The importance of inter-connections in a lead-acid battery formation department

If somebody was walking out of your factory with $800,000 for every five million batteries you produced, you’d want to protect against the event. Technical editor Dr Mike McDonagh concludes this connector-testing series by bringing all you need to know about such an event in one comprehensive article. The bottom-line savings and reduced carbon emissions will help you to make lead-acid an even more competitive battery technology.

Four years ago, in September 2016, I met up with an old friend and business colleague Mark Rigby of UK Powertech. We were on a stand in the suppliers’ exhibition hall, where he was displaying his company’s products, mostly lead-acid battery formation connectors. Rigby mentioned the problems of increasing battery and connector damage that he was finding in many of his clients’ formation departments. He had noted that the incidence of problems such as arcing between the connector and battery terminal had increased over the last few years. This coincided with the reduction of formation times and therefore higher formation currents used by his clients. The conversation concluded with an agreement to meet up in the UK to examine the problems in more detail.

Two years later, after the 2018 ELBC, we began to look at formation connectors returned from customers. It was noticed that they had some sort of corrosion layer on the inside of the connector head. This is the part that makes contact with the battery terminal throughout the formation process. It appeared that this corrosion layer would contribute to a higher resistance connection between batteries than a clean connector would. We decided to investigate this to ascertain the underlying factors causing these problems and the mechanisms creating them. We also needed to understand their consequences and measure the impact. The ultimate aim of the project was to find suitable solutions to overcome the problems, reduce the costs and improve the efficiency of lead-acid battery formation departments.

The programme of work

The starting point was to construct a programme of work. This is shown in Table 1. In this, it can be seen that samples of used connectors, supplied from the formation departments of UK Powertech’s clients, would be tested against new connectors to measure the difference in various performance criteria. These criteria were centred on the electrical properties that may contribute to the arcing and heat generation that was causing the expensive battery and connector damage.

The test apparatus used for the initial measurements is shown in Fig 1. In these tests, a perfect battery terminal machined from aluminium was used to measure the interface resistance between the connector and the terminal. We found that there was a highly variable resistance dependent on the age and condition of the connectors. In some cases, the interface resistance was more than a hundred times higher than that of the clean connectors. The temperature rise was also measured; this was significantly higher with the used connectors when compared with the new unused connectors.

Factory visits

The next stage was to visit some of the clients who had experienced high incidents of arcing to establish what formation schedules and working practices they were following. We were granted access to the departments and were able to observe several aspects of the process. These were:

• The operator function in the filling, then loading and connecting the batteries in the water-cooled conveyor tanks whilst the batteries were still moving
• The temperature of the batteries during formation
• The voltage differences between batteries in the series-connected strings
• The condition and treatment of the inter-battery connectors
• The incidence of arcing between the battery terminals and the connectors
• The complete formation programmes and their recorded and stored data

From these initial visits, it was evident that there was difficulty in properly fixing connectors to the batteries whilst they were moving. The sheer number of operations performed and the stretching required to reach the batteries would inevitably cause some operator fatigue— particularly towards the end of the shift. This could result in connectors being loosely placed on the terminals, which could create an electrical arc (sparking). In all of the factories, there was a fixed Ah input that was monitored and the process completed when it was reached.

In one factory, there was an additional refinement of a temperature limit that moderated the current flow. This meant that the programmed input current was reduced or switched off when the temperature of the batteries reached a pre-set value. This slowed down the formation process, as the current input was reduced over several of the circuits that we had observed. This resulted in the formation process taking longer to accumulate the pre-set number of Ah that were required in the programme (Fig 2). The other issues of concern were the conditions in the working environment, and the storage of the connectors in a wet, acidic condition.

One recurring problem, reported in every factory, was that of arcing damage between the connector head and the battery terminal. This was responsible for the majority of scrapped and reworked batteries in all five of the factories’ formation departments. The formation profiles and the computer programming, recording and monitoring methods were noted in each of the factories. This was an important aspect, as the reporting of the results of the field trials— particularly the energy consumed— would lean heavily on the ability to record and store relevant data.

