All about AGMs - absorbed glass mat batteries

Oct. 28, 2014
Absorbed glass mat (AGM) batteries are nearly identical in appearance and chemistry to flooded lead acid batteries. 

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.

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  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.

The problem is, it’s irreversible. With the outgassing comes a loss of electrolyte rendering the battery a useless paperweight in most cases. Without electrolyte there is no chemical reaction to produce a voltage potential difference between the positive and negative plates. No amount of recharging can solve this type of problem. My student learned a costly lesson and got an education on AGM battery charging.

So how do you know what charging voltage / amperage is acceptable? That is a tricky question. If you ask Toyota how to charge the 12 volt AGM batteries in their Prius, Highlander or Camry hybrids they will give you three different answers. Basically, it depends. Remember Ohm’s law. Voltage and amperage are proportional if resistance remains the same. The higher the voltage your charger puts out the more amperage it puts out as well. Do you know how much voltage your charger delivers when charging? How about how much amperage it delivers? If not, you need to find out.

On some of the Toyota products the recommended charging amperage is 5 amps, an extremely low rate of charge. You need to be certain of the capability of your shop’s battery charger to charge these AGM batteries. Toyota and many manufacturers have gone to a smart charger such as the Midtronics GR-8 that implements a charging strategy based on the battery’s capacity, CCA rating and state of charge. This smart charger monitors battery temperature and condition during the process as well.

The beauty of this charger is that it removes the guess work. This may seem to be overkill but after you replace a few of these toasted AGM batteries you may see the light. AGM charging and incorrect charging system strategies are the leading killer of AGM batteries. If you are uncertain of whether a battery is an FLA or an AGM it is important to consult the repair manual to make a determination of the battery type you are dealing with and its recommended charging rate. Many shops I train for are switching to smart charging for every vehicle which helps reduce costly oversights by their technicians. There are many chargers on the market that are up to the challenge. Snap-On for example, makes a charger that is very close in operation to the Midtronics GR-8.

Some manufacturers had initially had issues implementing AGM charging strategies and one that comes to mind is the Toyota Prius. It seems that every one of these vehicles I see for what was thought to be a major hybrid system problem was in fact a bad AGM 12 Volt battery. For the most part most these issues are a thing of the past. Some manufacturers such as BMW have taken charging system strategies to new levels. Have you replaced a battery in a BMW lately? Did you know a good portion of these vehicles need to have the battery registered when performing a battery replacement?

BMW uses a charging strategy that varies based on the service life of the battery. In plain terms it has a different strategy for new batteries and changes that strategy as the battery ages. This registration procedure requires the use of a scan tool and you will need to determine if your scan tool can perform this function. Autologic’s BMW tool platform is one example that is up to the task. In fact, their tool will allow you to substitute a standard FLA battery and program the computer to charge it accordingly. Companies such as Snap-On and OTC are joining the European party as well but do your homework before you commit. If you are unfamiliar with coding and programming on European vehicles you may want to consider a platform such as Autologic as they offer an industry leading customer service and tech support by BMW master technicians to help you through these types of procedures.

When it comes to AGM battery testing you will be relieved to know that you can test these batteries in the same manner as you would test a traditional FLA battery with the exception of a specific gravity test. A load test or conductance test is perfectly acceptable. Remember that any testing you choose to use is not foolproof as there
are infinite variables that contribute to a batteries state of health but your tried and true method of testing should give you a good indication of the ability of the battery to do its job. Many of the contributors at Motor Age have visited the subject of battery testing that are very much applicable to AGM technology. Be sure to have a look in the archives for some more extensive battery testing information including some great videos and articles.

To summarize AGM battery diagnosis and testing remember the following points:

• Determine if the battery you are working on is an AGM of FLA battery. Remember, they may look identical.

• Do not attempt to add water or open the case of an AGM battery

• Do use your preferred battery testing method including load testing and or conductance testing

• Be sure to use a battery charger intended for AGM batteries.

• If you don’t have access to an AGM battery charger, be sure that the charger you’re using will deliver the proper voltage/amperage required by the manufacturer.

• Before changing an AGM battery in a vehicle such as a BMW, be certain that you have the capability to register the battery with your scan tool.

With a little knowledge and confidence, testing and servicing AGM batteries is well within your capabilities. There is a wealth of information within your reach at to supplement your knowledge of battery testing and service. The more informed you are the better prepared you are to serve your customers, so read up!

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