HIOKI RM3545 Resistance Meter User Guide

HIOKI RM3545 Resistance Meter

Why is it necessary to measure welding resistance

There has been an acceleration in the adoption of electric vehicles (EVs) and with the batteries growing in both size and capacity, current flow has grown as well. Batteries designed for use in EVs must deliver a sufficient level of quality to withstand large currents during rapid charging. Materials used inside batteries are welded together using metals through which current readily flows. When there’s a weld defect on a power line that carries a large current, the area around the connection defect will heat up and such heating will accelerate degradation which can lead to ignition and fires. In addition to batteries, the windings in motors that use hairpin stator are assembled using welding. An insufficient welding in windings causes an increase in energy loss which has a significant impact on motor performance. In this way, the condition of welds has a substantial effect on the quality and performance of batteries and motors. Measuring resistance of the weld is a key method for checking weld conditions and it’s extremely important to verify that welds have formed in a way that provides a low level of resistance. Targets

Difficulty of measuring welding resistance

The difficulty of measuring welding resistance is that the weld resistance is extremely low. To make pass/fail judgments about the condition of welds, it’s necessary to use a measuring instrument that has both high measurement precision and 0.01 μΩ-order resolution. In addition, resistance measurement is extremely susceptible to the effects of heat. It’s very difficult to accurately measure resistance after welding when the part is still hot or while temperature remains unstable during cooling.

Issue 1: Resistance values vary with temperature.
Effects of the temperature coefficient of resistance When dealing with a single metal material, a standard temperature coefficient can be used. However, when measuring a weld in which two different materials of different thicknesses have been joined, it’s difficult to ascertain an accurate temperature coefficient for the measurement range. As a result, use of temperature correction functionality introduces an error component.

Resolution hints
Measure the measurement target after its temperature reaches room temperature. Determine an accurate temperature coefficient for the measurement target by making measurements while varying the temperature.

Issue 2: Effects of thermo-electromotive force.
In resistance measurement, current flows to the measurement target, and the instrument detects the resulting voltage drop. If thermo-electromotive force occurs on the pathway used to detect this voltage, an error may be added to measured values. Thermo-electromotive force is a potential difference that occurs at points where different metals are connected. More specifically, it occurs at locations where the probes make contact with measurement targets and where an instrument and its measurement leads are connected. The magnitude of the thermo-electromotive force depends on the temperature of the measurement environment and it increases proportionally with the temperature difference. It’s typical for measurement leads to be made of copper wire, whereas connectors use materials like nickel plating. In this way, it’s unrealistic for connections between metals to comprise a single material. In addition, measurement probes and measurement targets are made of different materials at different temperatures. In particular, the effects of thermo-electromotive force cannot be ignored when measuring low resistance values, for example regarding components such as busbars, due to the extremely low detected voltages involved.

Resolution hints
Use the offset voltage compensation (OVC) function to realize stable measurement.

Issue 3: Effects of the measurement target’s potential (for batteries)
When measuring welding resistance in a battery, there may be a voltage at the measurement location. When the measurement probes make contact, the battery’s voltage will be applied to the instrument which could be damaged by the resulting overvoltage condition.

Resolution hints
Use an instrument with protective functionality. The RM3546’s voltage protection technology (VPT) protects the instrument against voltages of up to 60 V.

Resistance meters recommended by Hioki

This page introduces key considerations when choosing a resistance meter that’s suited to welding resistance measurement.

Solutions for Issue 1 & 2 
Check weld quality even in environments with large temperature variations.

OVC function
The instrument corrects for the measured value offset caused by thermo- electromotive force. The value shown to the right is displayed as the true resistance value based on the measured value RP when the measurement current flows in the positive direction, and the measured value RN when the current flows in the reverse direction.

A-OVC function
The instrument performs OVC measurement twice and performs calculations based on the amount of variation in the measured values.

Resistance can be measured accurately even immediately after welding, when the part is still hot. There’s no need to wait for the weld to cool.