The working practices of the operator were of considerable interest, as severe arcing was often caused by a loose fit between the connector and a battery terminal. It was noted that most operators were in an uncomfortable position, had to reach over a long distance, and apply sufficient force to push the connectors firmly onto the terminals. The operator also had to maintain a pace sufficient to keep up with the roller conveyor. This was a continuous process over the whole 8-hour shift. It was made more difficult by the condition of the battery terminals. There was a high proportion that had casting defects, which made correctly fitting the connectors more difficult and time-consuming.

Another observation of the visit was the treatment and maintenance of the connectors. There was, in all cases, significant tarnishing or corrosion of the inside of the connector head. This would certainly contribute to a high-resistance connection between the connector and the battery terminal. This, in turn, would push up the voltage of the series-connected circuit, and probably limit the initial current flow in the first 30 minutes of the formation programme. After that, the voltage would be higher on charge and, with a fixed-current programme, the total energy consumed would increase. This high resistance, found with used connectors, was the basis for our first battery-testing programme.

The testing programme

For our testing programme, we wanted to measure the effects of high-resistance connections, due to the corrosion layer on used connectors, when compared to new, unused connectors. The work began by measuring the differences in the interface resistance between the two connector groups when attached to a standardised engineered-aluminium SMMT automotive battery connector. These tests showed remarkable differences: with an average of 6.52 milliohms and a maximum of 11.46 milliohms for the used connectors, compared to 0.11 milliohms for new ones. It was these differences that led to the examination of the energy usage or losses of the formation process. On each battery, there are two connections, i.e. one for each of the terminals. This would double the average- and maximum-resistance values per battery using used connectors.

The implication of this is that the energy used to pass the formation current through the battery is increased. The amount is proportional to the higher voltage created by the higher resistance, V x I watts. The higher voltage is the product of the current and the higher resistance I x R. The resistance loss from unused standard connectors was deducted from the resistance value of the used samples, to calculate the resistance losses from the used connectors. This showed potential formation-energy losses for the lowest and the highest resistance values obtained (Table 2). The financial implications for lead-acid battery factories from these simple tests were enormous. The losses in energy alone, leaving out battery damage and lost revenue due to increased formation times, would be in the hundreds of thousands of dollars per five million batteries produced. These tests and the financial implications can be found in archived copies of BEST available on the website.

High resistance

The next stage was to investigate the cause of the high resistance in the connectors that had been in service. Physical examination showed that the connector heads were both damaged from arcing and were corroded (Fig 3). The corrosion product appeared to form a coating around the whole of the connector head interior. This meant that the contact between the connector and the battery terminal would always have this corrosion layer as an interface between them. If this layer was non-conducting, it would most likely be the cause of the higher resistance values found with the used connectors.

Cross-sections made of used connectors had not shown any deterioration of the cable bunch and the attached connector head— which logically meant that the corrosion layer, formed on the inside of the connector head, was the most likely cause of the high-resistance connections measured in the tests. At this point, it was decided to examine the corrosion layer in more detail to ascertain its chemical and physical construction. It was hoped that this knowledge would assist in finding a method of removal, a maintenance procedure and/or a connector design that would prevent its formation.

A chemical laboratory, able to carry out SEM and optical microscopy methods, was contacted to analyse the physical construction and depth of the corrosion layer. The SEM was also able to provide a quantitative XRD analysis, which would provide the stoichiometry for the chemical make-up of the layer. The results were as follows:

• Physical construction of the layer. Fig 4 shows the inside surface of a used connector head. The corroded outer surface is seen to be a duplex layer. There is a brittle outer layer containing cracks and a homogenous inner layer that has not yet started to crack or spall. Also noticeable is the absence of antimony in the outer layer. The first impression is that it resembles the corroded surface of a positive lead-acid battery grid that has been subjected to cyclic service duty. In this type of corrosion, the antimony is preferentially leached out from the corrosion layer due to the electrochemical action of the charging process. The inference of this is that the process forming this inner corrosion layer is electrochemical. The physical brittleness of the outer layer, and the fact that parts have detached, underline the potential for pitting corrosion and arcing. These are caused by gaps in the contact area between the connector and the battery terminal.
• Chemical stoichiometry. The corrosion layer was subjected to EDX analysis using the SEM microscope. This revealed the presence of sulfur, oxygen and lead in all parts of the corrosion layer. It also confirmed the absence of antimony in the outer brittle layer as observed in the optical microscopy images. It was possible to determine the % composition of the elements found in the layer, which was consistent with lead sulfate, PbSO₄. Table 3 shows the spectral data analysis with the ratios of 4:1:1 for oxygen, sulfur and lead respectively. With this information, it was conclusive that the layer was electrochemically formed in the presence of dilute sulfuric acid.

The chemical analysis was important as it proved that there must be acid ingress into the connector at some stage in the formation procedures. This includes pre- and post-handling operations as well as storage and in-process use of the connectors. Reference to a Pourbaix diagram for lead in sulfuric acid shows that this PbSO₄ corrosion layer is stable at very dilute acid levels and with a potential difference of less than one volt.

This has implications for the operating and maintenance procedures of formation departments. For the process implications, it would be necessary to see where the acid gained access to the inside of the connector head. This would start at the filling process, through the battery connecting and charging, and ending when the connectors are removed. Any maintenance procedures would need to include the handling and storage conditions of the connectors. This would consist of a washing or cleaning process to keep the pH of the surface liquid close to neutral. This, in fact, was part of an investigation during further factory visits.

Controlled formation trials

At this stage, we needed to start some controlled formation trials to observe the effects of using used and new connectors. This would both reproduce and quantify the energy losses, which we now knew to occur in all lead-acid battery formation departments. It would also enable us to determine the source of acid ingress into the connector heads.

At this point, Digatron supplied one of their formation test units to help in the testing, Model BNT 150-036-4 ME. With this sophisticated, programmable unit, we could reproduce any formation programme or field application. We could monitor all electrical parameters and operational temperatures during the tests. The most significant measurements were: the energy used, the temperatures reached, the voltages, the currents, the instantaneous current; as well as accumulated ampere-hours and the instantaneous and average values of all charging and discharging parameters.

This single measure of supplying the test equipment enabled the group to accurately predict energy losses, current absorption, Wh and Ah consumed; as well as temperature rises and formation programme interruptions, in any formation programme. It also enabled the accurate testing of the proposed design and maintenance solutions that would be devised as a result of the investigation.

With the Digatron test equipment, we were able to start the following programme:

• Measurement of the effect of higher-resistance connections on formation efficiency
• The Ah and Wh absorbed with constant-voltage (CV) and constant-current (CC) charging
• Simulation of formation programmes with different control parameters. These include:
  • Temperature limited control
  • Energy limited programme.
  • Fixed Ah input
  • Constant-current charging with maximum voltage restriction
  • Constant-voltage charging with current limitation

It was possible to simulate any factory formation programme and record all relevant parameters to compare connectors in laboratory conditions. All this was entirely necessary to fully understand the implications of high-resistance connections, as well as the mechanisms causing the high resistance. Standard tests were performed to show the effect of high resistance on the energy used and the Ah absorbed by batteries in formation conditions. Fig 5 is two graphs directly taken from the Digatron software. It collects and also processes the results of test procedures. These are from a voltage-limited, constant-current programme. It was designed to demonstrate the effect of circuit resistance on a battery’s ability to absorb coulombs during charging. It also enabled the measurement of the energy consumed to absorb the current.

Graph 5a shows the battery response to a fixed-current input during charge with high-resistance connectors. The graph is divided into three stages:

1. Constant-current
2. Constant-voltage with steeply declining current
3. Constant-voltage with gradually declining current

There are two important points here: the first is the steep rise of the voltage curve in the constant-current phase, the second is the time taken to reach the fixed voltage and the point at which the current starts to decline. This illustrates the fact that a higher resistance will create a higher voltage at a constant current. This higher voltage will reach the maximum peak sooner than in the case of a battery that is part of a lower-resistance circuit. This means that the maximum current will flow for a shorter period, which will reduce the coulombic input into the battery during this CC phase.

