Testing and Verification of Intercell Welds

Testing and Verification of Intercell Welds Tony Schröer, Jeff R.Snell Digatron / Firing Circuits One of the more critical processes in battery manufacturing is the inter-cell weld. At the same time inconsistencies in lead material and less than reliable testing methods complicate process control and verification efforts. Time proven methods to verify welds such as X-ray, Gamma ray and ultrasonic testing are either not suited for lead welds or impractical when considering the production environment. Only electrical tests are suited for 100% production lot testing of inter-cell welds. This paper attempts to highlight the weaknesses inherent in traditional electrical testing methods when using a tester integrated with the inter-cell welding machine. Unimpeachable data regarding the quality of inter-cell welds cannot be achieved using traditional electrical test methods. More reliable results can be obtained with an improved measurement system, which is separate and installed after the inter-cell welding machine. An optimized inter-cell test process and measurement techniques are discussed.

1. Introduction To achieve a battery voltage of 12V, six cells must be connected in series. Connections from cell to cell for automotive batteries are made within the polypropylene container through holes punched in the partitions. The positive strap terminal on one side and the negative strap terminal on the other side of the hole are pressed together and welded (Fig.1).

This intercell weld is one of the more critical and difficult processes used in manufacturing batteries. Thousands of batteries are destined for scrap if any one of the six intercell welds per battery (Fig.2) does not fulfill the specifications.

Fig.2: Automotive Battery

Fig.1: Positive and Negative Strap Terminal

Testing and Verification of Intercell Welds

The following will focus on the resistance welding process of lead, specifically lead-alloys.

-1-

There is very little literature available which discusses the behavior of lead materials when welded. What literature exists, only considers welding by open flame. There are two reasons for the limited literature about lead welding: - Only the battery industry employs the process of welding lead to manufacture it products - Lead welding is one of the most difficult processes to control. It requires control of several variables to achieve reliable results.

Over the past decades the welding process have been more focused on improving efficiency. It may not be practical to eliminate all subtle material and process variations. In some cases this leck of process control may result in marginal intercell connections which could lead to battery failure. Why is it difficult to weld lead? Instead of pure lead (Pb), lead-antimony (PbSb) alloy is used to increase the hardness of lead. But properties and particulary mechanical properties of low antimony alloys do vary and depend mainly on material history

Fig.3: Two-Phase-Diagramm

Lead is a very soft metal with a relatively low melting point at 327 deg.C With respect to the resistance welding process the melting point can be controlled by an appropriate amount of energy. But the control range is small and if the energy applied, slightly exceeds the maximum limit lead becomes liquid and will squirt out. Another important parameter is the hardness. One cannot state the general hardness of a lead-antimony alloy. The rate of hardening depends on the cooling process and the percentage content of antimony. Fig.4 shows hardness versus age.

Fig.3 to fig.6 will reflect the characteristics of lead-antimony alloy with 3% antimony. Fig.4: Hardness vs. Age Testing and Verification of Intercell Welds

-2-

It is obvious that newly casted lead behaves somewhat differently than lead which was stored. Newly casted lead is very soft and can be easily formed together. Stored lead – roughly speaking one day or more – is brittle and breaks with even minor deformation. Usually welding machines receive newly casted lead without the necessary hardness. The welding clamps will pinch the two cast on strap flanges and just the force of the clamps will join the parts and create an electroconductive junction. Even though welding in this state is possible the process energy must be increased to compensate for the low resistance junction. It is possible to conduct the welding procedure without welding the material. And the internal resistance measurement will display values within the limits and will let the battery pass.

As a result internal resistance measurement systems can only be used to compare resistance data within a production batch and do not accurately measure the real resistances of the intercell welds. The Oxidation curve (Fig.6) indicates that oxidation begins immediately after contact with air. Once a thin oxidation layer has been developed further oxidation is inhibited. The film has an isolating property. In accordance with equation d=m/OxV

where

m = oxidation [mg], O = 5400 mm2 v = 0.252 mm3/g

and

in our test a thin film of about 0.4µm to 0.6µm was developed after a few minutes.

