Microcat

Self-limiting catalyst for standby VRLA cells: prevents negative plate discharge and reduces float current in hot climates

When a Microcat® catalyst is installed into a VRLA battery cell, it changes the electrochemical reactions within the cell. This causes balance within the cell preventing the negative plate from depolarizing over time.

A healthy balance in the cell will be immediately obvious by a reduction in the cell's float current. The reduction in float current translates into: increased life, minimized water loss, maintained capacity, minimized positive plate corrosion, reduced cell heating, reduced risk of thermal runaway, and energy savings. All these benefits are enhanced in more demanding high temperature applications.

 

Features & Benefits

  • Reduction in Float Current: One of the most immediate, observable effects of installing a catalyst in a VRLA cell is a sudden drop in the float current. Typically float currents are one half or less when a catalyst is installed.

    A quick explanation of how this happens: In a VRLA cell, the negative plate does double duty compared with a flooded cell. In addition to normal negative plate functions, it also is the site where oxygen and hydrogen are recombined into water, making the cell maintenance free. When this process is too efficient, excess oxygen reaching the negative plate causes it to become depolarized. When the negative plate is depolarized, the charging system will supply more current in effort to bring the cell voltage up. The additional current becomes excessive overcharge on the positive plate, which has many damaging effects on the cell. (See How it works for a more in-depth technical explanation)

    Adding a catalyst to the cell prevents some of the oxygen reaching the negative plate and allows the negative plate to stay polarized. This means that less current needs to be supplied to the cell from the charging system, manifesting itself as lower float current, leading to further benefits:

  • Minimized water loss:Gasses are recombined into water inside the cell rather than exiting the cell. Too much gas leaving the cell can lead to premature dry -out and cell failure. Cell dryout has been the predominant cause of customer dissatisfaction with VRLA technology.

  • Increased life: There are many potential failure modes of VRLA cells. A number of these failure modes can be mitigated by the catalyst technology such as: Cell dry out, positive plate corrosion, thermal runaway, capacity loss due to negative plate depolarization.

  • Minimization of positive plate corrosion: A reduction in float current reduces the amount of overcharge on the positive plate which directly impacts the corrosion rate. The design life of a lead acid cell is based on the corrosion of the plate barring any other unforeseen failure modes.

  • Reduced cell heating: Any excess current above that needed to charge the cell is converted directly into heat. A reduction in float current means less heat produced. This can result in a cooler environment for batteries and electronics or a reduced load on HVAC systems.

  • Reduced risk of thermal runaway: Since heating is reduced and float current minimized there is less risk of thermal runaway.

  • Energy savings: Reduced float current directly translates into less power purchased.

  • Maintenance of cell capacity: Many VRLA cells in service are failing capacity tests because their negative plates are depolarized. In fact significant capacity increases have been seen on some cells just by installing a catalyst.


Anatomy of a Microcat© catalyst

Hydrophobic Membrane: Microporous barrier allows cell gasses to enter catalytic chamber and water vapor to return to the cell. This acts as a barrier to keep acid spray outside the microcat. This also regulates the rate of gas diffusion so that the temperature of the microcat never exceeds 200°F (93°C).

Poison Filter: Guard layer of a dual acting filter material protects the active material from poisonous gasses found inside VRLA cells. Our tests led us to discover that there is a reaction between the negative plate and sulfuric acid that produces hydrogen sulfide (H2S); this reaction is unknown in the battery industry. This was a concern because hydrogen sulfide is a known poison to precious metal catalysts.

The purpose of the filter is to prevent poisons from reaching the active material. The material we have chosen is a dual acting filter that is an advance over older technologies. It has been common practice to use activated carbon filtering to protect catalysts used on batteries from the common poisons of Stibine and Arsine; both termed electron receiver type poisons. However, activated carbon will not filter Hydrogen Sulfide, which is an electron donor type poison. Our patent pending design utilizes a Hydrogen Sulfide filtering material deposited on an activated carbon substrate in order to filter out both families of poisons

Body: Engineered high temperature plastic outer housing. Can withstand temperatures up to 500°F (260°C). Chemically resistant to sulfuric acid.

