TA 167 - Starting / Running current measurements

Hi there,

My first post in the Automotive section, I have also posted in the Pico technology section regarding a suitable scope / data logger for starting current and charging current data logging (amongst other channels / temp / voltages).

My question here is regarding the TA 167 Current clamp which I have just purchased, it is the first time I have used a current clamp so this post is probably a mixture of me learning to use it and my lack of understanding how a vehicle charging system works....

OK, So I attached the clamp to the negative cable with the +ve sign on the clamp pointing to the negative terminal (after zeroing it)
Clamp set on 2000A range

I am using a Fluke 179 multimeter (via a Fluke BP880 BNC to 4mm adapter) set to mV DC

I get an Ignition on reading of +6 mV = 6 Amps
I switch the HRW ON and get +22.5 mV = 22.5 Amps total (16.5 Amps for the HRW alone)
I switch the HRW OFF and turn the heated seat ON and get - +12 mV = 12 Amps total (6 Amps for the heated seat alone)

I repeat this for various items lights etc just to get a feel for the clamp and the various current draws...

I then set the Fluke to MIN MAX mode and carry out a few vehicle starts which show the MAX starting current to be about 340 Amps (+340 mV)

So all seems fairly logical...

Whilst the vehicle is running (still on the 2000A clamp setting) the multimeter shows -16 mV = -16 Amps (No accessories are switched on)

I then run through a similar process of switching the HRW, Lights, AC Unit, Heated seats ON and OFF but I am surprised to see that the values showing on the multimeter hardly change from the initial -16 mV reading that I got when nothing is switched ON, at most it goes down to -13 mV = -13 Amps with a few things switched ON.

Am I just seeing the alternator varying its output to keep a consistent charge on the battery (with -16 Amps) and this is entirely normal or am I doing something incorrectly?

Sorry if this turns out to be more an Automotive charging systems lesson than about the clamp itself, I just want to make sure I understand what's going on before I progress on to trying to datalog these signals

Cheers

You are not clear on vehicle/engine type or why you want to 'log' starter & charging current ?

I then set the Fluke to MIN MAX mode and carry out a few vehicle starts which show the MAX starting current to be about 340 Amps (+340 mV)

Starter lock draw, not cranking current !

When the vehicle is running you want to be on the batt+ as close to the alternator as you can, as when it is running the alternator does the work & not the battery.

at most it goes down to -13 mV = -13 Amps with a few things switched ON.

Because the battery isn't taking much charge.

Sorry if this turns out to be more an Automotive charging systems lesson than about the clamp itself, I just want to make sure I understand what's going on before I progress on to trying to datalog these signals

In that case then a course is called for, no point 'data logging' if you don't understand how it works.

Thanks for the feedback

Yeah, I thing the first thing is that the current probe will have to be in a couple of positions in order to capture everything I want. (Which is now obvious to me with hindsight)

Regarding the alternator charging behaviour with varying load, yes... now you state putting it directly at the alternator lead it is obvious. I originally placed it on the negative battery lead as I was looking at the current during starting and it is obvious why it is not showing the varying alternator output with the variation in electrical load while placed here.



So here is the datalog, it's only 4Hz logging rate but is more than enough to show me what I wanted, at about time = 1min 17sec I have pretty well everything switched on (I have since remembered I could have had the lights on main beam and the fog lights on...) and we are at about 97 Amps dropping to 95 Amps in the time it was measured which exceeds the quoted output for the alternator (90A) but again all is now behaving as I would expect and gives me a clear idea of the maximum load.

The last part of the log from about 2 mins onwards is when everything I have control over is switched OFF, so about 20A in the minimum condition, I guess the largest variable in this condition is the state of charge of the battery, if this more delepted this will increase.

So happy with that, thanks, I will do another quick data capture making notes and methodically switch loads in and out to have a record of each item.

Back onto the current during starting, back on the negative lead, I knew before I started that logging at 4Hz wouldn't be enough to properly capture the starting transients but again it has given me an idea.



I am planning on testing at a few ambient conditions and with different batteries, including measuring battery voltage at the same time.