The electrical resistance curve (Fig.5) typically shows the highest gradient at the melting point. Although most welding machines ensure rapid cooling after the high current welding process the electrical resistance may vary significantly due to the temperature gradient.

Fig.6: Oxidation vs. Age

Any measurement system whether integrated or separate from the welding machine should have probes which break through the oxidation layer to ensure accurate measurement.

Fig.5: Electrical Resistance vs. Temperature Testing and Verification of Intercell Welds

All these variables make it very difficult to control the welding process for lead alloys and to consistantly produce quality welds. -3-

Despite these issues the battery industry has proven that lead welding is possible. Todays welding machines consist of two hydraulic clamps for interconnection 1, 3, 5 and interconnection 2 and 4. These clamps are positioned by automated x/y-positioning systems [1].

Contact detection via voltage measurement Water cooling

Power electronics are used to provide welding currents in kA range using programmable controllers and a PLC is used to control the conveyor station and to position the battery underneath the clamps.

Weld electrodes with projection forming tools (thrust adapter) Fig.7: Welding Clamp Details

Fig.7 shows details of the welding clamps. The voltage sense connection provides the feedback for current control and also detects the initial contact made with the parent material. It is important to control the time between inital contact with the parent material and the start of the welding process. This time period determines the size of the contact area and the initial contact resistance prior to welding. Because this pause time is so critical special contact detection modules are used to precisely measure initial contact.

Fig.8 shows the strap connections before and after projection. If the initial contact area is too small the resistance will be too high and the welding process will generate too much heat, causing lead, to squirt out at the beginning of the welding process. If the initial contact area is too large a very low resistance electroconductive junction will result. This will always create a condition for cold welds.

Strap position before forming of the projection … and after projection forming

Fig.8: Projection Forming Testing and Verification of Intercell Welds

-4-

The entire welding sequence (Fig.9) will be explained in the following.

Fig.9: Welding Sequence

Fig.10: 1000 Hz MF-DC Inverter Power Unit

When the battery is positioned, the clamp contacts the strap terminal and closes the projection forming electrodes (phase 1).

Fig.10 shows the principle schematic of a 1000Hz DC inverter power module using pulse-width-modulation.

Phase 2 starts with a signal from the contact detection module. If the programmed pause time (phase 2) is complete, the weld current starts (phase 3). Current amplitude and current time are controlled by the welding controller. The lead will melt and due to pressure applied to the strap terminals the material will be pressed through and fill the partition hole. In phase 4 the hold time allows the lead to cool (phase 4). After the hold time the clamps will open and the contact detection module indicates the end of the welding process (phase 5) Dynamic regulation is required to control weld energy and particulary weld current. As a result switched mode power supplies are typically used in modern welding machines. Testing and Verification of Intercell Welds

The 3-phase AC signal is rectified to provide a DC link voltage for the IGBT inverter. The IGBT inverter is controlled by 1kHz frequency and provides 0.5 ms control cycle. The welding transformer generates the high current output and after rectification by the two diodes a true DC signal is applied across the welding electrodes. Even considering the equipment and process improvements developed over the recent years there remains significant variation in both, process and materials which can result in substandard welds. As difficult it is to weld lead, it is more difficult to verify lead welds. The next pages will investigate and analyse existing methods of quality inspection for weld processes for use in battery assembly lines. -5-

2. Experimentals

Methods used in the industry to verify welds are: X-Ray, Gamma-Ray, Ultrasonic, Electric Inspection such as Resistance Measurement or Resistance vs. Welding Time and Destructive Inspection.

2.1 Destructive Inspection

Fig.12: Twist off tool kit with torque display

Strap terminals are twisted off using a torque wrench to measure the maximum torque value prior to failure (Fig.12). It is assumed that the welding quality correlates to the torque value. Fig.13 shows torque data, deviation and distribution of 28 samples

Fig.11: Adapter to prepare sample weldings

Destructive testing of welds is performed on a periodic or sample from a lot bases. The results can be used to adjust the intercell welding machine. A special adapter is used to produce weld samples to avoid wasting entire cell packages (Fig.11). A fixture keep the strap terminals in the right position for welding. There are cavities in the upper platform to position the terminals. Different battery sizes can be accommodated by the same fixture using the telescope.

