Clamp on Ammeter question

icarusicarus Solar Expert Posts: 5,400 ✭✭✭✭
Here is a question that I should know the answer to,, but since I don't where else to turn but here!

I (at Bill's suggestion) went out and bought the inexpensive Sears clamp on ammeter. Doing some testing with in on a friends dual battery RV set up I came across an interesting thing that I can't figure out.

With the battery on the on board charger, clamping the meter on the Positive lead, I get say 7amps (2 6 volts in series) Clamping it on the negative lead I consistently get about 1/2 the current, say ~3.5 amps. Clamping it on the series cable between the two batteris I get the same 6 amps. Intuitively my mind says I should have equal current on both conductors. What am I missing?

The negative lead in this case is just a simple 12" cable bolted directly to the negative post, and then to a ground bolt.

Once again, thanks in advance for any answers.



  • PhilSPhilS Solar Expert Posts: 370 ✭✭✭
    Re: Clamp on Ammeter question

    Prob'ly not your situation Tony, but my Fluke clamp-on ammeter is directional with an arrow for current flow molded into the clamp.

    Just had to check, I've never seen the Sears' model.

  • icarusicarus Solar Expert Posts: 5,400 ✭✭✭✭
    Re: Clamp on Ammeter question

    You're correct Phil, it is directional. Clamp one way and you get -amps, clamp the other way and you get + amps.

    As a correction, the cable between the 2 batteries shows 1/2 the amperage of the positive.

  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: Clamp on Ammeter question

    What goes in one battery terminal must come out the other. Did you zero out before making measurement? If there was truly 5 amps charge and meter started out unzero'd at +2 amps you might see something like that reading. One reading positive 7 amps other reading -3 amps for reversed clamp.

    Check it on the same lead wire forward and reversed on clamping. Should be same number with one negative and other reading positive. If you zero'd it out first (with jaw closed) and it reads different forward and reverse (with neg. sign and positive sign) then there is a problem with meter.

    Unless you have two batteries in parallel, or another leakage path on one of leads.
  • icarusicarus Solar Expert Posts: 5,400 ✭✭✭✭
    Re: Clamp on Ammeter question
    Unless you have two batteries in parallel


    Bingo! You win todays forum Kewpie doll!

    I went and looked and indeed there is the vehicle starting battery wired in parallel in such a way as the current flows from the charger to the positive post of the vehicle battery, though the battery isolator and then on to the the house batteries. Checking everything shows equal current in and out as it should!

    I just misunderstood the way the battery and the charger were wired! Duh!

    Thanks again for the help,

  • JamesJames Solar Expert Posts: 246 ✭✭
    Re: Clamp on Ammeter question

    Icarus, are you satisfied with the meter? Would you recommend it as a "middle of the road" quality/value?
    I was thinking about buying a clamp-on dc meter and had read this thread. Thanks
  • icarusicarus Solar Expert Posts: 5,400 ✭✭✭✭
    Re: Clamp on Ammeter question

    For ~$60 it seems to work fine. It has a temp probe that might be useful for some things as well. It does amps to two decimal places which is good enough for me. Having a hand held clamp on is pretty handy I think. For example you can test the output of any given component as well as look for stray current if something is amiss. At half the price of the Fluke I think it is a pretty good buy.

    My one complaint is that zeroing it seems to be a bit funky.

  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: Clamp on Ammeter question

    It is an EXTECH MA220 ($99) with Sears Craftsman label ($59) on it.

    I have a earlier model. For the price, it is pretty good. I would not recommend leaving it out in the garage subject to humidity. I had a little problem with touchy on-off slide switch.

    I get about a 0.2 to 0.3 amp offset on 40 amp DC current scale when first turned on. You just push the zero'g button to cal out the offset. You got to make sure the jaws are solidly closed before zero'g and making measurement.
  • BB.BB. Super Moderators, Administrators Posts: 30,493 admin
    Re: Clamp on Ammeter question

    I played a bit with the Craftsman version I just bought a few days ago. The day before the battery died on my Honda Van. :roll:

    Used the voltmeter and DC clamp meter to check out the van and it showed everything working well... After about an hour of running, the battery was charging at 14.14 volts and 10 amps. About what I would expect.

    I think the "problem" is we are driving short city trips about once a week and waiting (with the radio on) for a few hours... I think that we are deficit charging the battery (Hmm... Battery monitor on van--naw, that's crazy talk).

    I agree with Tony that the DC zero on the DC clamp meter is a bit random... Sometimes I have to push the button two or three times to get a solid zero (00.00 amps).

    Also, the thermal couple seems to be off a couple degrees F at room temperature (mine reads high). Also, I will have to glue the plastic cap on the meter end of the thermal couple wiring plug--The yellow cover popped right off.

    Overall, it does seem to work well. And it is really nice to be able to directly measure DC current. Finally (I am terminally cheap:p).

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