Voltage drop

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stmoloud
stmoloud Registered Users Posts: 111 ✭✭
in Solar Beginners Corner #1
Inverter meter reads 12.5V, another meter I have measures DC only, it reads 12.25. But there is only a little more than 3 metre / 10 ft between it and the battery bank.  Is a quarter volt acceptable over this short distance?
760W panel array, 4 x 6v 220 ah Crown batteries, Tristar TS-45 PWM controller,  no name 600 PSW inverter. 
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #2
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    The voltage readings of most inverters is a rough guide at best, 12.5 V could be anywhere between 12.46 and12.5 V. Using the other  meter measure both ends, this will be a more accurate means, assuming the meter is accurate. There can be no voltage drop without current flow, simple Ohm's law, V= I × R, if current " I " is zero, resistance " R " is 1 ohm the voltage " V " would be 0 × 1 = 0. 

    The voltage drop therefore increases in direct proportion to current at a given voltage and conductor resistance. As current flows through a conductor it's temperature increases, this in turn increases it's resistance thus increases the voltage drop. Using larger conductors reduces these effects as will keeping them as short as possible, especially between batteries and inverters due time the inherent high current values.

    So to answer the question if 0.25 V is acceptable as a voltage drop. The value would be dependent on how much current is flowing as well as conductor size , since two factors of the equation are missing, it is impossible to answer.

    This voltage drop calculator is handy to check if the voltage drop is acceptable https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.8152&voltage=12&phase=dc&noofconductor=1&distance=10&distanceunit=feet&amperes=10&x=37&y=24
    The lower the % the better, 3% is considered the maximum acceptable, 3% of 12 V is 0.36 V., feel free to ask questions.

    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • stmoloud
    stmoloud Registered Users Posts: 111 ✭✭
    #3
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    Something definitely wrong then, presently am pulling 1.7A on 12 AWG.  Should only be dropping 0.057 of a volt. So am losing some from somewhere. Not that it's making a difference, just trying for efficiencies. Thanks for the calculator. Curious, why should inverter voltage reading be inaccurate (if it is)?
    760W panel array, 4 x 6v 220 ah Crown batteries, Tristar TS-45 PWM controller,  no name 600 PSW inverter. 
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #4
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    Did you measure both ends with the same meter?  What current are you referring to AC or DC ?
     If a 1.7A load is on the AC side the load on the DC side would be approximately 10 times or ~ 11.7A, assuming 120 VAC, the inverter itself will draw some current and is not 100% efficient, using those numbers the DC voltage drop would be something close to what you are seeing.

    Inverter volt meters are generally inaccurate because an accurate meter costs too much to make and would drive the cost up. My Fluke 179, which is a good quality accurate meter costs ~ $350, as the price goes down so dose the accuracy and resolution from thousandths of a volt to tenths of a volt, in the case of an inverter.

    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • stmoloud
    stmoloud Registered Users Posts: 111 ✭✭
    edited July 2019 #5
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    The 1.7A is a DC load. I use the inverter volt meter to read the battery volts using the wired remote display as installed using the manufacturers directions.  The DC goes through another meter, the resolution is 0.01V.  I figure the inverter volt meter is accurate enough for solar because when the Tristar indicates going into absorb (14.6V - green LED blinking), the inverter reads the same battery voltage and when the Tristar drops out there is a similar alignment.
    Actually the problem I believe is one of resolution. The inverter meter is only 0.1V resolution and I am trying to read that against a 0.01V resolution DC meter. If I apply a digital meter of 0.01V resolution  directly to the battery terminals and then subtract the 0.01V resolution measurement of the DC meter, the voltage drop is much less.
    760W panel array, 4 x 6v 220 ah Crown batteries, Tristar TS-45 PWM controller,  no name 600 PSW inverter. 
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