Fig.13: Production lot test results

Another criteria to verify the welding process is optical inspection.

After welding the operator lowers the upper platform to get access to the strap terminals. Testing and Verification of Intercell Welds

-6-

Destructive Inspection, optical evaluation

Fig.14: Ideal Welding

Fig.15: Corona welding

Fig.16: Blow holes

Defective areas can be seen clearly as displayed in figures 15 and 16

2.2 Non-Destructive Inspections There are several non-destructive inspection methods typically used in the industry.

Unlike X-Rays or Gamma-Rays, Ultrasonic waves accurately detect the smallest of air gaps, which create an unbridgable barriers completely reflecting signals.

As a result the high degree of absorption, X-Ray and Gamma-Ray cannot be used for inspection of lead welds. The reason is the attenuation effect whereby the atomic number is cubing and multplied by the density of the material in question. As a result the degree of absorption for lead is much higher when compared to other metals. It is technically inpossible to use X-Ray or Gamma-Ray to find irregularities in the intercell welds. Fig. 17: Ultrasonic inspection, pause time too short

Ultrasonic inspection would be an ideal method to determine irregularities in intercell welds. The main advantage is the straight-lined propagation of ultrasonic waves through the medium and very good diffusion [2]. In this regard ultrasonic waves are comparable to X-Rays or Gamma-Rays. Testing and Verification of Intercell Welds

Fig.17 shows an ultrasonic image of an intercell weld where the pause time was too short. One can see in the image that there is no coherent area of welding. It is assumed that blow holes have been generated. -7-

In general blow holes will be generated if there is too little material available. This will be the case when pause times are too short and when the welding process starts immediately after the two terminals are in contact with each other.

Fig.19 shows the result if the welding current is too high. The lead squirts out and leaves blow holes.

Welding current below the minimum required will result in an incomplete and poor quality weld.

Fig.18: Ultrasonic inspection, pause time too long

If the pause time is too long the material will develop an electroconductive junction and the resistance will be too low to produce the necessary welding temperature. Fig.18 shows a corona welding at 1000 ms pause time. The next two figures will show the results when varying the welding current.

Fig.19: Ultrasonic inspection, welding current too high Testing and Verification of Intercell Welds

Fig.20: Ultrasonic inspection, welding current too low

In Fig.20 it can be seen that the areas at 1, 4 and 7 o‘ clock are not welded properly.

Fig.21: Ultrasonic inspection, welding current with ramp -8-

Fig.21 shows a substantial result. Whenever current ramps were used better quality welds resulted. The major advantage of a current ramp is the preliminary warming of lead which will create uniform material and process conditions. Thus the influence of material hardness can be minimized. The current time was set to 240 ms whereas 180 ms were programmed as a ramp. Fig.23: Ultrasonic inspection, current time too short

The next figures show the variation of current time.

If the current time is too short the weld is incomplete. Fig.23 shows welded material in the center only. Outside the center the weld is very shallow.

The next figures will show the importance of correct pressure when welding the strap terminals

Fig.22: Ultrasonic inspection, current time too long

If the current time is too long the energy applied to the weld will be too high and material will squirt out. After coagulation blow holes and hair line fractures will be found. As it can be seen in Fig.22 only a small area between 11 and 12 o‘clock is welded.

Testing and Verification of Intercell Welds

-9-

Fig.24: Ultrasonic Inspection, pressure 2.5 kN

Fig.25: Ultrasonic Inspection, pressure 3.8 kN

Fig.24 to 26 show results generated at 2.5 kN, 3.8 kN and 4.7 kN. Too little pressure will not create the desired electroconductive junction, where resistance is low enough so as not to produce excess heat. The lead will squirt out and will leave intercell welds with blow holes (Fig.24). Beyond a certain force applied, the results will be consistant with Fig.25 and Fig.26. This supports the theory that increasing pressure should be applied up to that point just before structual damage to the partition or hole might occur.

