Traditionally, I had not given too much consideration to milliohm (mΩ) values thanks to using volt drop techniques instead!

For example, when starting an engine, we discover the cranking speed is low or laboured and may choose to measure the volt drop across the starter motor ground cable. Once we hit above 0.25 V (e.g., 1.4 V drop) we know we have an increase in resistance resulting in low current flow

We use this technique in our PicoDiagnostics Battery test and demonstrate using PicoScope here viewtopic.php?p=101584#p101584

Given we capture the current through the entire starter motor circuit whilst simultaneously measuring the voltage across our ground lead, we can determine resistance using ohms law. It is here where I have not paid too much attention to mΩ as my focus has always been on the recorded volt drop value.

If you have ever wondered about the resistance of a starter motor ground cable, (I know I should get out more) here are some real-world good examples calculated using ohms law

The average of all the above equates to 1.070 mΩ which does not sound like any resistance whatsoever!

Roll onto the present day with EV’s that may well draw current at a constant rate of 100 A under certain driving conditions, suddenly, milliohms become

**more than relevant**

Every connection/terminal has a resistance regardless of the material used and the contact tension within.

Let us image a high voltage battery bus bar with a contact resistance of 10 mΩ (0.01 Ω) instead of the desired 1 mΩ (0.001 Ω) transferring a current flow of 100 A.

Volt drop = A x R

With our 10 mΩ (0.01 Ω) contact resistance we have 100 A x 0.01 Ω = 1 V drop

With our 1 mΩ (0.001 Ω) contact resistance we have 100 A x 0.001 Ω = 0.1 V drop

Can you now image the heat generated around the bus bar contact when 400 A is flowing (4 V drop)

Factor in “heat” whereby our 10 mΩ resistance will increase in proportion to rising temperature and we really do have

*food for thought*. (Not to mention other HV connections within this circuit on route to the 3-phase motor)

As you can see from the figures above, trying to measure resistance with milliohm precision is going to require

**precise control**over the test conditions along with the supplied current and voltage beyond the realms of a conventional multimeter

This is where our MT03A comes into play using the “Kelvin sensing” 4-wire measurement technique

More information on Kelvin sensing can be found here https://en.wikipedia.org/wiki/Four-term ... connection.

Below is a diagram highlighting how the 4-wire measurement principle is applied to resistance measurements. Note how the current flow via the resistor under test is a function of the measurement device via test leads “Amp 1 & Amp 2”

With a known voltage & current flow, resistance is measured via test leads “Volt 1 & Volt 2” at the

**contact point**either side of the resistor under test. (Akin to how we measure volt drop across our ground lead using the starter motor analogy above)

The MT03A utilises Kelvin crocodile clips whereby each jaw is insulated from the other. A current lead (Amp 1 above) is connected to one jaw and a voltage lead (Volt 1 above) to the other. In this scenario one crocodile clip incorporates 2 leads (Amp & Volt) so reducing the number of clips and leads required to make a 4-wire measurement

With MT03A, both Amp & Volt leads are bound into a single cable terminating in a Kelvin clip at the measurement point and 2 x 4 mm banana connectors at the measurement device

In addition to the above, the MT03A triplicates the above test lead/clip design in order to deliver a highly accurate 3-point resistance measurement solution (ideal for 3-phase motor windings)

See image below for connection method for 3-phase motor winding test

Note above how the MT03A incorporates a temperature sensor which connects to the rear of the tester via a 3.5 mm jack plug (See below)

Incorporating temperature of the resistor under test allows for accurate compensation of the resistance measurement. For example, manufacturers will specify the phase winding resistance of their motors at 20°C for copper windings.

Given resistance increases with temperature, if we are measuring above 20°C the non-compensated measured value could appear out of specification, resulting in the

**incorrect replacement**of a 3-phase motor (This could include motor & transmission combined!)

The MT03A and combined software will display both the measured and compensated resistance value to prevent such an error highlighted above

MT03A Software

The software for the MT03A will be available for downloaded here https://www.picoauto.com/downloads once released, but in the meantime, the following video summarises what to expect when carrying out a 3-phase motor winding resistance check https://youtu.be/J54C2V1OysE

Note during the video above how current is

**reversed**through each winding in order to remove the effects of Thermal EMF that can be generated between contact points of dissimilar metals (i.e., the Kelvin clip to the point of measurement) An average resistance value is then obtained using the formula:

Resistance with forward current + Resistance with reverse current / 2

The screen shot below is an example of a good motor winding test where the bar graph and “Phase resistance balance” summary, confirm a maximum deviance (deviation) between phases of 0.2 mΩ which equates to 1.1%

Notice above how we have 2 resistance values for each phase winding, “measured” and “temperature compensated”

Given the motor temperate was 13.3°C at the time of measurement, and the reference temperature for the resistance of copper winding is stated at 20°C we apply the following compensation

Temperature coefficient of copper = 0.00393 @ 20°C

Copper has a positive temperature coefficient (PTC) where resistance increase with temperature

E.g., Copper wire resistance at 20°C = 1 Ω

What is the resistance at 22.7° C?

