This is a regular column of George Zguris the Senior Scientist and Technical Director of Holingsworth and Vose. George is an expert in the area of separators for electrochemical devices. Questions regarding this topic may be E Mailed to [email protected]. Copyright by George Zguris, with reprint permission to BD.

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 (July 2004) Question – Can the material used by a separator manufacturer influence the separator’s influence on battery performance.

Answer – The raw material and the source of the raw material that is used to manufacture a separator can influence battery performance.  The separator has been quoted as being the third electrode for VRLA batteries.  In an ad from the May 1928 issue of The Battery Man, (see ad), the separator’s importance is reflected in this quote, “Greater care must be taken in preparing separator for your battery than is necessary in making the plates.”    As the ad demonstrates, the importance of separators and how they are made and process have been known for many years.  In the same magazine, an article entitled, “The Wood Separator, A Discussion by a consulting engineer on this vital part of a storage battery-giving his opinions as to the construction in use of the same” by Gustave H. Rabenalt, discusses how the species of wood will impact performance of a battery.

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Q: I am going to a higher surface area AGM separator, what should I look for?

Answer: It depends on the change and your battery design, but in general, the following eight factors give a general idea of  what a battery manufacturer would experience.

1. Smaller pore structure, thus slower/harder acid filling.

2. Increased strength such as tensile and puncture strength.

3. Lower initial thickness, fine fiber tends to make a denser separator.

4. Slower drip speed and rate of wicking, relates to finer pore structure.

5. Decreased stratification.

6. Increased separator electrical resistance.

7. Increased cycle life.

8. Decreased high rate performance.

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Q: If I had one variable to control regarding physical characteristics of a VRLA microglass separator, what should this be?

A: Grammage of the separator is probably the key variable to control. The grammage will control the force within the cell. The grammage of the separator will control the inserted group porosity of the battery. This can influence the amount of acid and saturation level in the plates. Changes in these conditions can lead to changes in cell performance. A reference to this can be downloaded at http://www.tms.org/pubs/kournals/JOM/0101/Nelson-0101.html. This paper, The Basic Chemistry of Gas Recombination in Lead-Acid Batteries, was authored by Dr. Robert Nelson. It discusses reasons to support the concept of why uniform grammage is critical. Dr. Nelson in his paper teaches “ Electrolyte-fill volume is critical with VRLA products, requiring an amount great enough to provide the desired discharge capacity and saturate the separator at an 80–95% level, yet be small enough so that the separator is not fully saturated and free electrolyte (in capillary contact with the separator) does not exist to any significant extent within the cell. Small differences in fill weights from cell to-cell could cause imbalances in top-of-charge Voltages, which is a shortcoming of recombinant systems. It seems that the desired operating range for recombinant cells is somewhere in between flooded and starved, yet this area is the one where apparently insignificant changes in cell materials and amounts can be translated into widely different recombination behaviors”  As can be seen from the quote from Dr. Nelson’s paper, grammage variation will be a critical attribute. Some think that thickness should be the controlling variable.  Thickness will not influence the cell as much as grammage. It must be remembered that the separator will usually be compressed from the incoming thickness by as much as 40%. Thus, the stated thickness will not be the inter-plate compressed thickness. The inter-plate thickness based on the plate thickness, number of plates and battery case dimensions is fixed. The grammage which is the variable that influences changes in porosity among the plates does not change.     

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 Question: What is the future of AGM VRLA Batteries?

(April 2004) Answer: The future looks very good for VRLA batteries. Every day more applikcaitons are moving from flooded design to a VRLA design.

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 Question: I am adding sodium sulfate to my AGM VRLA battery to prevent dendrites.    What influence may the addition of sodium sulfate to the sulfuric acid in a VRLA battery have on electrical performance?

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 (December 2003) Question: I want to decrease separator stratification. Will adding Silica to the separator help by improving the surface area of the separator.

Answer: No, this is not a correct assumption.  The below graph shows a hybrid control that had a BET surface area of 1.65 m2/g and to the same furnish a 10% silica addition was added resulting in a BET surface area of 6.9 m2/g. As can be seen no appreciable change was obtained as tested. This was a gravimetric test done under compression using water. The addition of the silica raised the separator density from a 136 to 142 (g/m2)/mm. This is a common affect of adding a particle to a fibrous web.

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 (November, 2003) Question:  Is surface area a good indicator of RBSM quality?