The ultrasonic inspection of intercell welds yields significant and reproducable results without damaging the test object. But problems occur with regard to the testheads used to transmit and receive ultrasonic signals. Testheads are only available for larger objects. The space required to properly position a testhead is not available. Another problem is to

Testing and Verification of Intercell Welds

Fig.26: Ultrasonic Inspection, pressure 4.7 kN

establish a good ultrasonic junction between the testhead and the intercell weld. In typical applications water, grease or oil are used as a transfer medium to achieve the necessary contact. It is difficult to believe that this could be implemented in an assembly line for batteries.

Beyond destructive inspection techniques, electrical testing is the only suitable method to verify intercell welds, in production environments. Several battery manufacturers implemented two tests:

have

The first using a tester integrated into the cell welding machine and also the second test after the battery has left the intercell welding machine. In the following the advantages and disadvantages of both systems will be discussed.

- 10 -

Voltage sense

V

Intercell weld

The internal microprocessor calculates the resistance which - at this stage – does not represent the resistance of the intercell welds. First, the parasitic resistances must be eliminated from the calculation. Parasitic resistances are:

Welding current

Fig.27:4-Wire Current-Voltage Measurement

The general principle is shown in Fig.27.

- electrode resistance - contact resistance - resistance of the lead material

The principle is based on the 4-wire current-voltage measurement. A power source will provide constant current to the test object and voltage will be measured across the test object. A high impedance voltage input is required to measure voltages in the microvolt range. The microprocessor will then calculate the resistance.

R1: electrode resistance R3: contact resistance R6: material resistance R5: contact resistance R7: material resistance R4: contact resistance R2: electrode resistance

Fig.29: Parasitic Resistances

Voltage sense

V

Fig.28: General Assembly of a Welding Clamp R1

The general assembly of a welding clamp is shown in Fig.28. The voltage sense leads are connected to the clamps. After the welding process and a cool down period an additional measurement current is programmed to pick up the voltage drop across the clamps. Testing and Verification of Intercell Welds

R3

R6

R5

R7

R4

R2

Welding current

Fig.30: Real Resistance Model

The real resistance model of the integrated resistance measurement is shown in Fig. 30. - 11 -

Some machines measure the offset resistance and store this result for use as a compensating value when calculating the intercell resistance. However in production conditions the offset resistance may increase over time due to contamination that accumulates on the contact surfaces of the thrust adapter (Fig.31).

cleaning

cleaning

Fig.31: Contamination of Thrust Adapter

Contamination of the thrust adapter will result in increased resistance between the electrode and the lead material because of a layer of contamination accumulates between the electrode and lead material acting as an insulator. Additional heat will be generated at the electrode/thrust adapter, not at the lead material. Therefore this heat will not contribute to the welding process.

The parasitic resistances mentioned on the previous pages will form the major part of the total resistance. Irregularities in the intercell weld will not represent a significant portion of the total resistance and will typically be within the tolerance band. Another fact to be considered is the resistance measurement takes place when the the test object is subjected to pressure from the welding clamp. Since lead is a soft metal the pressure applied is capable of creating a low resistance elctroconductive junction even without welding. Experiments confirmed that resistances of both welded and unwelded strap terminals can be within the tolerance band when using a clamping pressure of above 3.5kN. Subsequently the integrated resistance measurement can only be used as comparative data. It does not reflect the real resistance of the intercell weld. Another option would be to measure and record resistance during the welding process. This is commonly done when welding steel or sheet metal [3].

Heat and high pressure join to create a thin lead film on the upper thrust adapter. Any accumulation of lead will tend to accelerate this plating process. Poor intercell welds will result. Fig.32: Master resistance curve for steel and sheet metal welding

It is necessary to periodically clean the surface of the thrust adapter to ensure a consistant welding process. Testing and Verification of Intercell Welds

- 12 -

Welding steel and sheet metal master curves are used to control the welding process.

The results of all the records are shown in Fig.34.

The first part of the curve should be ignored as the data will vary significantly depending on material conditions under pressure. The important part of the curve is the shaded area. This area indicates the acceptable limits of time and resistance that will result in an appropriate level of energy applied to the weld. The question is: Can this control method be used for lead welding as well? To answer the question 3000 curves have been recorded.

