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Dinosaur

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Joined: 12/08/2011
Location: Australia
Posts: 336

Posted: 06:02am 07 Jul 2025

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Hi All

My solar panels when connected to my BM system have a single pair Red / Black.
When I measure the current using my Tongs I get a difference of 1.5 A between reading the
red or the black.This is in the 16 A range.

Even when I measure a load in the mA range the same way, there is always a difference.
Granted my Tongs are no-brand, but am I missing something.

I am careful about having a nice loop of cable and away from others when I measure.

Regards
Hervey Bay Qld.

phil99

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Joined: 11/02/2018
Location: Australia
Posts: 2601

Posted: 06:42am 07 Jul 2025

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If you have the panels connected to a switch-mode MMPT or charge controller the high frequency may be affecting the accuracy of the meter.

An old analogue Hot Wire ammeter will give true RMS from DC-to-Daylight.
It uses a thermocouple connected to a short piece of resistance wire.
Slow response and +/- 3% of full scale are its limitations.

Dinosaur

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Joined: 12/08/2011
Location: Australia
Posts: 336

Posted: 07:06am 07 Jul 2025

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Hi All

Quote If you have the panels connected to a switch-mode MMPT or charge controller the high frequency may be affecting the accuracy of the meter.

My BMS is a Rpi switching SSR's, so the only time the SSR switches is when target Voltage is achieved.
I would think the error would be the same on both leads.

The tongs actually show the Red lead as Negative current and the Black lead as positive current. Either way whatever comes out of one, MUST go back through the other.

So perhaps the accuracy of measurement differs from one direction to the other.

Regards
Hervey Bay Qld.

phil99

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Joined: 11/02/2018
Location: Australia
Posts: 2601

Posted: 07:29am 07 Jul 2025

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Turning the meter around at the same place on the same lead should produce the same results if that is the case.
You probably need to check it against a known good meter in both directions.

Dinosaur

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Joined: 12/08/2011
Location: Australia
Posts: 336

Posted: 07:34am 07 Jul 2025

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Hi All

Good suggestion, will try that tomorrow.

Regards
Hervey Bay Qld.

KeepIS

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Joined: 13/10/2014
Location: Australia
Posts: 1871

Posted: 11:01pm 07 Jul 2025

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If by "Tongs" you mean current clamp tester then yes, I find that most DC clamp meters can give a slightly different result in one direction (-/+) and a slight difference between clamping the positive and negative leads.

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