Lead battery formation back to basics— the start

Having already saved the lead battery industry six-figure sums, the same team of Dr Mike McDonagh, Mark Rigby and the Digatron engineers now begin the next phase of their project to improve the formation process.

Here, in the first part of a series, the team examines the fundamental mechanisms that lead to inefficiency in the formation process itself. The potential outcomes are further energy reductions and reduced process times leading to lower costs and higher productivity.

The formation process for lead-acid batteries consumes at least 50% of the total energy used by manufacturers in the production of lead-acid batteries. The drive to maximise output and minimise capital expenditure has resulted in the current practice of pushing as much energy as possible into the green lead-acid cell or battery in the shortest time achievable. Because this gives maximum ROI on the equipment, and pushes up sales with no additional investment, it is considered as a viable commercial tactic.

The downside, which is the detrimental consequence for battery production efficiency, in terms of an increase in energy used and reject or warranty return rates, is considered as acceptable collateral damage. But is it inevitable that these formation consequences are unavoidable, with the higher currents and lower process times now commonplace, particularly in the SLI industry?

Following on from the successful two-year formation connector project, undertaken by UK Powertech, Digatron, BEST and Ecotech Energy Solutions, the same team are now moving on to examine the formation process itself. This new project, which starts now, examines the possibility of reducing the energy wasted and the time of the process, by forming green batteries in a more efficient and cost-effective way. In this article, the first stage of the new project— “Efficient fast formation processes”, the team has begun probing an SLI battery’s voltage response to a pulsed current input, at different stages of formation. As with before, there is a strong connection with the industry, and several global manufacturers are partners in providing both resources for the R&D stage and also facilities for the project field trials.

The project

The first stage is to look at what is happening in the lead-acid battery formation process. To more thoroughly understand this, we need to consider the acid-filling stage as the starting point. This is the process whereby the newly assembled battery with cured AM plates is filled with dilute sulphuric acid. The active materials of the positive and negative plates are chemically similar at this stage, with very little difference, other than a small carbon content in the negative, to distinguish between them.

The basis of all batteries is to create a potential difference between plates immersed in electrolyte. After acid filling and before formation, both plates have similar chemistry, mainly lead oxides and basic lead sulphates. At this stage, there is almost no voltage and no energy difference to create an electron flow when the plates are connected. Once a current is applied, the polarity is chosen to ensure that the electron flow is in the right direction to pump electrons from the positive into the negative plate. In the case of the lead-acid battery, we have to create that imbalance by forming two different chemical species for the positive and the negative plate, i.e. PbO₂ for the positive and Pb for the negative.

Referring to previous articles in BEST, we know that the actual structure and the chemistry of the dry cured paste is complex. The dry cured structure, before acid filling, will have an effect on the properties and performance of both the Positive Active Material (PAM) and the Negative Active Material (NAM). However, the basic reactions and the electrical energy required for conversion can be illustrated with reasonable simplicity if we take the plate chemistry after soaking in sulphuric acid.

Although there is a difference between the 3BS and 4BS cured plates, in general, the conversion of the cured active mass to lead sulphate is restricted by the penetration of sulphuric acid into the particle. The larger the particle the more time that is required for the acid to penetrate. Additionally, the larger the particles the lower the temperature rise due to a smaller surface area which reduces the rate of the reaction in the active mass. The formation reactions generate heat that results in a temperature rise in the battery and the formation tank. The temperature continues to rise until the rate of the reactions slows to a level where the heat loss to the atmosphere of the system balances the internally generated heat of reaction. The maximum temperature reached depends upon the acid strength, the chemistry and the physical structure of the plates.

The sources of inefficiency in charging, stem from the sources of resistance, which prevent, slow down or raise the energy of the conversion of unformed material into the PAM and NAM of the battery. It is necessary to understand the physical and chemical architecture of these mechanisms in order to devise a more efficient method of putting in the electrochemical energy to convert the material. Authors such as Detchko Pavlov and Hans Bode, have written a lot on the phase changes in the material as it progresses through the formation programme. For the purposes of this article such complexity is not required. We simply have to look at the bulk mechanisms affecting the ability of chemical species to migrate to the correct place in order to form the active material of the battery. In a simple form there are two elements to the formation reactions:

• Solid-state reactions within the active material particles.

• Solid/liquid interface reactions occurring at the surface of the particles.

Fig 1 is a PAM particle immersed in an electrolyte of dilute sulphuric acid. It is representative of a particle at the beginning of the formation process. The PAM of a battery plate is connected by networks described by Pavlov. The connection of the PAM to the grid is initiated by the curing process and firmly established during formation.