Fig.34: Intercell Weld Resistance Curves (yellow curve)

The resistance curves for lead are almost constant throughout the process. There is no significant event during the process that could surve as a direct or indirect indicator of a good or bad weld.

Table 1 shows the deviations in voltage, current and resistance as measured during welding of 3.000 samples. Fig.33: Expected Resistance Curve

The typical resistance curve is a combination of contact resistance and material resistance vs. welding time. At the very beginning in the welding process contact resistance will be very high generating heat up to the melting point of the lead material. Contact resistance will decrease, as material resistance increases due to rising temperatures(Fig.33). Testing and Verification of Intercell Welds

Process time

0.01s

0,02s

0.09s

0.17s

Voltage deviation

+/- 12% +/- 10% +/- 5% +/- 5%

Current deviation

+/- 5.9% +/- 5%

+/- 5% +/- 5%

Resistance +/- 9.3% +/- 8.2% +/- 5% +/- 5% deviation Table 1: Deviation of voltage, current and resistance - 13 -

The flat resistance curve can be explained by two variables which tend to compensate for each other: Although the resistance of lead increases with higher temperatures, resistance also decreases as the distance between the electrodes is reduced.

The integrated resistance measurement system cannot reliably determine the quality of strap terminal welds. In addition this method will not differentiate between welded and non welded strap terminals. It should only be used to capture possible trend data such as the resistance increase due to contamination

3.2 Separate Resistance Measurement 3. Results 3.1 Integrated Resistance Measurement

To measure the strap terminal resistance more accurately a separate system is required. This system ensures real 4-wire measurement (Fig.35).

To summarize the investigation of the integrated resistance measurement one can say the advantages are: - High pressure contact allows for high test currents which generate higher voltages across the clamps improving signal to noise ratio measurement. - No additional equipment investment is required

The disadvantages are: - Resistance measurement takes place when the test object is subjected to pressure from the welding clamp. Since lead is a soft metal the pressure applied is capable of creating a low resistance elctroconductive junction even without welding

- No real 4-wire measurement. Parasitic resistances will form the major part of the total resistance. Irregularities in the intercell weld will not represent a significant portion of the total resistance and will typically be within the tolerance band Testing and Verification of Intercell Welds

Fig.35: Separate Resistance Measurement

Contact is made by two current probes and two voltage sense probes. The current probes are spring-loaded to ensure safe contact. However, unlike the integrated resistance measurement system, this method does not clamp the terminals together. In contrast the probes apply force in the opposit direction attempting to separate a poor weld. - 14 -

The voltage probes are also springloaded and have tapered contacts which penetrate the oxyd film layer to accurately measure voltages. Five of these contact couples are connected in series assembled in one common testhead. The machine interface should allow fast change over of testheads to adapt to any battery type.

Fig.37: Effective Welded Area 104 mm2

The maximum current is limited by the design of the spring-loaded current probes.

Fig.38: Effective Welded Area 145 mm2 Fig.36: Testhead of Separate Resistance Measurement

Tests were conducted to determine the optimum current amplitude required to generate significant variation in measured resistance which then correlates to percentage completeness of welds. Welded terminals were modified by drilling holes of increasing diameter through the weld nuggets.

Fig.39: Effective Welded Area 245 mm2

It is clear that test currents well in excess of 100A are required for meaningful results (figures 37 to 39).

Testing and Verification of Intercell Welds

- 15 -

Unimpeachable data regarding the quality of intercell welds cannot be achieved using the integrated electrical resistance measurement

In general the addition of a separate Welding Resistance Tester would be an enhancement to any battery assembling line.

More accurate results can be obtained with an improved measurement system, which is separate and installed after the intercell welding machine

Digatron / Firing Circuits has literature available describing the Welding Resistance Tester Product. Please contact [email protected] or [email protected]

References: [1] Oliver Roehl, Bielomatik Leuze GmbH + Co; Intercell Welding Machines August 2004 [2] Burghard Danch, Study of the Intercell Welding process, October 1993 [3] Matuschek Messtechnik GmbH, Adaptive Control for Resistance Welding in the Automotive Industry, August 2001

Testing and Verification of Intercell Welds

- 16 -