Current flow / Lack of knowledge

Hello all,

I have a question about the wiring of my battery bank and the flow of current.

The bank consists of (4) 6volt 420ah U.S. Battery L16HC XC. The interconnects are all within a 1/4" in length of each other for similar connections. The parallel connections are twice the length of the series, and the connections going to the busbar are for the negative 18", 14" for the positive.

I have included a diagram showing the way the bank is wired, and the current readings using an Extech Clamp Meter, the one that Bill recommends in his posts, it works great.

My lack of knowledge is with the current readings. As you can see the readings are all different. That part I can understand, the meter readings were not steady / constant, but how much of a difference is acceptable. And why such a difference between the individual series / parallel runs. I would have thought that if 25 / 26 amps of current were running through the batteries, I would see that on every interconnect??

Do the numbers look ok, are they tolerable. Thanks for your help / insight / thoughts.

*** The readings do not add up exactly, I recorded them as I was moving between interconnects and they were jumping by +- 1 amp all the time. They are as accurate as I could get.

Glen
currentn.jpg

Comments

  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: Current flow / Lack of knowledge

    You are probably using a clamp on ampmeter. The variations you are seeing in the same series string is just due to meter accuracy or variation in time between measurements based on a declining battery voltage.

    The hook up looks good. Objective is to have the same wire length to all batteries series, which you have.

    As to the variation between the two series strings, the numbers you are showing are not too bad. No batteries are exactly matched. You are getting about 11.5 amps on one string and 13.7 amps on other.

    You may see this vary over discharge cycle with one string initially giving more current then later on the other giving more current.

    Same thing will happen for charging current.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Current flow / Lack of knowledge

    You can also use your volt meter to measure the voltage across each battery (look for weak battery--low voltage when discharging, high/low voltage when charging).

    Also, set the meter to 2 volt or 200 millivolt full scale and check the voltage drop across each piece of wire/connection. If you find any voltage drop that is high--check that the connection is tight and clean (and that the crimp is done correctly).

    But--as RCinFLA says--getting exact current match is never going to happen.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Muskoka
    Muskoka Solar Expert Posts: 103 ✭✭
    Re: Current flow / Lack of knowledge

    Thanks for the responses,

    I guess what I don't really understand is what is going on inside the battery with respect to current flow.

    Why is the series current flow different between pairs of batteries and thus the parallel strings different. I was thinking that the current would be even between series and parallel connections. They seem to be constant differences, and the opposite between pairs. I was thinking that Series 1 would be 11'ish, - Parallel would be 13'ish, Series 2 11'ish, + Parallel 13'ish. Hope that makes sense.

    Series 1 -11.7
    Series 2 -13.6
    Parallel 1 -13.8
    Parallel 2 11.3

    Also, why are three of the readings negative, one positive. I wouldn't expect to see negative on the positive parallel connections, but would have thought that one of the series connections would have been positive? Does this have anything to do with the flow of electrons from negative to positive. Getting too technical / fussy now, just want to understand.

    Thanks Glen
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Current flow / Lack of knowledge

    I am not quite sure I understand your use of series vs parallel current measurements in your question... Unless some of those readings are in amps (current) and others are voltage (Volts).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Muskoka
    Muskoka Solar Expert Posts: 103 ✭✭
    Re: Current flow / Lack of knowledge

    Hi Bill,

    All the readings are amps. I was measuring current through the interconnects, not voltage. On the meter when I put the clamp over the interconnects, 4 of them give negative readings on the meter and 2 of them give positive readings. That's when using the 40amp setting on the Extech MA220.

    Also, why are the readings 2+ amps difference between interconnects. I might expect the "longer" interconnects to give a slightly lower reading but even that is not constant with the readings, just curious. It's not a big deal, in the end the batteries are getting charged, although not much today, should be 70+ amps not 26. I guess I want to know how much variance is acceptable before I need to be concerned about the battery interconnects. At what point would you say an interconnect is faulty.

    Glen
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Current flow / Lack of knowledge

    Glen,

    The DC Clamp meter "cares" which direction the current flow is... So, if you "flip" the meter you will see a sign change...

    I went back to the drawing and looked---Basically, the battery set in series is ~11 amps for one string and ~13 amps for the other...

    In the series connections, the current is identical no matter where you measure it--so the difference of ~0.2-0.4 amps or so is just measurement error (variation in current flow over time, meter calibration--DC clamp meter calibration drifts pretty quickly, and possible stray magnetic fields including the Earth's magnetic field, as well as meter accuracy)...

    As long as you hold the meter (relative to +/- of the battery bank, the current should all have the same sign--unless there is a reverse current flow due to, for example, a shorted cell).

    From an engineering point of view--basically stuff within a factor of 2x or less, is pretty close. Anything with a factor of 10x difference--you can usually ignore the smaller value).

    My suggestion was to go through and measure battery voltage, battery specific gravity, and even the voltage drop across the wiring and connections.

    If your batteries have different voltages--the lower voltage one may simply need more charging and the lower voltage is simply taking more current at this time. You can confirm by measuring the resting specific gravity... If the readings between the high cell/low cell in your bank is 0.030 or greater, you should "equalize" the battery bank (~15 volts for 30-60 minutes, measure S.G. and keep charging until the cells do not go up in voltage any more--and log this as 100% charged... Also make sure you have a good hydrometer and there are no bubbles on the float).

    Trojan has a nice PDF download battery manual.

    Trojan Battery 20 page Maintenance FAQ (PDF).
    Trojan Battery Maintenance FAQ in español (PDF).

    Some general background information on batteries:

    Deep Cycle Battery FAQ
    http://www.batteryfaq.org/
    www.powerstream.com/SLA.htm (gassing voltages)

    Current sharing in a parallel battery bank is not perfect and will change over level of charge, temperature, current levels, etc... What you are generally looking for are "gross" differences from when your bank was in good shape...

    So, currently you see a few amp difference between banks (less than 2:1 ratio). If you ever see >2:1 ratio (or what you consider "normal" variations), you can start looking for poor electrical connections and check the specific gravity (dead/open cells).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Muskoka
    Muskoka Solar Expert Posts: 103 ✭✭
    Re: Current flow / Lack of knowledge

    Thanks Bill,

    I learn something new every time I come to this site. I purchased the clamp meter per your numerous links to the unit, it works great. It's the first time I have ever used one and didn't know it cared which way I held it. Just tried it and sure enough it changed. Should probably read the instruction manual.

    I will try the other suggested tests tomorrow.

    Thanks again

    Glen
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Current flow / Lack of knowledge

    You are very welcome Glen.

    I am very glad this meter worked out well for you---I do recommend shopping around and asking further questions before purchasing (if you have any questions).

    I just hate it when somebody says get a "DC Clamp Meter" and I have no idea what that tool is--hence the links to something that works well and is not too expensive as a starting point.

    -Bill

    PS: I guess I should have added a link to Hydrometers in the previous post too. ;)
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Kamala
    Kamala Solar Expert Posts: 452 ✭✭
    Re: Current flow / Lack of knowledge

    As BB. suggests in post #3 you can use your battery interconnect cables as a shunt. Use an mV meter to take the readings. Compare each battery interconnect with each other.

    I'm tired..........