Answer:  Surface area can be a measure of separators quality or functionality.   This must be considered along with an understanding of the class of separator.  When a 100% microglass separator is being used, surface area can give an estimate of the average fiber diameter of the glass fibers being used.  In general ( with 100% microglass separators) the higher the surface area the finer the average glass diameter.  A finer average glass fiber diameter would mean more fibers, thus more springs or cross over points . A finer diameter usually results in higher tensile properties, better compression characteristics, and a smaller mean pore size. Therefore, in a all microglass separator a higher surface area usually means that the separator would provide better compression force against the plates: the fiber pore structure would suggest better (less) stratification.

Adding or modifying the glass can change these facts. Higher surface area can be derived by adding silica or other fine particulate. The glass fiber could be etched or salts can be added to increase the surface area. In these cases higher surface area may not mean better performance but less performance inside the battery. Hybrid separator ( glass -organic blends) are another class of separators where the surface area of the separator is not that valuable as  a key classification tool.  A hybrid composed of 15% synethetic fiber may actually have better pore structure and finer average diameter glass fiber's, than a  100% microglass separator that has a higher surface area.  In today's complex separator world,  surface area must be used only with a full understanding of how the separator is manufactured.  If unsure, scanning electronic microscopes can be used to better understand whether some additive or other ingredient is being used to influenced the surface area.

(October, 2003) Question: What is happening in Alkaline Separator testing standardization?

ANSWER: ASTM task group D.09.19.02 is set to ballot three separator draft test

methods. Interesting individuals can request a copy from Christi Sierk

([email protected])Manager - Technical Committee Operations (D9).

Contributions can be made without being a ASTM member.

The three methods are:

* Standard Test Method for Determination of Ion Exchange Capacity in Grafted

Battery Separator.

* Standard Test Method for Determination of Ammonia Trapping in Grafted

Battery Separator.

*Standard Test Method for Determining the Electrical Resistance of Alkaline

Battery Separator.

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Underlined questions are linked to a full page answers. click on the link to go to the page.

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(August, 03) Question: What is the impact of compression on AGM separator density.

ANSWER: The following table is provided to answer a portion of this question.  The critical factor is the amount of glass (separator) between the plates.  This could be obtained either by higher compression or higher density separators.  This must be done carefully; a very dense separator must be manufactured with good practices to provide the expected wet force inside the battery.

(July 03) Question: Is the Brunauer, Emmett and Teller (BET) method (1) Surface Area a Good indicator of quality for a RBSM?

Answer: In the early days when almost all the separator for VRLA batteries was 100% microglass a determination of a BET surface area gave a very nice benchmark on the fineness of the microglass used and general quality of the separator.

Properties such as the pore size of the separator was also indicated. With

the growth of hybrid separators ( less than 100% microglass) the

interpretation of what a surface area means has different relevances;

depending on what the other ingredients are, besides the microglass fiber.

As hybrids grow, the use of simple quantifiers such as BET will be diminished

in importance.

Reference:

1. Brunauer, Emmet, Teller, Journal of the American Chemical Society, Volume

60, 1938, p 309.

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 (June, 03) Q: What has happened with the ASTM (American Society of Testing Materials) Alkaline Test Method Task Group?

A: The task group ASTM D.09.18.02 met recently at the ASTM  committee week in Kansas City on March 19th. The task group discussed three draft methods. The meeting resulted in two of the methods being sent to the subcommittee (D.09.18) for initial balloting. The two methods are:

 1.) Standard test method for the determination of ammonia trapping in grafted battery separators.

2.) Standard test method for determining the electrical resistance of Alkaline battery separator using a Palico measuring system.

At the present task group level, anyone is free to input corrections and suggestions. Added information can be obtained by visiting the ASTM website, www.ASTM.org. The staff manager for committee D.09 on Electrical and Electronic Insulating Materials is Christine Sierk, whose email is  [email protected], telephone number is:610-832-9728.

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(May 2003) Q: Give some insight into the new ASTM subcommitee which will persue 42 Volt insulating systems?

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(April 2003) Question: What relationship exists between batteries and accumulators? This month question relates to more of request from the readership to provide information on the relationship between electric accumulators and batteries.

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Question: What is the standard density for RBSM (AGM) separators?

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Question? - Is testing thickness at 100 kPa of any value for AGM separators?

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(July 2002) Question:Is it true that a lot of investigative work is being done to control / influence the oxygen recombination in a VRLA battery?

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(June, 02) Question: Is there a group to set standard test methods for alkaline type separators?

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(May 02) Q: How does the AGM separator impact the hi-pot testing of batteries?