It also demonstrates that the voltage will be higher during the CC phase. In other words, we have fewer coulombs or Ah going into the battery; it also takes more energy to push them in. This is borne out by Graph 5b, which is obtained from the same programme but using lower resistance connectors— in this case, new, unused, standard connectors.

Following the voltage curve in Phase 1, the voltage rises more slowly and reaches the ceiling many minutes later. More Ah are input, with less energy consumed before the current begins to decline. In the second and third phases of the programme, the current declines as the battery becomes more fully charged. It then begins to level out at a smaller value. The accumulated Ah in each phase was higher for the unused connector.

These results are summarised in Table 4. In this, there are two tables, which highlight three factors: Ah input, Wh input and connection resistance. Both tables compare used connectors with new connectors. In the upper table, the connection resistance of the used connectors is significantly higher. This points to the existence of an insulating layer inside of the connector head. The energy consumption in phase one is higher for the used connectors despite accumulating fewer Ah. This shows the inefficiency of putting in coulombs when you have a high resistance. The last column shows the total Ah input which is significantly higher for the new connectors. The second table shows the percentage difference of Wh and Ah absorbed.

The big take-aways are:

• The new connectors are able to absorb 5-10% more Ah on formation than the used connectors
• Less energy is consumed, particularly in phase one

A lot more data was generated from these trials which are reported in archived copies of BEST.

Formation programme effects

The next stage was to run a real formation programme to see the effect of the higher resistance connections. For this trial, one factory donated green, lead-acid SLI batteries in order to run a complete formation programme. The batteries were flooded, 12V 90 Ah C20 with a final electrolyte SG of 1.280 – 1.290. There were two batteries connected in series per circuit. One circuit used standard design new connectors; the other had used connectors in average condition. These were taken from the donating factory’s formation department.

The programme chosen was a temperature-limited programme as used in the same factory. The nominal run time was 20hrs and the voltage was limited by the maximum output from the Digatron test unit. This accurately reflected the situation in the participating factory, which had a maximum voltage per circuit from its charging rectifiers. Fig 6 shows the results of the Wh accumulated by each battery circuit over the total formation cycle.

The programme was constant-current, with a fixed Ah input and a nominal time of 20hrs to completion. The total time to complete the programme for the new connectors was 18hrs. The Wh accumulated for the new and used connectors are clearly diverging at a constant rate over this time period. At the end of the formation period, the total difference was 3.65% less energy used by the new connectors compared with the old design.

Connector redesign

At this point there was sufficient information to provide the basis for redesigning the standard connector. It was evident that there were three clear objectives:

• Construct the connector head in a way that allows for battery terminal defects whilst providing a high contact area (Fig 7).
• Prevent acid ingress into the internal surface of the connector head.
• Have a more ergonomic holding point to prevent operator fatigue.

Fig 8 shows the new design. The new TSC connector has a split lead-alloy head. Each of the six segments has sufficient flexibility to move and accommodate casting defects. The movement is small and within the elastic limit of the lead alloy. The outer insulating sheath is able to maintain pressure on the connector head to sustain the grip provided by the six segments. There is also a longer handle, which provides a more generous grip for the operator to make fitting and removing less strenuous.

Field trials

The next step was to arrange field trials with participating factories. Five lead-acid battery companies had agreed to take part in monitoring the performance of the new design connector and unused standard connectors, in field trials. In these, we compared the performance of the new TSC connector and the standard T-connector against connectors that had been in service for several months. There were three parameters to measure: the energy used, the incidence of arcing (battery damage) and the ease of use (ergonomics)

Out of the five battery companies, two were able to measure the energy used and three measured the incidence of arcing. All companies made three dedicated circuits available from standard production rectifiers in their formation departments. In all cases, the circuits consisted of up to 20 x 12V batteries, connected in series strings and contained in water baths. The circuit voltages and applied currents were limited by the rectifier characteristics. The average voltage limit for the circuits, in all companies, was approximately 315 volts, with a supplied current of up to 50 amps.

