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A goal of any progressive battery manufacturer is to shorten formation time while reducing energy consumed during the process. The requirement to increase capacity while reducing operating expense is a common request. Battery manufactures are looking for innovative solutions from formation system suppliers that provide a competitive edge. That edge may be lower cost to manufacture, improved product quality or some combination of both.

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Digatron Power Electronics formation charging rectifiers provide high current output and allow PC controlled formation processes to takeadvantage of the efficient cooling methods optimizing the formationprocesses.

There are two leading power conversion technologies used in formationcharging rectifiers

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The objective of this paper is to evaluate the two power conversion technologies in terms of the following characteristics:

- Process efficiency

- Battery quality and performance

- Crystalline structure of active material

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The traditional SCR technology uses phase angle control to regulate DC output. The DC output signal contains a characteristic 300 Hz current ripple component with an almost constant RMS value within the output range of 10 to 100%.

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For this reason ripple as a percentage of total DC output is greatest at low current levels.

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There is a direct correlation between power factor and output voltage. When battery string voltage is low with respect to full scale output the power factor will also be low. As battery string voltage increases the power factor improves in an almost linear relationship.

Phase angle control technology requires reactive power which must be compensated for, with Power Factor Correction (PFC) equipment.

SCR based rectifiers introduce undesirable harmonics to the AC power line which must be considered when specifing PFC equipment.

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The IGBT based switch mode technology uses pulse width modulation to regulate DC output.

Filter components such as chokes and capacitors of a given package size are significantly more effective when used at 20 kHz than when used in SCR circuits at 300 Hz.

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A characteristic of IGBT circuits is that current ripple is very low, typically less than 1% throughout the output range with worst case ripple at 50% of full scale output and best case at the output extremes.

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The power factor of IGBT circuits is constant at 0.98 throughout the output range.

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This slide displays the individual harmonics at full output for SCR and IGBT circuits. It is obvious that the 5th, 7th and 11th harmonics are the most significant.

Harmonics must be considered when specifing Power Factor Correction equipment.

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This presentations attempts to answering the following common questions:

Is one power conversion technology inherently

better suited to shorten formation time ?

more efficient than the other ?

Does power conversion technology

influence battery performance ?

influence heat generation in batteries during formation process ?

influence the crystalline structure of active material ?

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One formation process was started with DIN EN 630xx truck batteries (130Ah) optimized for cold cranking applications

using 6x 60A/360V SCR charge circuits, each circuit 18 batteries and 6x 60A/360V IGBT charge circuits, each circuit 18 batteries.

One formation process was started with DIN EN 640xx truck batteries (140Ah) optimized for cycle life using 6x 60A/360V SCR charge circuits, each circuit 18 batteries and 6x 60A/360V IGBT charge circuits, each circuit 18 batteries

For each battery type processes were started at the same time with identical programs

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One 3-phase power net analyzer was connected to the AC input of the SCR rectifier and another was connected to the AC input of the IGBT rectifier. The analyzer was used to evaluate AC data for real power, reactive power, cosphi and total harmonics distortion (THD).

Battery Manager PC software was used to control the formation process and to evaluate all DC data and electrolyte temperatures

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5 sample batteries per formation batch were selected for test and shipped to an independant battery laboratory. One additional sample was selected for scanning electronic microscope (SEM) analysis.

The tests were conducted in compliance with DIN EN 50342.

Three cycles were completed consisting of capacity C20 test and a cold cranking test followed by a single charge acceptance test.

Data from each 5 sample batch was averaged.

Testing generated 7 data files per batterytimes 5 batteries per test batchtimes 4 formation batches consisting of the following:

- DIN EN 630xx battery formation using SCR rectifier

- DIN EN 630xx battery formation using IGBT rectifier

- DIN EN 640xx battery formation using SCR rectifier

- DIN EN 640xx battery formation using IGBT rectifier

which resulted in 140 datafiles to be evaluated.

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Water baths were filled with 20°C water to establish a common temperature at the start of test.

Separate water baths were used to isolate the test samples from the formation system water bath.

Battery in bath 1 was connected to the formation string of the SCR rectifier.

Battery in bath 2 was connected to the formation string of the IGBT rectifier.

Battery Manager PC software was used to control the formation processes and to monitor electrolyte temperature data.

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This graph represents the DC output power profile generated by the charge regime defined in the Battery Manager program editor.

This profile is identical for both SCR and IGBT circuits.

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When we compare the DC output power profile to the AC input power profile we see how insignificant the losses are with IGBT technology.

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When we compare the IGBT AC input power profile to the SCR AC input power profile we can see the efficiency advantage gained with IGBT technology.

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In addition to real power the SCR circuits consume a significant amount of reactive power which will require special power factor correction (PFC).

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During the 12h formation period the charged energy was 1170 kWh and the real energy consumed was 1242 kWh yielding an average efficiency of 94,2%.