Answer: Many companies have experienced difficulties in doing hi-pot testing with a battery using AGM media. In many cases the Voltage that is used is equivalent to the Voltage required to actually test transformers, although we are only using existing batteries with 12 Volts or less. It is a an important requirement for quality control to determine whether there are dead shorts built into a new battery. In some cases, just slightly lowering the Voltage to a more reasonable level results in a test that can detect shorts but eliminate false alarms.

The glass used in the AGM media, if slightly wet, will have a very low Voltage breakdown, i.e. low hi-pot withstand. If one would wet the separator over this slightly damp condition, then excellent dielectric strength would be obtained, passing the hi-pot test requirements. This slightly damp condition represents moisture under one percent. This moisture could be introduced into the separator by excess moisture in the plates during wrapping/stacking or could be due to very high humidity conditions within the plant during the assembly operation. One-way to help eliminate this problem would be to study room conditions when the manufacturing processes has greater hi-pot rejections. If the high humidity correlates to greater hi-pot failures, some form of drying of the separator before its use could lower the number of false alarms. A solution to the problem could be accomplished by storing separator materials in a dry room (oven), or by installing some form of heating (drying the separator) just before wrapping the plates. For example, infrared lamps could be installed during the run of a separator before the plates and the separator meet.

One could investigate this phenomenon by hi-pot testing only the separator media at different levels of moisture.

Problems sometimes arises when the lugs are fluxed and the flux ends up wicking into the separator. Remember the separator is designed to be a super wick. Another possibility would be that all the shorts are in the lug area.

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(April,02) Q: Can an AGM separator influence the thermal properties of a VRLA battery?

Answer: One of the technical differences between a VRLA AGM battery and a flooded Lead-acid battery is the difference in thermal performance. Each battery type has a different amount of acid. Having less acid in the VRLA battery results in less heat transfer capacity. Therefore, in designing the separator, if one allows for more acid, then the heat capacity of the battery will increase. The fringe area of the separator and the wrapping of the separator will influence the thermal management. In an AGM battery, free acid is not available to contact the jar. Therefore, it is important that when the group is inserted into the jar that an adequate separator-to-jar contact is obtained. If the separator does not touch the end walls of the battery case, an air insulating interface results which will lower the heat transfer to the case. Thus, ‘how the separator is used’ will impact the thermal properties of a VRLA battery.

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(March,02) Question: For AGM separators, does the core size have an influence on the separator?

Answer: The core size will not have any influence on the physical properties of the separator, provided the separator is slitted upon a suitable slitter. If a manufacturer is using a very thick separator and is also specifying a three-inch core, it is possible that the separator might curl as an operator gets close to the core. In some cases this could make the separator close to the core unusable. When thick separators are used, a six-inch core should be specified to eliminate this issue.

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(December, 01) Q: Will requirements for separators change for 42 V systems as compared to the 12 Volt VRLA battery?

Answer: The basic requirements should not change although it is believed greater emphasis will be given to separator quality such as consistency of grammage, uniformity and purity. It has been reported that greater reliability will be needed from these batteries. As the battery takes on increased car functions and as a car’s electronics moves toward drive-by-wire, it is not hard to understand why the battery must be very robust. Thus, the separator’s robustness will have to be investigated, also. This may be accomplished by the use of a hybrid such as EnergyGuard (TM) or by the systems recently highlighted in a published Panasonic patent which discussed use of a tough rather porous nonwoven with a fine glass AGM. (See Patent -EP 0 821 422 B1, Sealed Lead-acid batteries, Matshushita Electric Industrial Co. Ltd., Takashi Hottori.) The value of Lead-acid batteries has allowed this technology to be in the forefront for the car manufacturers. Having a more robust separator system should also help the battery manufacturer decrease costs by allowing for fewer scrapped batteries and higher assembly speeds, even though these technologies or systems are more costly than a typical pure glass AGM.

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(February 01) Question: Will separators be made of microglass in the future?

Answer: Microglass separators will be used for some time to come into the future. The cost benefits of microglass still can not be touched by other materials. Glass fibers (made of wool) have been used in Lead- acid batteries for over 80 years.(1) Glass fibers, due to the high density of glass, offer a wettable fiber that provides for a high porosity.

A lot of research is going into making non-wettable materials wettable. Applications such as plasma treatment, sulfonation, and acrylic -acid grafting are being explored, but these new potential applications add cost to the separator. Most polymers which might fit the application have densities around one and therefore occupy 2.5 times the space that a glass fiber would occupy; this added space utilized by the polymer results in less space for the needed valuable acid capacity.

(1) This reference was found on page 57 of Automobile Battery Care and Repair by Manly, CR 1920 and 1922.

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