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.

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.

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.

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.

Hi All

Good suggestion, will try that tomorrow.

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.

Hi All

Definitely the Tongs.
Just rotating the meter, but keeping the cable nicely in the center,
gave me 1.8 Amp difference at 16 A this morning.

The only other Clamp meters I have access to, one is the same brand as mine
and the other is an old analogue only for AC.

The more expensive units are not as sensitive to exact cable position or the polarity of the cable. High current units sometimes indicate the correct position of the cable for best results, lower cost units are not too bad it you use the same cable positioning each time, of course that needs to be established against a known amp meter, after that they can be quite accurate  

BTW These are just my observations after using a number of different units over the years.

Hi All

Appreciate the advice.
It's made a mess of my Battery capacity calculations though, as now I don't know what to believe.
May have to use my Fluke at 10 A limit to re-calc.

The "clamp"  method measures the magnetic field from the current. There were some
that used a ferrite toroid to concentrate the field, usually fine but I think the
core can become slightly permanently magnetized sometimes which would give
zero level offsets and so on. Maybe try demagnetizing the clamp and re-zero (?)

  zeitfest said  

Maybe try demagnetizing the clamp and re-zero (?)

Yes!  Check if the clamp is magnetized.   A tiny bit of magnetization isn't much of a problem.  But if you can pick up and hold small pins or needles with it, you won't be able to measure accurately with the clamp.

The big problem I've had is it is really hard to demagnetize those clamps.   The hysteresis values are pretty high.   And you have to take it deep into hysteresis (in the right direction) to undo the magnetization.
Edited 2025-07-18 13:28 by EDNEDN

Hi All

Thanks for the heads up on that.
One of the reasons I can't use traditional current measurement with a Multimeter
is that I am measuring current flow into a Lipo4 cell which has an internal
resistance if 0.2 milli ohm.
Anything in series with it causes an increase in resistance and drops the current.
Thus I don't have a true measurement.

I will see if I can borrow a better clamp meter.

More important than the internal resistance of the cells is the total series resistance of the charging circuit. Any voltage droop at the charger output is equivalent to series resistance.
Reqyiv. = (Vnoload - Vload) / Iload
As long as the shunt resistance is significantly less than the total it will have little effect on the reading.

If you are using a constant current charger the effective series resistance is almost infinite.

If you have a good millivolt meter you just need a shunt rather than a whole ammeter.
If you can accurately measure the resistance of one of the charging cables that could serve as a shunt, though accuracy will be affected by temperature.
Edited 2025-07-18 22:53 by phil99

Hi All

Good suggestion Phil.
My charging circuit (as noted in a different thread) is a simple SSR ON/OFF.
The Rpi reads the cell voltages (using a ads1115) and makes decisions based
on the voltage of all the cells.
Once I reach 95%, or one of the cells goes high, then I change to individual cell charging
using 4 x isolated DC step down converters.(to 3.6v)
These are NOT constant current,so it is up to me to find the cell voltage where I can safely
switch on the converters without them going over their current limit.(they shut down to a "hiccup" mode if over current)

I have a Battery internal resistance meter that appears quit good, so it would measure a shunt in 0.01 milli ohm and also measures voltages.(don't recall if less then 1 mV)

Sometimes I just can't see the wood for the trees.