24 Volt battery testing.

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Steve Smith
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24 Volt battery testing.

Post by Steve Smith » Mon Nov 09, 2015 5:33 pm

A recent enquiry into support@picotech.com surrounded 24 volt battery testing using Pico Diagnostics which certainly got me thinking about further diagnosis if the battery test returned a “Failed” result.

Unfortunately the Pico Diagnostic battery test can only indicate the overall performance of both batteries combined as one unit, where the specification (EN, SAE or DIN) does not exceed 2000 A (The limitation of the current clamp)

If the combined specification of both batteries exceeds 2000 A then the Pico Diagnostics battery test cannot be performed accurately on these vehicles.

Should the combined specification of both batteries fall within the maximum allowed specification of 2000A then the battery test can be performed using Pico Diagnostics with a TA167 current clamp.

This will then produce a pass or fail result but will not indicate which of the two 12 V batteries are at fault. In this scenario the batteries would then have to be disconnected and tested individually.

Assuming both batteries are the same age / specification and subjected to identical service life on the same vehicle, there is a good case to replace both batteries.

With PicoScope (Not Pico Diagnostics) we could identify the contribution from each battery (whilst connected) based upon the fall in battery voltage during cranking. (Including the efficiency of the battery to battery link lead)

I mention the link lead as the vehicle in question is actually an agricultural vehicle where the batteries are mounted at the LH and RH chassis members and linked via 3 metre battery cable. A link lead operating in such an environment is vulnerable to corrosion culminating in volt drop

Using a 4 channel PicoScope it is possible to calculate the voltage of both batteries and the volt drop across the link lead whilst connected during a cranking test. (non-intrusive test)

Using the maths channels we are able to calculate the all-important value of each battery and the link lead between the two.
Channel A blue lead to battery 1 POSITIVE terminal. Ground lead to chassis ground

Channel B red lead to battery 1 NEGATIVE terminal Ground lead to chassis ground

Channel C green lead to battery 2 POSITIVE terminal Ground lead to chassis ground

Channel D current clamp around main 24 volt battery cable (Positive cable to starter motor)


A trigger is then set on D to capture the inrush current into the starter motor and so record the voltage drop values of Channels A, B, and C

Channel A indicates 24 V and total volt drop of both batteries (24.7 V to 17 V cranking)

Channel B indicates 12 V of battery 2 and includes the link lead between both batteries (12.46 to 8.85 V cranking)

Channel C indicates 12 V of battery 2 minus the link lead. This should match Channel B if the link lead is OK

Once the capture is complete the maths channels are used to calculate the following.

Channels A-B calculates the voltage value of battery 1 accounting for the link lead.

Channel A-C calculates the voltage value of battery 1 NOT accounting for the link lead

Channel C-B calculates the volt drop across the link lead only
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I hope this information is of some help, take care......Steve

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Re: 24 Volt battery testing.

Post by FioranoCars » Fri Nov 13, 2015 6:13 pm

Hi Steve
Thanks for another interesting and useful post. We did look at something like this about 5 years ago, but so rarely see 24v product I can't remember what we did, or the outcome!

I'm surprised that a 3m long link cable is only suffering a Volt Drop of 0.3V (well 293mV) during a peak 1000amp in rush, that seems low, but I've not seen the cable size :-)

The "scaling" of the batteries separate voltage traces needed to be changed (scale the math channel of A-B by "3.5" to match Channel C to allow direct comparison (or scale C by 2 and A-B by 7)) and you can see very near identical performance from each battery. I wonder if this is normal, and if the "amps contribution" is unequal or again very equal (can it be unequal? Amps always being equal in a circuit?), especially if there is a less good battery in the pair?

I might be tempted to add a Maths Channel ="(C+(A-B))/2" to show the average of the two 12v batteries (ex link cable), which would highlight how much 1 battery is under performing verse the other, but not sure if it would really help too much with only 2 batteries ...