Use these resistance meters recommended by Hioki to check weld quality without sacrificing takt times.

RESISTANCE METER RM3545

• Minimum resolution: 0.01 μΩ
• Maximum measurement current: 1 A
• Measurable range: 0.00 μΩ to 1200.0 MΩ
• Contact check function

WELDING RESISTANCE METER RM3546 (coming in 2022)

• Minimum resolution: 0.001 uM
• Maximum measurement current: 1 A
• Measurable range: 0.000 uQ to 1200.0 MQ
• Contact check function, A-OVC function, VPT function

What kind of measurement leads are suited to welding resistance

measurement

Probing is a key consideration when measuring weld resistance. This page provides an easy-to-understand introduction to hints for probing based on measurement target width and thickness and key considerations when selecting probes that are suited to weld resistance measurement.

Difficulty of probing
When measuring minuscule resistance levels, the measurement environment and probing can cause measured values to vary. As a result, it’s very difficult to get accurate repeatable measurements.

Issue 1: Measurement points (probing locations)

• When the measurement target is wide or thick, for example a plate or block
• When measuring a current detection resistor (shunt resistor) with a resistance of less than 100 mΩ

In situations like these, it’s difficult to get accurate measurements using clip-type leads. If using such leads, measured values will fluctuate from several percentage points to dozens of percentage points depending on contact locations.

Resolution hints
It’s ideal to eliminate any changes in contact locations by using single points on the measurement target for the SOURCE pin, which applies current, and the SENSE pin, which detects voltage.

Issue 2: Characteristics of 4-terminal measurement (effects of potential gradient)
In low-resistance 4-terminal measurement, minuscule differences in pins’ contact locations can cause large variations in measured values if the distance between the SOURCE and SENSE pins is small. This occurs as a result of large variations in potential near the current application point (SOURCE pin), which create a steep potential gradient. (See Figure 1.) To avoid this effect, it’s necessary to separate the SOURCE and SENSE pins. (See Figure 2.) N

Resolution hints
Separate the SOURCE and SENSE pins. (It’s desirable to separate the pins by at least 3 times the measurement target’s width or thickness.)

Issue 3: Effects of external noise
When the measurement current is small, measurements will be more susceptible to external noise, which may cause measured values to exhibit instability. If measurement is performed near equipment that uses large currents, for example welding machines, noise from the equipment can contaminate the measurement signal and affect measured values. When fabricating your own cables, it’s necessary to take steps to keep external noise from affecting measurement.

Measurement leads recommended by Hioki

Key considerations when choosing measurement leads suited to welding resistance measurement

• Solution for Issue 1 Align the SOURCE and SENSE pins on a straight line if the measurement target is wide or thick.
• Solution for Issue 2 Separate the SOURCE and SENSE pins to increase reproducibility.
• Solution for Issue 3 Twist cables together to avoid the effects of external noise.

In addition to the above, cable durability and the ease with which pins with worn tips can be replaced are key considerations if you need to test large quantities of batteries

PIN TYPE LEAD L2100
Max. rated voltage: 1000 V DC (Hi-to-Lo), 1000 V DC (voltage to earth)

PIN TYPE LEAD 9465-10
Max. rated voltage: 60 V DC (Hi-to-Lo), 60 V DC (voltage to earth)

PIN TYPE LEAD L2102
Max. rated voltage: 60 V DC (Hi-to-Lo), 60 V DC (voltage to earth)

1.  Dimensions other than overall length include typical values.
2. Caution is necessary when measuring high resistance values of 1 kΩ or greater with the L2100 and 9772 as those parts lack guard pins.

Suggestions for customers who wish to automate welding resistance measurement

• Multichannel, high-precision measurement
• For measuring cylinder-shaped batteries

All information correct as of June 1, 2022. Contents are subject to change without notice.

Note: Company names and product names appearing in this brochure are trademarks or registered trademarks of various companies.

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