Zoomed in clearly shows the lack of detail / resolution, looking in your waveform library the start 'event' is typically 1 second in duration (so the same as this) and I think at least 20Hz would be required to have something a little more representative.

Steve,

Do you think having the clamp on the negative terminal with the engine running is a reliable indicator of the amount of charge that is being applied to the battery, In particular I am interested in the maximum likely alternator charging current when the state of charge of the battery is relatively low

Thanks again for the help

Cheers

Thanks again for the feedback...


At the moment I am limited to a single channel and the 3 - 4 Hz logging rate (3 to 4 S/s capture rate) for the data I am collecting, which is purely to try and establish that what I am doing is vaguely correct and the results look sensible before I consider what and if I need to buy in order to conduct the exercise properly with a variety of batteries / temperatures / setups.


So just concentrating on the charging current and not the transient starting current for now (mainly due to my limited data logging rate).

I have collected:

Charging data from the alternator from the time of initial engine startup up to the point that the required charging current appeared to plateau. (Red trace)

(On a seperate day) current data from the battery negative cable collected over the same period as described above > again to the point where the current was seemingly near a plateau. (Blue trace)

I appreciate that collecting these two 'pieces' of data at anything other than the exact same time will lead to errors from a variety of sources, zero offset of current clamp, different ranges used on current clamp, differing state of charge in the battery hence charge demanded by the battery, ambient temp etc...

In both instances, to the best of my knowledge everything I can switch off was off so as to at least be consistent from this point.

However the comparison still seems useful if you look at the graph below;



The red trace is the alternator current output, the blue trace is the battery negative cable current measurements and the green trace is the delta between the two traces.

Again all of the potential errors mentioned above apply but the green trace is quite close to being a constant charging rate after time = 4 seconds, suggesting that the battery negative cable current is providing a pretty close representation (under these conditions) of the battery charge current.

The data in the first 4 seconds or so is likely to be effected by the current demand of various sources such as the fuel pump, ignition coil/s, injectors etc etc all settling down to a constant level - so is not a total suprise that this region isn't a constant current rate.

So it's sort of what you suggested but just captured at different times, so hopefully when the data is collected simultaneously we will see a near constant charge rate represented by the delta between the two currents data sources, but until then it at least looks plausible.

Thanks again

Hi Steve,

Following on from a post on the test and measurement forum which discusses which of the Pico Data loggers may be suitable for this kind of data capture...

As far as the 4 channel Automotive scopes go, I assume I could measure / capture the channels you suggested plus a couple more?

Engine RPM (Mainly inerested in this during start up)
Analogue input for 'Battery current' measurement - Needs to be able to deal with +/- 0.5V (o/p from current clamp) (20 S/s minimum)
Analogue input for 'Alernator current' measurement - 0 to 2.5v (10 S/s minimum)
Analogue input for battery temp measurement in the range -10 to 60 Deg C, K type is probably not ideal for this, maybe a NTC thermistor and resistor setup to be fed in as an analogue voltage?

How is RPM measured with Automotive scopes is it derived from one of the measured channels or via specific sensor input?

However I think the problem is that whilst I could capture and view these channels in real time with an Automotive scope how would I get on just trying to log this data for analysis after the event for say up to 1 hour of data collection?

Is it possible to lower the data capture rates with the Automotive scopes on a channel by channel basis or gloabally? And then log the data directly to a laptop, say down to 200 S/s or so. Otherwise the logged data file will become too big, temps could probably be down to 5 S/s or lower?

Also, even when I am just logging the battery charging current, the current clamp needs to be in position while the engine cranking takes place otherwise I will miss the maximum current peak charging the battery which occurs as soon as the starter motor stops. Could the TA 167 current clamp be used in 200A setting during start up (-250A to -280A) without any neagative effect on the clamp or generating any measurement offsets as a result to try and get better resolution for the battery charging phase measurements?

Otherwise I have an engine ECU / data logger which I could try and use just as a data logger but I'd prefer not to as I will need to make a suitable loom / break out box to suit.