The first thing to note is the conversion of lead sulphate to lead dioxide at the surface. At this stage the reaction is hindered only by the diffusion of SO₄^– ions or H₂SO₄ molecules away from the reaction site plus diffusion of oxygen ions/molecules to the reaction site. Taking a single particle of PAM (for simplicity of illustration) we can look at the progress of formation at two points during the process. Fig 1 illustrates that within the first few minutes, we have a very thin layer of conducting PbO₂ around the surface of a particle. The resistance of the particle is quite high at this point, being largely made up of lead sulphate. The resistance of the cell will also include the metallics (grid plus plate connections) Rm and the electrolyte, Re. Where the total battery resistance Rb is:
Rb = Rm + Re + Rp.

A simple equation to show the resistance of the particle Rp as formation time progresses could be: Rp = xR1 + yR2 + zR3. Where:

• R1 = resistivity and x the linear depth of penetration of the PbO₂ layer

• R2 = resistivity and y the linear depth of penetration of the PbO₂/PbSO₄ interface layer

• R3 = resistivity and x the average linear dimension of of the PbSO₄ particle

Where R3>R2>R1

We know that a battery’s internal resistance will decrease during the formation process as the NAM and PAM particles are converted to Pb and PbO₂ respectively. The interface layer within the PAM particle has been identified by Pavlov as PbSO₄.xH₂O + PbO₂

Fig 2 shows the situation of the same particle well into the formation programme. In this, X and Y increase with time whilst Z reduces. As this mechanism progresses y reaches a more stable value as its length reaches equilibrium, whilst x continues to grow at the expense of z. Because R3 has the higher resistivity, the total resistance of the particle decreases as the formation process progresses.

However, as already mentioned, the AM of the battery is not the only source of resistance. We have to add in the electrolyte and the metallic conductors of grid, plate straps, terminals and intercell connection metal. Again, for simplicity we can remove the contribution of the metallics, as these will stay constant (adjusted for temperature). The electrolyte resistance will vary however, due to the increase in acid density as the sulphate is removed from the particles, to react with water and form sulphuric acid. Fig 3 represents the source of resistance from an increasing acid concentration. As can be seen from the three time points chosen in the formation process, as the bulk acid concentration increases, the concentration gradient between the particle surface and the bulk solution decreases. This means that there is a lower driving force for the sulphate to diffuse away from the particle surface into the bulk electrolyte. A lower driving force means that there is more resistance and that a higher voltage is needed to overcome this increasing impedance to AM conversion.

So, what has all this got to do with the price of lead? Very simply, we are looking for indicators to tell us what stage the formation process is at. By eliciting a response from the battery in the form of a voltage pulse, we can link this to the state of battery charge under these conditions. Since voltage is related to resistance then could we simply use voltage as an indicator? The straight answer is no! Early on in the schedule this value reaches a plateau caused by parasitic reactions, mostly hydrogen and oxygen evolution. The ratio of gas evolution and heat compared to the progress of AM conversion, increases as formation progresses, making the process more inefficient the longer it goes on. However, this does mean that there is another indicator to use, based on the rate of voltage increase and the rate of voltage decline when a fixed current is switched on and off. Putting together the total voltage rise due to electrolyte density change and the speed of the voltage response to a fixed current pulse, we can perhaps map out a profile for a pulse that would characterise the different formation stages. It would also give some insight into how to vary a current input to obtain maximum efficiency by minimising gas evolution.

Testing programme begins

We are now at the first stage of the formation review programme; this initial study has two parts: first it logs the data obtained from forming batteries using a commercial formation schedule. This will then be used to compare with results from further schedules designed to improve the process efficiency. Secondly it probes the battery’s voltage response at each stage of the process in order to understand how the chemistry alters from the start to the end of the schedule. From this we should build up a picture during the process of how the efficiency might be improved. This could take the form of a time and/or an energy, a water loss, a temperature reduction, whilst still achieving the same amount of AM conversion.

To start the testing programme, we have batteries donated by a collaborating partner that manufactures SLI lead-acid batteries. They have also supplied their standard formation schedule as a baseline against which we can measure our progress during the coming months. In parallel is another battery on a formation schedule that inputs the same number of Ah and contains a section designed to probe the batteries response to current pulses during its progress. For reasons of commercial confidentiality, none of the details of any of the formation schedules will be published. However, there will be descriptions to explain the reasoning and methodology used to construct any schedules used during the test programme.

• The battery test samples have a nominal 75Ah capacity at the C20 rate

• The equipment used is the Digatron C600 test unit with four 150A, 34V outlets

• The Digatron unit records current, voltage, instant and cumulative watt-hours and Ampere-hours, temperature and time of each phase in the programme

• The batteries are water cooled in a closed container with a pump to circulate the water giving an even temperature distribution within the bath

• A digital oscilloscope displays the battery voltage during the pulsing phase of the test schedule. Screenshots are taken at different phases of the schedule to compare changes in the voltage pulse response at these different phases

Test schedules

There are two formation schedules to be compared.