I'd also question as why bother with the Maths Channel A-C, it's an aggregation of 2 components which are best looked at alone, for a production template to use day in day out, I'd also hide channel B too but I'm being pedantic!

Here's my simplified view:

The template is a great starting block, and I'm sure will save lots of people time and mental anguish! ... just a shame we don't have 6 channels so we can add + and - volt drop at the starter motor all in one capture :-) (when's that 8 channel scope coming? !)

This template does not allow for the negative battery lead volt drop if you ground your leads to chassis earth rather than "Battery 2" negative terminal directly (your lovely graphic does not show this), although with the new floating independent channels on the 4425 you could read each battery voltage directly, without even maths channels or we could use that 8 Channel scope ...! (oh, I so want one ... a 4825 please! Please, Santa hear this!)

However I have a question about terminology (sorry, now off topic from 24v to more general 6/12 or 24v batteries) ... while I'm very happy to use the term "Volt Drop" to describe the drop between two ends of a cable etc, I think using the same term to describe the "depression" in voltage of the battery(s) to be confusing for many technicians let alone customers ... I have no idea if there is a separate official term or if Volt Drop is the proper term to use ?

To help explain to customers the difference between battery issues and cable/connector issues we avoid the term "volt drop" for the battery element, as for us a "good battery" might see voltage dip to 10.5v during cranking (at the peak in rush when 600amps is drawn which is say 2.1v dip from the 12.6v unloaded level), but if the start motor cabling is suffering a 1.5v Volt Drop that would be significant, and require repair, even though our cables are typically front to back or vice versa, so 4-5m long, 0.75v drop is about the max we'd expect to see.

There are lots of good explanations of CCA, Ah etc:
https://en.wikipedia.org/wiki/Automotiv ... nd_ratings

But does anyone know of a proper term to describe the battery voltage level during load?
What other terms do people use informally to describe this to clients?

Thanks again, Steve

Richard Lukins

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Re: 24 Volt battery testing.

Post by Steve Smith » Wed Nov 18, 2015 6:58 pm

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:


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

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Re: 24 Volt battery testing.

Post by Robski » Wed Nov 18, 2015 7:08 pm

However I have a question about terminology (sorry, now off topic from 24v to more general 6/12 or 24v batteries) ... while I'm very happy to use the term "Volt Drop" to describe the drop between two ends of a cable etc, I think using the same term to describe the "depression" in voltage of the battery(s) to be confusing for many technicians let alone customers ... I have no idea if there is a separate official term or if Volt Drop is the proper term to use ?
You could use 'available voltage under load' ?

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Re: 24 Volt battery testing.

Post by FioranoCars » Wed Nov 18, 2015 8:16 pm

Thanks for the cross post reference, and I'll keep talk about multi channel scopes there!

For the Battery Voltage depression verse volt drop I have no real idea, I was hoping you'd come back with a wonderful and smart answer to save my brain :-)
available voltage under load
... is good, and a fairly accurate term for the purpose, as you can talk about the battery unable to maintain 10.5V threshold under the starter motor load etc ... Thanks Robski

I'm still happy with the battery voltage being depressed by 3.7V during cranking verse the 2.2V (12.73 representing 100% charge to 10.5 the min a battery should maintain) ... but all this gets confusing as rarely is the battery at 12.7V, often more like 12.5-6, so talking "relative" in any sense can be confusing, where as absolute figures have less room for confusion ... ie the battery maintained 10.7V

Your performance value idea is equally good, again how do you set the bar / base line for the % ... from 12.7 to 10.5 so 10.5 = 0% and 12.7=100% (or starting battery voltage being 100%)? Will think on this approach ...

By the fact no-one has jumped up and set the record straight with an official term, I guess means that this is one area where you can use whatever you prefer without risk of being "wrong" !

I'd be keen to see some faulty battery traces of the dual battery test, as the real world sometimes shows how theory is more complex than first thought, and I have a feeling that where different batteries (ie not identical, and of identical charge) will be providing different outputs, and will potentially be "depressed" by different values (% or absolute). All very interesting!


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