with, as matt said a proper drop tester.
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  15. #15
    Chilling Out jonathanh1989's Avatar
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    Would some mechanics have the equipment or am I after a place that specificly fits batteries?

  16. #16
    Heavenly Creature realnutter's Avatar
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    Quote Originally Posted by jonathanh1989 View Post
    My cells are definetly tired today!

    As for my battery cells... If they are tired why is my multimeter lieing?
    I was all ready to buy 2 Trojans to replace them but the multimeter has thrown me into uncertainty.
    You're multimeter isn't lying, just telling half truths.... It takes virtually no current from your batteries, so gives you no idea about how your batteries behave with modest or large currents being taken....

    If you want to fully understand the concepts, you need to get your head around Norton's Law (summarised as current into any node is equal to current out of that node), And Thevenin's law (summarised as the sum of all voltages around a circuit = 0).... links here:

    https://en.wikipedia.org/wiki/Norton%27s_theorem

    https://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem




  17. #17
    Radiant Being patchypete's Avatar
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    Think i'll get Rick and Realnutter to do my electrics , they be Wizards
    "The best way to get something done , is to begin "
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  18. #18
    Heavenly Creature realnutter's Avatar
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    Quote Originally Posted by jonathanh1989 View Post
    Would some mechanics have the equipment or am I after a place that specificly fits batteries?
    Typically, a little, long established independant car parts place will have one.... Halfords won't...
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  19. #19
    Comfortably Numb Rick69's Avatar
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    Lol halfords have one, and you can nearly guarantee that it will tell you they are fucked and you need some new ones, which they just happen to sell
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  20. #20
    Non of this matters NomadicRT's Avatar
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    Quote Originally Posted by jonathanh1989 View Post
    My cells are definetly tired today!

    As for my battery cells... If they are tired why is my multimeter lieing?
    I was all ready to buy 2 Trojans to replace them but the multimeter has thrown me into uncertainty.

    I think.before you do anythingvto your system you should do as Matt suggested and get your batteries checked since theyre the oldest link in the chain

    You need a car parts place or battery centre with one of these
    Hebridean at heart..everywhere else is just somewhere on the way back there...
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  21. #21
    Heavenly Creature realnutter's Avatar
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    Yeah, that's the baby!!!! If the needle stays in the green, with one of those, you've got a good battery!!!
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  22. #22
    Chilling Out jonathanh1989's Avatar
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    Thanks guys, I'll get phoning around and find somewhere to get them tested.

  23. #23
    Chilling Out jonathanh1989's Avatar
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  24. #24
    Non of this matters NomadicRT's Avatar
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    Quote Originally Posted by jonathanh1989 View Post
    Thats the more modern american style equivalent....yes it will do the job.
    Last edited by NomadicRT; 24-02--2017 at 11:19 PM.
    Hebridean at heart..everywhere else is just somewhere on the way back there...
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