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Ask George The Expert on Separators 02


George Zguris, Technology Director/ Sr. Scientist, Hollingsworth & Vose Company

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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.

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

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

(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?(07-02BD76-13)

Question: Is there a group to set standard test methods for alkaline type separators? (06-02BD75-15)

Q: Will requirements for separators change for 42 V systems as compared to the 12 Volt VRLA battery?(12-01 BD69-14)

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 cars electronics moves toward drive-by-wire, it is not hard to understand why the battery must be very robust. Thus, the separators 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.

Q: How does the AGM separator impact the hi-pot testing of batteries?(05-02 BD74-15)

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.

 Q: Can an AGM separator influence the thermal properties of a VRLA battery?(04-02 BD73-15)

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.

Question: For AGM separators, does the core size have an influence on the separator?(03-02 BD72-13)

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.

Question: Will separators be made of microglass in the future? (02-01,BD70-14)

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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