The first is a standard industry schedule with temperature limits incorporated. The schedule is entirely analogue using constant current input with a minimum formation time of 10.39 hours, provided the temperature limits are not exceeded. If the limits are exceeded, then the current is reduced or switches off, until the temperature drops below the limits. In this instance the schedule time would be extended beyond the minimum value. In the factory that provided the samples and schedules, there are efficient, refrigerated water baths that effectively control the temperature. In our case, we do not have refrigeration but, instead, rely on a high water volume to number of batteries ratio, to achieve the same end.

The second programme is devised to extract information from the battery during its formation schedule. It should theoretically supply the same number of Ah into the battery in approximately the same time as the industrial programme. However, from an early stage it will provide a constant current pulse at half-second intervals i.e. 2Hz frequency, equal on-and-off time. By analysing the pulse voltage response from different stages in the schedule, it is hoped to get some insight into the ability of the battery to accept useful, AM transforming charge, and how much of the current input is consumed in parasitic, energy-wasting reactions.

Results of tests

The results are in two sections: the first section contains the calibration tests, which were designed to find out how accurate the recording of energy and ampere hours would be when measuring pulsed data. The second is the current batch of results: these are the data used to assess subsequent formation profiles, and which also provide some indication of how to structure future charging profiles to improve their efficiency.

Section 1— Proving trials

Fig 4 shows that the measurement of Wh and Ah is dependant on the pulse frequency and shape. A regular on-off pulse with equal time interval for both conditions Fig 4a, is easiest to monitor and all data points are captured. An irregular pulse, Fig 4b, has fewer data points captured and may require a higher sampling frequency. For the initial trials a 500ms on and 500ms off pulse rate was chosen, which needed a relatively low 500ms sampling rate to capture all the pulses.

Using this sampling frequency with a regular half-second pulse it was possible to obtain values for the accumulated Wh and Ah that were within 1% of the calculated values. With this information, we are able to confidently construct formation schedules with appropriate pulsing algorithms, and able to accurately measure the energy and ampere-hours consumed in the subsequent process trials.

Section 2 – Baseline (control) trials

These tests were twofold:

• To determine the baseline efficiency and effectiveness of the standard industrial formation schedule for SLI batteries

• To ascertain the battery responses to a fixed current pulse at different stages in the formation schedule

The data capture graphs in Fig 5 show the changes in voltage as well as the increase in Ah and temperature during the two formation schedules. Table 1 gives the summarised data for both schedules. As can be seen, the Ah input was identical; the temperature rise was similar, with the pulsed version being almost 20C higher than the standard. Likewise, the energy input was 2% higher from the pulse schedule. The times are interesting. The pulsed version took 50 minutes less to complete the schedule, which means the standard method takes 8% longer. The pulsing method had to put in double the current for the majority of the programme, in order to get the Ah input in the same time as the standard programme, which had a 28 amp analogue input.

Fig 6 shows the voltage response to a 2Hz constant current pulse of 58 amps. The first oscilloscope trace (Fig 6a) was taken after just over two hours into the schedule, the secondcloser to the end at eight hours. As can be seen, there are significant differences between the two oscilloscope traces from near the start of the schedule to near the end of the schedule. In Fig 6a the baseline is at 13.2V c.f. 15.8V for Fig 6b and the maximum peak is at 18.2V C.f 14.5V. This means that the battery is always substantially above the gassing voltage right through the charging period. The other point to note is that the shape of the voltage response curve is also different. The gradient on the slope to reach maximum voltage in Fig 6b is far steeper (almost 90°) than that of the voltage slope in Fig 6a. In fact, the pulse is virtually at the peak voltage for its entire duration. The reason for this stems from the increasing resistance values due to the mechanisms occurring at the AM particle/liquid electrolyte interface.

The consistent high voltage of the formation latter stage current pulsing, shows the difficulty of obtaining a higher efficiency of current input towards the end of the process. The next tests will be probing these latter stages in more detail with higher frequency pulses. It is fairly clear that, in order to input the current more efficiently, we need to alter the schedule to take into account the higher resistance and voltage response of the battery as the schedule progresses from beginning to end. With this method of interrogating the battery, we have, in fact, identified the limits of the magnitude of current input as a function of the formation reaction progress to completion. The shape of the voltage response pulse is vitally important in ascertaining the type and rate of reactions occurring during the AM conversion in forming the battery plates. The next set of tests will examine the pulse morphology as a function of state of formation progress.