Active Material: Precious metal catalyst dispersed on a granular substrate, which recombines Hydrogen and Oxygen into water vapor.

Base: Can be custom molded with a variety of different attachment methods for easy mounting on customer vent cap.

 

How does it work?

The VRLA cell was designed to correct all the problems of flooded technology. All the gas produced inside the cell was intended to recombine back into water on the negative plate in a very efficient oxygen cycle. In an ideal world there would be no negative plate self discharge, no positive plate corrosion and no excess charge current needed. Batteries would last forever and no gas would be released from the cell.

In the real world, chemistry dictates that negative plates do self-discharge and they do this more when impurities are present in higher quantities. In our experience the typical high quality, long life (20 year) VRLA cell has a self discharge rate equivalent to 80 ml of Hydrogen gas per day per 100 Ah. Oxygen, produced from a variety of processes on the positive plate, will recombine with this hydrogen on the negative plate and cause it to depolarize.

In the real world positive grids also corrode. When a positive grid corrodes at a relatively high rate, it absorbs the oxygen produced as the lead grid turns into lead dioxide; leaving no oxygen to depolarize the negative plate. In this case, the negative plate stays polarized and all the hydrogen will vent. Unfortunately, a positive grid that corrodes at the required rate will last much less than 20 years. Designers have done what is typically done on flooded designs for long life and reduced the corrosion rate of the positive grid. Typical state of the art designs will only absorb 10 ml of oxygen on the positive plate instead of the 40 ml needed to counter act the hydrogen generated on the negative. This is the paradox of VRLA design. A "better" positive grid can actually impair the life of the design.

This leaves an unbalanced situation with a strongly depolarized negative plate. The charging system will compensate with more current which will lead to excessively high polarization on the positive plate and damaging effects on the cell due to the excess current. Electrolysis will generate high amounts of gas leading to water loss.

The dilemma for the battery designer is in achieving perfect balance in the cell. If the positive plate corrosion rate doesn't correspond exactly to the self discharge rate of the negative, then negative plate depolarization becomes an issue. To achieve balance, one can either make an extremely pure negative or have a relatively high corrosion rate on the positive. The purity required may be prohibitively expensive, and a high corrosion rate precludes a long life design.

Adding a Microcat® catalyst to the cell gives the battery designer a new tool to break out of the deadlock. The catalyst will absorb free oxygen in the head space and recombine it with the abundant hydrogen always present in the cell. This drastically reduces the amount of VRLA gas venting from the cell, but most importantly this prevents oxygen from reaching the negative plate and buffers the negative plate self discharge reaction from the positive plate corrosion reaction. Now that the cell is in balance the negative remains charged. The charging system responds by only sending the small amount of current needed to keep the cell charged.

The catalyst equipped cell has a healthy set of polarizations while the non catalyst cell does not. The reward for this balance is that the float current drops dramatically, usually by half or more.

Note: For a more in-depth technical description of the reactions see the Telescon '97 See Paper

 

Background

VRLA technology was launched as an improvement over standard flooded technology which had been extensively proven in long term service. As with any new technology there were inevitable problems as the technology evolved. A brief history of the development milestones will aid in understanding why many current VRLA designs can be improved with a catalyst.

1982: Stationary VRLA born
The large Valve Regulated Lead Acid (VRLA) battery was launched by GNB (now part Exide Technologies) in 1985. Telecom customers immediately liked the new design because it was maintenance-free, safer, and compact. Over the next decade, all the major manufacturers in the US, Europe and Asia were making and selling VRLA batteries into the Stationary/Standby market.