The minimum efficiency of 85,5% occures only during the inital phase of formation when battery string voltage and charge currents are low .

High efficiency is achieved if battery string voltage is above 50% and current is around 75% of full scale output.

Maximum efficiency of 97.6% was recorded at 262V and 44.75A.

Because the reactive power is so low it is not compensated for or considered in this calculation.

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During the 12h formation period the charged energy was 1170 kWh and the real energy consumed was 1283 kWh yielding an average efficiency of 91,2%.

The minimum efficiency of 89,9% occures only during the inital phase of formation when battery string voltage and charge currents are low .

High efficiency is achieved at maximum DC power output.

Unlike the IGBT circuits the reactive power is so high and must be compensated for. The inefficiencies associated with PFC compensation are not considered here.

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The graph compares the measured efficiency of SCR and IGBT power conversion technologies throughout the DC output power range.

IGBT circuits are up to 5% more efficient than SCR circuits within the output range from 30 to 80%.

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The graph compares the measured cosphi of SCR and IGBT power conversion technologies throughout the DC output power range.

The power factor for the IGBT circuit is significantly greater than the SCR circuits throughout the output range and approaches a factor of 1.0 from 40% to 100% of full scale power.

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This graph shows the measured power factor for each technology throughout the 12h formation profile.

The average power factor for the SCR was 0,66 and for the IGBT 0,98.

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This graph shows the measured total harmonic distortion (THD) for eachtechnology throughout the 12h formation profile.

The average THD for the SCR was 32% and for the IGBT 13%.

With Digatron transformer techniques the harmonic characteristic of an SCR rectifier can be reduced down to 20% THD.

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The data collected here goes against the widely held asumption that there is a strong correlation between the output ripple typical of SCR circuits and heat generated during the formation process.

One must conclude that there is no significant difference between SCR and IGBT circuits relating to heat generation during formation for the battery types tested.

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The data here indicates there is no siginificant difference after the third cycle in discharge and recharge capacity for both battery types.

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Again the data indicates there is no significant difference between the two technologies in the C20 discharge test.

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The 10s voltage data during cold cranking provides similar results.

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And also cold cranking discharge time to 6V.

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Unlike the previous tests we do find a significant difference in charge acceptance performance of batteries formed with IGBT circuits.

This data is especially significant due to the small standard deviation in the data collected.

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SEM analyzis of the crystalline structure of active material revieled no significant difference in crystal size, quantity and surface area.

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Based on the test results we can conclude the power conversion technology does not influence formation time.

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Formation process time is determined by:

- charging factor

- final capacity to be achieved during the process as set by themanufacturer

- formation process methods such as cooling, varying acid density,increasing charging currents

- battery chemistry and use of additives

- the reliability of the process control equipment

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The table shows that the SCR circuits will consume 3.8% more energy than an IGBT circuit during the formation process.

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There is no significant difference between SCR and IGBT circuits relating to heat generation during formation for the battery types tested.

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There is no significant difference between SCR and IGBT circuits relating to battery performance except charge acceptance for the battery types tested.

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SEM analyzis of the crystalline structure of active material revieled no significant difference in crystal size, quantity and surface area.

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Ultimately it is the goal of this presentation to reduce the test results and conclusions to a set of practical guidelines that can be applied by battery manufacturers when making decisions regarding the selection and purchase formation rectifier equipment.

The test results indicate there is no significant advantage to either SCR or IGBT technology when considering key factors including process control, battery quality or battery life. That given one must consider the financial aspects when making a purchase decision.

If your production line includes SCR rectifiers with significant service life remaining there is no financial justification for replacement with IGBT. The 3-5% energy savings with IGBT will not result in positive return on investment in the near term. If you local utility has required PFC that investment has already been made and cannot be recouped. Maintenance personnel are already quite familiar with SCR technology, PM procedures, repair processes and typically there has been a significant investment in spare parts inventory that would not be compatible with IGBT rectifiers.

If your formation line includes IGBT rectifiers we recommend continuing with this technology. There is the benefit of energy savings and you will avoid the need for additional PFC equipment when increasing capacity. If your objective is to outfit a new facility we recommend you consider IGBT technology for the same reasons.

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Digatron Power Electronics is a leading supplier of both SCR and IGBT formation rectifiers. SCR rectifiers have been furnished to hundreds of battery manufacturers worldwide over more than three decades.

Our first installation of IGBT rectifiers was commissioned some 8 years ago and has proven extremely reliable.

Technical data for IGBT rectifiers is as follows:

Standard current ranges from 30-60A, other ranges available

Voltage output to 440VDC

Up to 16 circuits in a cabinet

Efficiency up to 98%

Power factor up to 0.98

Isolated secondary for each circuit to eliminate circuit interactionand enhance safety

Paralleling of circuits for higher current output

Constant current pulse discharge and depolarization discharge

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