Cheers

Hello Steve,

Thanks again for taking the time to respond and for your time in actually capturing test data to quantify the file size

Based on your feedback there is no question that (finances aside) a 4 channel automotive scope would be an invaluable asset, even as a 'hobbyist' and to be fair I don't think it is expensive for what it appears it can do.

The custom probe option just sounds like the sensor calibration that I would normally put into a data logger setup, can these settings be input as a tabular transfer type function or does it have to be an expression / function such as y = mx + c, I imagine it can be tabular in order to capture something like a non linear NTC thermistor (my likely choice for the temp sensor, as a K type would have poor resolution for the -10 > 60 Deg C range I expect)

For the RPM signal I would really need the ability to capture / display a real transient RPM signal. With the fixed frequency / fixed RPM example that you gave I assume is only useful in a steady state measurement where you manually calculate the RPM or could you reliably derive the instantaneous RPM for the whole data capture as a maths channel and have it visible as an RPM trace?

In the past I have had problems trying to 'piggy back' onto an existing crankshaft sensor on the basis that it has compromised the original signal that the ECU needs to run the engine. How reliable do you find the derived RPM techniques you mention, although I think I would end up short of channels to do this if the RPM is not derived;

1. Battery charge current
2. Alternator output current
3. Battery voltage
4. Battery temp
5. RPM

The current transducer used at the battery -ve cable is something of a dilemma, for the sake of a 1 second event (starting current = 280A) I will end up compromising the resolution for what will be 50A max for say 20 sec at worst and then quickly less than 5 Amps for the remainder of the 3600 seconds.

Another problem has just dawned on me, the TA 167 does an auto zero on power up?, my intention was to have the clamp/s secured in place within the engine bay for the test runs, but they will auto zero on power up when there is in some residual current, I guess I could try and remove the offset after the data is captured but the vehicle has a coolant circulation pump which runs for at least 10 mins after shutdown making it difficult to try and establish a 'zero' current reading to use as an offset.

Here are my latest voltage readings during vehicle start up showing the same lack of logging rate, these tests are using two different battery types, hence the apparent difference in voltage drop during starting.



EDIT: The title looks like -5 deg C, it was +5 deg C, the '-' was just intended as a separator but looks like a negative sign.

All good examples of how those without Pico Scopes see the lower resolution world

Looking back at the Pico Scope Automotive training video which shows a battery test, I notice that this test reports a minimum voltage of 7.64 Volts, I have achieved similar figures during some cold start testing albeit only this low for a couple of tenths of a second, in your experience do most OEM ECU's have enough what I would call a "Capacitive buffer", i.e. sometimes by design and sometimes otherwise there is enough voltage stored in capacitive elements within the ECU that such a voltage drop out doesn't cause an issue due to the short duration of the drop out as the ECU stays 'energized'?

Cheers

Spencer

OK, so job done... (Well as far as current measurement goes)

I have researched for more time than is healthy and I have purchased an OEM "battery management" current sensor to act as my additional current sensor as I couldn't justify adding another conventional current clamp meter to my Pico TA 167 just yet.

There was a little extra effort required due to needing a 5v regulator to the sensor supply as this is a ratiometric sensor and it needs to be as near as damn it to 5v, but that aside it was relatively painless.

My various fluke DMM's suggested it was at 4.99# volts so I was happy to proceed...

The Pico TA 167 clamp meter is measuring the output from the alternator and the OEM current sensor is measuring the battery charging current via the negative battery cable.

The graph below confirms what I have previously shown, but now the data was 'collected' synchronously at the same time...



The blue curve is the "human data logged" by me using the TA 167 and a Fluke DMM (Bear in mind that the display is constantly flickering so I have to estimate any value that I 'log'), this is the total output from the alternator from the instant that the vehicle started

The green curve is the OEM current sensor at the negative battery terminal at 3 - 4 Hz ( 3 - 4 S/s) measuring battery charge current.

The red curve is just the blue curve (alternator charge curve) offset by 17.5 Amps and it is a a very good fit to the alternator charging curve...

So I am happy the negative cable gives the actual battery charge current as it is just a pure 17.5 Amp offset from the overall alternator current measured values.

Apologies this is not entirely Pico related but it confirms one of my original questions from this post

Cheers