New DC Amp Meter

Wilis
Wilis Solar Expert Posts: 84 ✭✭✭✭
in General Solar Power Topics #1
Today I received my clamp DC Amp meter. I measured my 48v battery bank for the first time. What readings have others found using this type of meter? Knowing the difference could be in the cable or the batteries, (8 series and 2 parallel strings of T-105 Trojans) I am not sure just what to expect? In my system all cable are the same length between batteries, AWG 2/0 cable made by NAWS. How close should the readings on the different parallel strings be? What difference between batteries should there be? Anyone using this type of meter please post. Thanks Wilis
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: New DC Amp Meter

    I would suggest that experiment with measuring your car's electrical system first.

    Note that the +/- sign on the current meter is sensitive to current direction... If you flip the meter "backwards" on the cable with current flowing, you will notice the sign flip.

    Same thing with charging discharging. Turn on the head lamps, then start the car, you will see the current value change and again the +/- sign flip.

    And, there is a zero button... With zero off, the meter will not necessarly read zero amps... When you Zero the meter, it takes what ever the present reading is and subtracts it from the display. You have to do this because the magnetic field transistor that measures the field does drift... So, disconnect the meter, press zero (to turn off Zero--if needed) then press zero again and the meter should read zero DC amps. Then connect to the cable you want to measure current on.

    Note that this would also be true for measuring AC current in a lamp cord--You can take a short extension cord and separate the wires so you can measure the current through each separately.

    They also make a plugin adapter that splits out the three wires for an AC cord (see below).

    480172 - AC Line Splitter

    Also note that a current clamp meter measures the sum of the current going through the hole. If you measure the + cable, you will get +4 amps. And if you measure the - cable, you will get -4 amps. Put put + and - cables in the clamp, and you will measure 0 amps (-4 amps +4 amps = 0 amps).

    When measuring parallel current flow--ideally, you should measure equal current flow in each branch... In reality, you may not... Batteries do change voltage and current a bit, so if everything else is OK, then you may see one string carry a bit more current for a while, then the other will catch up.

    If your difference in current is less than 2x or 1/2 (i.e, 100 amp current, 60 amps on one string and 40 amps on another parallel string--you probably are OK. If the difference is greater (like 90 amps and 10 amps), you probably have a real problem (bad cell, bad connection, etc.).

    At least, that is where I would start.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Fullpower
    Fullpower Solar Expert Posts: 69 ✭✭
    #3
    New DC Amp Meter

    The simple answer is that the current in OR out of your two parallel battery banks SHOULD be exactly equal.
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  • Wilis
    Wilis Solar Expert Posts: 84 ✭✭✭✭
    #4
    Re: New DC Amp Meter

    Thanks for the replies. Yes, BB I did see a little difference current between the parallel strings. I will continue and record my reading for the next few days. Some when they are solar charging and some at rest or grid charging. Everything seems to be working fine now so I want to know what is normal so if or when a problem comes up I will know it.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #5
    Re: New DC Amp Meter

    If the panels are identical, and not getting any shading, the current should be within the make tolerance of the panels AT MAXIMUM ILLUMINATION POWER. 3% to 5% difference would be possible. Check your panel's manufacturer specs. At lower levels of illumination the leakage effect on each cell starts to come into play and drives a greater divergence in panel matching.

    If one panel gets more dust and dirt collecting on its surface it will effect it.

    As for absolute number, that depends on accuracy of your clamp on meter. Again, check its spec. Unless it is an expensive meter it is likely +/- 3%. The $60 Sears meter (Extech 220 copy) is +/-3% plus last digit +/- 0.01 amps uncertainty on 40 amp scale.
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  • Wilis
    Wilis Solar Expert Posts: 84 ✭✭✭✭
    #6
    Re: New DC Amp Meter

    The meter is a Triplett Model 9320-A true RMS AC/DC clamp-on.
    http://www.electro-meters.com/Triplett/T_M/Clamp_Meters/9320.htm

    Seems to be working better now I am zeroing it out before I use it, just as BB suggested. (Also in the instruction book) Still seeing a few amps different between batteries in one series string. Will be doing a SG to see if I need to equalize. Was about 2months ago when I did equalize. At that time everything seemed to show within spec. Nice to have a new tool to check my batteries. Seeing this is the first battereis I have had a learning curve to understand how they work.
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  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    #7
    Re: New DC Amp Meter

    I've been looking at the Fluke 374 http://www.fluke.com/Fluke/usen/Electrical-Test-Tools/Clamp-Meters/Fluke-374.htm?PID=70402

    Does inrush current, true RMS voltage readings, and auto-ranges so you only have to set the mode.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
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  • Wilis
    Wilis Solar Expert Posts: 84 ✭✭✭✭
    #8
    Re: New DC Amp Meter

    Now I have had a few day to work with my meter I still have a question? When the system is charging, this maybe from the PV's or charger, I see a different amps value on my 2 parallel strings. Bank 2 is always charging at a higher rate. Example: When bank 1 was charging at 2.5 Amps bank 2 was charging at 4 amps. This value will change depending at how charged the batteries are, but always a difference between them.
    All wires within each individual string show amps within 1 tenth of each other. This should show that the connections are tight and not having resistance.
    If I check when no charging is going on and some amps are being removed they seem to be discharged at the same rate.
    The 16 batteries were purchased within 1 month of each other but were not made on the same day. There are 8 on one day and 3, 3 and 2 on other days. Would this make enough difference in charging characterizes to cause this to happen?
    I don’t seem to be having any problems with SG or keeping my system charged.
    Word of wisdom would be helpful. Just trying to learn what to expect is normal so if things start to change I will know when I have a problem.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: New DC Amp Meter

    A few days difference in the manufacturing date will not cause any significant performance issues. A year's worth maybe.

    Check the obvious things first: how exactly are they wired in parallel? http://www.smartgauge.co.uk/batt_con.html Should be method 2. If the inverter 7 controller are connected to different positive/negative posts that could alter the current flow between the two strings.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #10
    Re: New DC Amp Meter

    You do want to be sure to have the same cabling and connector/connection resistance on all strings.

    It is common to have varying charging and discharging matching between parallel strings. One string may start off taking a larger percentage of current then over charging/discharging time, it will back down and another string will have high current.

    You would like to see the currents not vary more then 30%. If one string is continously taking most of the current then there is likely a problem with one of the batteries or connections, that could be anywhere in the battery array.

    The best way to test connections is with a moderately high, constant load. With a DVM check voltage directly on battery posts then from post to post to check voltage drop of interconnect wire and connectors.
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  • Wilis
    Wilis Solar Expert Posts: 84 ✭✭✭✭
    #11
    Re: New DC Amp Meter

    This is how I have linked my batteries. Not sure how to do it different to get 48v?Attachment not found.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #12
    Re: New DC Amp Meter
    Wilis wrote: »
    This is how I have linked my batteries. Not sure how to do it different to get 48v?Attachment not found.

    That's the right way! Especially the part about equal length wires.
    So now what you want to do is keep an eye on it: check the string difference at varying times and conditions. See if there is a consistent discrepancy or, like RC said, it shifts between one and the other. The latter may indicate some high resistance in a connection, such as a bit of corrosion or a loose crimp or bolt.
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