Pulsing Battery with a current

[xaccto]

I am trying to understand an aspect of my measurement of a current pulse into a battery.



so the oscilloscope ground is at the battery -Ve terminal. The pulse is from a capacitor being dumped.
so the negative going Ch B which is the voltage across a resistor shows a direction I understand - the current
is flowing INTO the battery.
What I don't get is why the Ch A connected to the positive terminal is also showing a negative going pulse,
I would have thought it should be positive going pulse.

 

[BradtheRad]

Yes, the way you describe your setup, there should be a positive pulse at ch A.

This is a remote possibility, but by some chance could there be any crosstalk between ch A and B?

Since the B spike goes many volts into the negative, it might influence A to decrease a few volts, if there is crosstalk.

 

[FvM]

The schematic suggests a floating current source, no ground connection of the switcher circuit. Is it true?

 

[xaccto]

reluctantly I must admit to a measurement error,lol. Whilst I connected the scope probe earth lead to the indicated position (-Ve bat terminal), I forgot about the earth lead from the
scope still connected to the ground circuit of the pulser, so the scope sees ground loop current of sorts. That impressive looking current pulse is sadly no longer the case, I was so
impressed with it I didn't want to believe it was wrong, yet there was that -ve going voltage pulse - I was blinded to it for quite some time

 

[BradtheRad]

Even the best of us can overlook a thing like that.

You saw something unexpected and questioned it.

Many important discoveries have been made by one individual who recognized something was out of the ordinary, and who dug deeper to discover the cause...

Yet the same thing had happened in front of other researchers' eyes, but they failed to notice it.

 

[iop95]

There are not any error. Pulse current of around 116A lead to negative pulse of about -41V on + terminal of battery because internal resistance of battery; your battery seem to have about 0.35ohm internal resistance. This pulse current pass from - terminal to + terminal of battery so will lead to a positive voltage on - terminal and vice-versa to + terminal. After capacitor discharging, current go to zero and of sure measured voltage also.

- - - Updated - - -

There are not any error. Pulse current of around 116A lead to negative pulse of about -41V on + terminal of battery because internal resistance of battery; your battery (if have 12V) seem to have about 0.45ohm internal resistance. This pulse current pass from - terminal to + terminal of battery so will lead to a positive voltage on - terminal and vice-versa to + terminal. After capacitor discharging, current go to zero and of sure measured voltage also.