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All about AGMs - absorbed glass mat batteries

The proper care and servicing of Absorbed Glass Mat batteries.
Tuesday, October 28, 2014 - 07:00
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Looks can be deceiving. Just because something looks like a technology we are used to seeing doesn’t necessarily mean that we can test it and service it the same way we always have. Case in point: AGM batteries. AGM batteries, a battery technology in the valve regulated lead acid (VRLA) family, often look identical to their flooded lead acid (FLA) counterparts. Battery manufacturer’s such as Johnson Controls, are revving up their production of AGM batteries to suit the needs of today’s high tech vehicles and are producing millions of AGM batteries annually. This is a sure sign that you will be seeing these batteries in more applications in the near future. In this article we look at what makes these batteries different from traditional lead acid batteries and most importantly, how to test and service them.

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Absorbed glass mat (AGM) batteries are nearly identical in appearance and chemistry to flooded lead acid batteries. Flooded lead acid batteries are the batteries we all grew up with and are familiar with servicing. If you need to brush up on your battery chemistry knowledge, take a look at my blog in the Motor Age Community Workshop at searchautoparts.com.  The internal difference between AGM batteries and their FLA counterparts is a fibrous glass mat material separator that acts as a sponge for the electrolyte around the plates which provides electrolyte coverage to a greater surface area of the plates. This “absorption” of electrolyte allows for more efficient use of the plate surface area.

The result of this efficiency is a smaller, lighter, more effective battery. In most AGM designs the mat is wrapped around the positive plate which protects the plate from vibration and extended cycling. For this reason alone, battery manufacturers claim that AGM batteries have double the overall cycle life of the traditional FLA battery. The AGM cells are housed in a sealed, maintenance free, spill proof housing with a one way pressure relief valve. AGM battery manufacturers use what is known as “recombinant” technology that takes the oxygen produced on the positive plate and combines it with hydrogen to produce water (H2O). For this reason AGM batteries are self-watering and do not typically vent Hydrogen making them a great candidate for placement inside the vehicle or wherever the engineer may want to place it for weight distribution purposes. Note: The battery cannot be placed in a sealed compartment in the event that the battery outgasses hydrogen through the one way relief valve due to overcharging.

Let’s summarize the advantages of AGM Batteries:

• The AGM battery can be made smaller and lighter than a flooded lead acid battery while maintaining or even increasing the battery capacity and cold cranking amperage comparatively

• The AGM battery is spill proof allowing for mounting in a variety of locations and positions

• AGM batteries are shock and vibration resistant

• AGM batteries boast a  service life that is nearly double that of traditional FLA batteries

• AGM batteries are maintenance free

With every new technology there is a downside. I like to think of this as more of a learning curve. Once you know the ins and outs of this technology you can proceed with confidence. Let me share just one example. Several years ago a student approached me after class to ask me about a problem that he had encountered with his Optima AGM battery. He had purchased the battery for his off road vehicle that was driven only on occasion. The vehicle was used so infrequently that each time he went to start it, it needed to be jumped (I would imagine he also had several parasitic draws on this vehicle with his addition of numerous aftermarket accessories). On one occasion he decided to charge the battery before attempting to start the vehicle and his problem got worse. He said “How is it that this ridiculously expensive battery is such a piece of junk!” 

His problem might be one that you have encountered in your shop in that AGM batteries require a very specific charging strategy. Because AGM batteries use the process of recombination, the charging rate has to be limited. Using just any old battery charger will typically result in a phenomenon known as thermal runaway. What happens here is the excessive voltage / amperage charging rate increases the temperature of the plates and results in a production of excessive hydrogen that cannot be recombined. The result of thermal runaway is an outgassing of hydrogen. Remember the one way pressure relief valve? It is installed specifically for this situation.

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