Measuring PWM Current with Analog Meter

[metrologist]

I wanted to measure the current in a solar PWM battery charging system. I found an old analog panel current meter originally for an RC NiCd charger/discharger and put that inline with the solar charge controller and battery. Since this is PWM, the current pulses and I wonder if that will ruin the meter very quickly? The needle is bouncing about as fast as one could wiggle their finger...

[danadak]

Its may be a jeweled movement, but I suspect the prolonged stress
on the movement might fail sooner rather than later.

If its an ammeter you could use the shunt to develop V and then the
meter as a V meter with RC network to change the pulse to a DC value
dependent on duty cycle. The meter now measures the output of the
RC network V, rather than shunt V directly.

Regards, Dana.

[metrologist]

Thanks. Probably not jeweled. Sort of reminds me of a 80's style automotive white panel meter.

[oldway]

I think your choice is right because charging current of a battery is the average value of the PWM current and that's what your analog ammeter is measuring.

But PWM frequency is generally well above 20Kh and the needle of your ammeter cann't bounce at such a high frequency.

If it does, there is something wrong with the PWM waveform, such as a low frequency modulation.

[metrologist]

I've read that many PWM charge controllers are in the range of 10's of Hz and up. I'm guessing mine is around 25 Hz. It's fixed.

I thought about adding a capacitor to smooth that out, but also read about the benefits of using PWM to charge lead-acid batteries. I don't really need an accurate current measurement, but wanted to know if the controller was charging the battery and if it was in a full charge mode or trickle, or if the battery was being discharged.

[Ian.M]

The controller may be periodically varying the PWM duty cycle to dither around the max power point for MPPT tracking.   

If the meter has a separate shunt and a resistor in series with the movement to trim the sensitivity to match the shunt you *MAY* be able to damp it enough with a very big electrolytic directly across the movement, but if its the typical dirt cheap moving iron movement with a high current field coil and no shunt, you don't have any options to damp it short of filling it with silicone oil!

[oldway]

Ammeter with moving coil measure average current, that's the charging current.
With moving iron, it measure the RMS current, it is higher than average value.

[metrologist]

I try to make an animated gif of my meter. If it does not move here, then at least this is the meter I was using...

Yeah, it moves if you click the image. It's just a bit faster in this image, but not too much faster than reality. Sometimes the controller connects the solar panel direct DC, so no needle movement. That is a rarer case when the full power from the panel is used.