1995: Fundamental problem comes to light.
At Intelec, Dr. David Feder presented a controversial paper (see paper) on the results of a study of 24,000 cells that ranged from one to nine years old. The cells were produced by nine different manufacturers from around the world and were in service in benign temperature controlled environments. It was found that 68% of these cells failed to meet their capacity requirements. More alarmingly, cells that were three years old had a failure rate of 35%. Also this year, Philadelphia Scientific presented a paper that established a water loss standard for VRLA cells to meet in order to achieve 20 years of life.

1995-1996: Field complaints rising
By the mid 90's, there were an increasing number of complaints from users regarding the unreliability of VRLA batteries. Defects reported included high float currents, positive grid corrosion, negative strap corrosion, capacity loss, thermal runaway and dry out. Though not understood at the time, all these disparate defects were actually closely related.

At Intellec '96 a paper (see paper) was presented that continued the tests from the 1995 paper and for the first time, identified that the central lingering problem with VRLA technology was negative plate depolarization. It also announced the beneficial effect of a catalyst on negative plate polarization.

1997: Understanding the problem
At the Telescon conference in Budapest in 1997, the entire problem was brought to light, defined and experimentally demonstrated (see paper). A serious problem lay concealed in the electrochemistry of the VRLA design. Many of these batteries were predisposed to an unexpected failure mode of negative plate self-discharge. The same oxygen cycle that provided the maintenance-free benefit was causing self-discharge and loss of capacity.

Most of the battery industry was unaware of the problem at this time. It was presented by comparison to the well-known flooded cell design where virtually all of the float current charges the negative plates so that they always stay fully charged. In VRLA cells, almost none of the current goes to charging the negative plates. Therefore, the negative plates self-discharge slowly on float, even when the exact same pure materials are used.

At the conference, three solutions to the problem were also proposed:

  1. Improve the purity of the negative plate and otherwise minimize its self-discharge rate. This requires the use of extremely pure lead, which may be cost prohibitive and may run counter to the imperative to recycle.

  2. Increase the corrosion rate of the positive. This was an acceptable approach on short life batteries but obviously not on long-life ones.

  3. Use a small internal catalyst to remove excess oxygen and permit the negative plate to recharge naturally.


1998: VRLA catalyst product launch
In 1998 Philadelphia Scientific began the manufacture of catalyst devices for VRLA batteries. One large manufacturer of VRLA batteries incorporated catalysts into one of their product lines.

At the 1998 INTELEC conference Philadelphia Scientific published the results of a definitive test showing dramatically how premium VRLA cells suffered almost 50% loss of capacity in a period of less than 2 years. The cells with catalysts installed maintained 100% capacity and had much healthier negative polarizations. (See paper)

2000: Next generation catalyst designs. The second generation catalyst design called the Microcat© was launched (Anatomy of a Microcat). This significant advance in the catalyst technology added poison filtering and temperature limiting features along with a more robust construction.

At INTELEC 2000, Philadelphia Scientific presented a paper that defined the standard of purity required for a high quality VRLA cell. (see paper) Since normal spectrographic tests were not adequate to measure such low levels of impurities, the standard was based on a test method developed by Chloride in the 1980's. It gave manufacturers a simple, reliable and low cost method of measuring the purity of their final products.

Summary
Flooded lead-acid batteries have been with us for over 100 years. VRLA technology was born in the early 1980's and was not well understood. The new technology was prone to unique failure modes, which had to be diagnosed and corrected. (It is especially difficult to prove a 20 year design when the oldest cells are only now just approaching their twentieth birthday.) Many improvements have been made to VRLA technology to get better jar to cover seals, improved post seals, an end to strap corrosion, improved cell compression, etc. When these more immediate failure modes were addressed it then became possible to discover a major root cause of shorter life - negative plate self discharge. As noted, this was and is a failure mode specific to VRLA battery designs due to the gas management issues inside the cell. The catalyst was shown to be one of the three fundamental solutions and for many applications it is the most practical.