The circuits were arranged so that one of each was dedicated to an individual connector. The arrangement was: one circuit with unused new design TSC connectors, another with unused standard design T-connectors and the third with used standard T-connectors. The used T-connectors had been in service for several months.

This was the starting point of the trials. The connectors have not been changed since the trials began. The trials are still ongoing and have completed between nine and 18 months testing service. During this period the new TSC and T-connectors have not been changed.

The purpose of the trials is to ascertain how these connectors perform in several aspects:

• Limiting acid ingress from liquid or mist to prevent the formation of a corrosion layer
• The ability to maintain good contact over the life of the connector
• The number of arcing incidents and damage caused by the new TSC design in comparison to the standard T-design connector
• The ease of use for the operator to enable a good connector-to-battery-terminal contact to be achieved under normal working conditions
• How long the connector will last until its performance declines to match that of the standard used connector
• The useful life of the connector over which it is still providing cost savings
• Finding the ROI of the TSC to compare with the standard T-design of connector

As previously mentioned, three of the companies were unable to record the energy used during the normal production operations. They concentrated on the productivity, rework and scrap rates arising from the three test circuits. The incidence of arcing was used as a measure of the possible scrap/rework generated; and the ease of fitting the connectors. The other two companies were able to monitor these parameters, but in addition, they were able to record the energy used in their trials.

Potential savings confirmed

The summary of the test activities of the factories, identified as F1 to F5 is given in Table 5. Each column shows a different parameter measured during the trials. The savings columns for energy and arcing damage are self-evident. The confirmed cost-savings column lists those parameters that were directly measured or calculated directly from the results. For factories F1 and F2, it was energy that was directly measured and listed in the confirmed savings.

The arcing damage is based on a figure derived from verbal confirmation of normal scrap rates in the relevant formation departments. The percentage of production is based on discussions with the participating companies and on personal experience of more than 40 years in lead-acid battery manufacture.

It can be seen from the first column that energy savings were not monitored in three of the factories. These were estimated from the results of factories F1 and F2 and are included in the final column. The incidence of arcing was measured in all factories and the cost savings were calculated.

The basis of these costs is given in Table 5. This is a full explanation of the values used to make the calculations. The costs of battery manufacture have almost certainly been underestimated at $40 for a 75Ah 12V battery. The scrap allowance is also very generous, leaving a net cost of $18 per battery scrapped. Likewise, the cost for battery rework is based on the factories’ own estimations.

Because the factories have different throughputs and different product ranges, the results are normalised to a five million battery per annum turnover. The energy costs are standardised to a 75Ah 12V battery in order to make valid comparisons between participating factories.

In the case of factory 1, they had already started testing in 2018 after reading the articles in BEST magazine. They had simply followed the advice to keep the standard T-type connectors already in service, clean and free of acid. They measured the energy use of the factory over 12 months, from 2017-2018. They found that the energy used in the formation department was considerably less as a result of this measure. In fact, they had made a real saving of around $300,000 in those 12 months of monitoring. This was before starting the trials of the new TSC connector and measuring the parameters listed above.

In the other factories, these tests, which began between the Summer and Autumn of 2019 are still on going. Since April of this year there has been a small hiatus due to the Covid-19 pandemic. However, the trials have fully resumed and there are sufficient results to make a useful progress report.

It should be appreciated that comments regarding ease of use are mostly subjective, and so are not included in the results summary. However, the comments have been noted, and are being taken into account to decide if any future design modifications may be necessary. At this stage, there has been no evidence from the reports that would support making further design changes.

Table 5 gives a useful prediction of the potential savings for practically any lead-acid battery manufacturer, by upgrading the connector design used in the formation department.

