Why does Pos and Neg battery cables read differently?

Azel
Azel Registered Users Posts: 9
I was testing a new clamp meter I purchased to check the amps coming out of my battery while underload and I got a reading I did not understand. On the Positive side of the battery cable I read 13.1 amps DC and on the Negative side I read 14.1 amps DC.

I thought they would read the same. Why is there a difference in readings?

And, normally which one should you read with DC current, Pos or Neg cables?

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Why does Pos and Neg battery cables read differently?

    Normally the current is exactly the same on either. But operating DC clamp-on Ammeters can be a bit of an art. Unless these were consistently repeated results (with proper zeroing of the meter in between readings) I wouldn't worry about a 1 Amp difference. In fact the current may have actually changed that much between when you took one reading and the next.
  • Azel
    Azel Registered Users Posts: 9
    Re: Why does Pos and Neg battery cables read differently?

    I did zero the meter and took the reading over and over many times just to be sure. There is a difference of 1 amp for some reason. But I was only curious anyway. It’s not important.

    I posted a question last week about how long a 50 Ah battery would run my computer and monitor. (I had figured it out on paper to be about 30 hours. What a block head. LOL)

    Someone suggested I take a amp reading off my battery cable, so I bought an Extech clamp-on DC amp meter. I had been wanting one anyway, so it gave me an excuse to buy one.

    After using the meter I found the computer and monitor draw 14.0A DC, so I divided 25 Ah (50% DOD) by 14.0A and got 1.78 hours. So I guess that is how long my computer and monitor will run. (Not the 30 hours I had thought.)

    I also tested the DC/AC Inverter and it draws 1.2A DC when it is not powering anything, which was good to know. These clamp-on DC amp meters are really handy when working with DC power.
  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    Re: Why does Pos and Neg battery cables read differently?

    I also tested the DC/AC Inverter and it draws 1.2A DC when it is not powering anything, which was good to know. These clamp-on DC amp meters are really handy when working with DC power.That is not a very good figure for an inverter unless its larger than 1000w .. Most good inverters below that draw half an amp or less.
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Why does Pos and Neg battery cables read differently?
    Azel wrote: »
    I did zero the meter and took the reading over and over many times just to be sure. There is a difference of 1 amp for some reason. But I was only curious anyway. It’s not important.

    But we are curious too!

    Can you confirm that there is only one wire/cable attached to each battery post, or are you just assuming that only one of two wires will be carrying any current?
    Have you tried turning the meter to face the other way on the wire, just to rule out any effect of a strong local magnetic field?
    Is there enough clearance that the clamp is able to close freely and completely around each wire?

    Is there a place where you can clamp the meter around both + and - cables at the same time? The result should be zero.
    SMA SB 3000, old BP panels.
  • Azel
    Azel Registered Users Posts: 9
    Re: Why does Pos and Neg battery cables read differently?

    Since you asked, I went back and measured it again with the Extech MA 120 clamp meter, and it read 14.0A DC on both battery cables this time. There was no difference between the reading of the two cables. I can’t say what caused the difference in the readings from earlier today. It was probably caused by some way I was taking the reading. The main thing is I know now that they should be the same, so I feel I have learned something.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Why does Pos and Neg battery cables read differently?

    The Extech meters have a really "strange to me" way of zeroing the meter.

    If the meter display does not say "ZERO" in the upper status line--Then the meter will have some sort of offset (perhaps an amp or more) from zero.

    When you push the ZERO button, the electronics make whatever the clamp is reading zero (and a "ZERO" will be on the display status line). And any current flow will add or subtract from "zero" setting (remember, for DC amps, the meter will display + or - depending on the directly of current flow through the clamp.

    After a few minutes, you may see some other number than 0.0 on the display when no current is flowing--pushing the ZERO button twice (once to turn off zero function, and again to set zero and turn on the function) to get 0.0 again.

    It is the drawback of many DC current clamp meters.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset