Hello Richard and thank you for this valuable feedback.
I agree, looking back at my screen shot it was crying out for a simplified view but I thought it best to cram all the information onto one screen. One look at my image and you may not want to use your scope again!
I guess what it does highlight is the wealth of information that can be added for the technician’s reference only, as customers would simply glaze over.
The image/psdata file included was taken from an Iveco truck we used to simulate the concerns raised by one of our agricultural customers; hence the “Volt Drop” of 0.3 Vpk over a link lead approx. 30cm long.
Thanks for the tips with the scaling as this does bring the waveforms together nicely for comparison of both batteries.
I could not understand why the A-B scale (purple) was so far off A-C (Black) until I realised the A-B maths channel used was a “built in” maths channel with auto scaling (Lesson learned for me again)
I do remember both batteries were fully charged and equal in specification, hence the near identical “voltage” performance which I would expect to be normal (all things being equal, which they rarely are)
Theoretically, if battery 1 had a faulty cell, the total voltage for battery 1 would be lower than the voltage of good battery 2. The total voltage of both batteries could be 22 V rather than 24 V.
However the current passing through the whole circuit (during cranking) would remain equal throughout, based upon the total resistance of the circuit (including the faulty cell) regardless of where the current clamp was placed.
I think it is safe to presume if both battery voltages are performing equally under cranking load (same specification) then the current contribution from both batteries should be equal. Total current measured divided by two equals the current contribution from each battery.
The same could not be said in our example, “battery 1-faulty cell”! In this scenario the total current measured would be lower as a result of our under-performing (under contribution) from battery 1-faulty cell
We can only measure the total current even when the current contribution from each battery is not even.
However, we can measure the voltage at any point within our cranking circuit to determine where the failure lies.
In this scenario (battery 1-fault cell) would be identified using maths channel A-B assuming a good link lead
There may be a formula based upon the voltage performances of both batteries during cranking to reveal the theoretical current contribution of each battery! (Not sure about the accuracy?) I feel another maths channel coming on!
I do like the (C+ (A-B))/2 for the average voltage of both batteries, nice touch and good from a routine maintenance point of view for vehicles in storage or multiples of batteries etc.
You have made a very good point about test lead grounding as the image indicates the test leads are connected to chassis ground and so do not return to battery negative, missing any possible volt drop issue in our negative battery lead. I will ask for the drawing to be modified, this is my mistake.
I am glad you mentioned the 4425 as you are correct it would remove the need for the maths channels, I think another drawing would help here to show how the new generation PicoScope could be connected.
Either way the drawing above covers all our automotive scopes which will help to avoid confusion.
I hope Santa does hear you and your request for an 8 channel scope (Dreams do come true you know)
For now we do have the 4824 Test and Measurement scope but please read the thread below before purchase:https://www.picoauto.com/support/post32215.html#p32215
Moving onto terminology I can see how “Volt Drop” may be deemed a derogatory term when referring to good battery performance under cranking load.
Not sure I know to an alternative or a more politically correct term as it certainly does fit the description as we except “pressure drop” in our pipework when we open the tap.
Perhaps we should lose the term “Volt Drop” in favour of a percentage performance figure to describe battery efficiency under cranking, and use “Volt Drop” only to describe circuit errors.
Would you agree?
“Your battery test returned a performance value (under load) of 77%” or,
“The voltage drop across your battery under load was 3.6 V”
Both indicate a good battery but does one sound more serious than the other?
Thank you for the feedback and input, take care…….Steve