Testing results

The confirmed results clearly show that preventing the scrapping and rework of batteries, damaged by connector arcing, has the highest impact on the potential cost savings. This is approximately two to three times more than the energy savings. That is not to say that the energy savings alone are not substantial. As you can appreciate from the last column, there is a pretty big saving potential from all the cost-saving measures.

All factories are showing potential savings of more than $800,000 per annum. This goes directly to the bottom line of the factories’ annual accounts. However, the savings do not stop there.

The energy savings also directly translate to reduction in carbon emissions, to radically reduce the carbon footprint of those participating companies. Fig 9 shows the potential annual savings of CO2 emissions for companies adopting these connector-based cost-saving measures.

Looking in more detail at the energy savings, Fig 10 shows the results from factory two as a graph. This compares the energy consumed, under the test conditions described, by the different connectors used in the trials. This shows a comparison of energy saved using the new and standard design TSC and T-types. These are compared with the standard used connector.

The percentage saving is the energy used by the old connector for a specific battery type, minus the energy consumed by both TSC and TS new connectors. In this graph, the energy consumed by the used connector is a straight line at zero. The Y-axis is the percentage cost saving compared to the used standard T-type connector. The bottom line is each individual formation batch. The gap in the results was due to a maintenance break on those circuits using the new TSC and T-type designs.

Initially, the new TSC and T-types gave similar results but, after several uses, began to diverge. The last results we have show that the new design TSC connector is starting to positively diverge from the standard design with time in service. The standard T-design will eventually become the same as the used standard connectors. The most likely reason is that the standard T-type connector does not exclude acid from forming a corrosion layer on the contact surface. This confirms the advantages of the new design in excluding sulfuric acid from the connector head.

The trials are on-going and will continue for some time. This is partly to ascertain the life of the TSC connector and its contribution to operator comfort. However, the results obtained so far are sufficient to make accurate estimations of the potential savings for lead-acid battery manufacturers. The savings can be made by reducing or eliminating battery damage from arcing, and providing better overall energy efficiency, due to maintaining low resistance connections in formation circuits.

Positive conclusions and future plans

At this stage we can draw some definite, positive conclusions:

• UK Powertech, Digatron, Energy Storage Publishing and Ecotech Energy Solutions have identified the causes and measured the costs of energy losses and battery damage that are inherent in most lead-acid formation departments
• The standard design of connectors used in a formation department suffers from poor battery terminal fitting and an insulating corrosion layer, formed by electrochemical action in the acid environment
• Both of these factors are responsible for significant levels of battery damage due to arcing and formation energy losses of 3-5%
• In field trials UK Powertech’s new formation connector designs and maintenance procedures are proven to save lead-acid battery factories more than $800,000/annum per five million batteries. The savings are the result of preventing lost energy and connector/terminal arcing damage
• Digatron have made significant modifications to their formation rectifiers to prevent fires, reduce downtime and minimise spares inventories

Considering this work began with a conversation at the 2016 ELBC in Malta, the group of UK Powertech, Digatron, Energy Storage Publishing and Ecotech Energy Solutions, along with the participating lead-acid battery factories, have come a very long way. The results of this work can only be of benefit to the industry as a whole, as it applies to every formation department in every LAB factory in the world. This process is a major, capital-heavy bottleneck in practically all LAB factories. It is responsible for at least half of the total energy consumption of a lead-acid battery factory. By taking advantage of such simple and cost-effective measures, factories can reap substantial rewards. These include: lower energy costs, smaller carbon footprints, less scrap and rework from arcing, lower operating temperatures to improve battery quality and better throughput efficiency. That’s the good news.

The even better news is that the same team are now working on the formation process itself, in order to further increase energy efficiency by the use of novel formation programmes. This has just started, and although progress has been hampered by Covid lockdown, early results are encouraging. These are showing lower operational temperatures, reduced water loss, lower gassing rates and lower energy usage. This work was not possible until the variability of the connector resistance had been removed. With that part completed, we now have available the new 21st century connector design. With this, we really can make the formation process a far safer, cleaner and more cost-effective method for manufacturing lead-acid batteries.

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