22.7°C – 20°C = 2.7°C (Temperature change)

Temperature change x Coefficient of Copper x Measured resistance (Ω) = Change in resistance

2.7°C x 0.00393 x 1 Ω = 0.010611 (this is 2.7°C above 20 °C so add to 1 Ω)

1 Ω + 0.010611 = 1.010611 Ω (This would be the resistance of our 1 Ω copper wire at 22.7°C)

Moving onto our test results for U to V phase above @ 19.2 mΩ (0.0192 Ω) at 13.3°C

What is the resistance value at 20°C? (Temperature compensation value)

Once again:

Temperature change x Coefficient of Copper x Measured resistance = Change in resistance

(20°C – 13.3°C) * 0.00393 * 0.0192 Ω = 0.000505 Ω

This value needs to be

**added**to 0.0192 Ω measured at 13.3°C (Added due to PTC)

0.0192 Ω + 0.000505 Ω = 0.019705 Ω = 19.71 mΩ

19.71 mΩ is our temperature compensated resistance value for our winding measured at 13.3°C to display what the resistance value would have been if measured at 20°C

The displayed Deviance (Deviation) and percentage values are calculated as follows:

Deviance (Deviation)

Max value – Min value (We will use the temperature compensated value)

19.7 mΩ - 19.5 mΩ = 0.2 mΩ

Percentage maximum deviance

100 – (Min value (mΩ) / Max value (mΩ) x 100)

100 – (19.5 / 19.7 x 100)

100 – 98.98 = 1.1%

Customer copies of the test results can be saved using the “Print” function and “Microsoft print to PDF”

Below is the PDF print out from our initial motor test at 13.3°C

**A word to the wise,**like all tests, our results are only as good as our connection and with milliohm testing this has never been more relevant. Just think about continuity testing where we approve 0.5 Ω as being “acceptable” if we assume we have 0.1 Ω resistance in our test leads and point of contact

To now put 0.5 Ω into perspective, this is 500 mΩ and in the world of high current transfer

**this value is huge**! I guess the message I am trying to get across is we need to think in

**mΩ**and not

**Ω**so we can grasp the enormity of low resistance (I hope that makes sense)

Moving onto Milliohm testing

As you will be aware from the title “Milliohm and Motor tester,” we can also use the MT03A to measure resistance across items such as earth boding leads and bus bars to name but a few applications.

Using your choice of 2 x Kelvin clips/leads from the MT03A we can accurately measured between any two points on a vehicle to obtain a highly accurate resistance value. This may be between a Hybrid transmission assembly and chassis ground, or, across a bus bar in the example below.

The real beauty of the MT03A is via USB connectivity only, we comfortably achieve the legislated test current required to carry our such tests as earth bonding: (See ECER100 extract below)

*ECER100 Earth bond test specifications*

5.1.2.2. The resistance between all exposed conductive parts and the electrical chassis

shall be lower than 0.1 ohm when there is current flow of at least 0.2 amperes.

5.1.2.2. The resistance between all exposed conductive parts and the electrical chassis

shall be lower than 0.1 ohm when there is current flow of at least 0.2 amperes.

The MT03A meets these specifications (200 mA below 100 mΩ) whilst also continuing to deliver sufficient current up to a maximum resistance of 2 Ω (2 Ω, the maximum resistance value the MT03A can measure)

Below we are

**logging**the resistance between our chosen test leads (U & V) to determine our bus bar resistance at 0.1 mΩ (0.0001 Ω or 100 µΩ)

Note in the image above how the Kelvin clips are attached to bolts passing through the eyelets of the bus bar. If I now slowly loosen one of the bolts, we can see how the resistance changes dramatically (This is ideal for wiggle testing)

The following video

https://youtu.be/OCJIPKNAG18 covers the 2 tests described above and help clarify the process of connection, set up and obtained results

Information on the purchase of the MT03A can be found here https://www.picoauto.com/products/milli ... tester-kit

A further in-depth video testing a faulty EV motor beyond milliohm testing can be found here https://youtu.be/YpgRD7Su1TI and will feature in an up-and-coming video case study in May 2023

I hope this helps, take care……Steve