VRLAs, AGMs and gels Before discussing AGM technology, it’s important to define exactly what it is and is not. AGM batteries use what’s known as valve-regulated lead-acid (VRLA) technology. The VRLA title encompasses all batteries that are “sealed” and operate on the gas recombinant principal. Instead of venting hydrogen and oxygen like conventional flooded electrolyte batteries, VRLA batteries recombine these elements during the recharging process, releasing virtually no byproducts into the atmosphere. This makes for an inherently safer battery; it’s spillproof and leakproof, virtually eliminating the possibility of a hydrogen gas explosion. Under normal conditions, VRLA batteries operate with a slightly positive case pressure to facilitate the recombination process. This pressure is regulated by a valve mechanism, which resides in place of a conventional battery’s cell access cap, hence the designation “valve regulated.” The electrolyte within VRLA batteries is either liquid suspended in fiberglass matting for AGMs or a jelly-like material created using fumed silica for gel batteries. All AGMs operate on the VRLA principal, as do gel batteries. Gels are also spillproof and unlikely to emit hydrogen gas. There are a few other similarities; however, this discussion will center primarily on AGM rather than gel technology. Flooded, AGM and gel are all considered lead-acid battery types. A letter from the front
As the manager of an active repair, refit and custom boatbuilding yard, I’ve seen and worked with AGM batteries basically since their introduction in the early 1990s. My experience with these batteries has been generally positive; however, a few months ago I received a letter from a reader whose experience with his AGM batteries was disappointing at best. This sailor, we’ll call him Cruiser X, described how their capacity was woefully inadequate and considerably less than their rated values. He was convinced they were inferior. Upon testing, Cruiser X’s 360 amp-hour bank showed a capacity of just 230 amp-hours after less than a year of use. Furthermore, Cruiser X went on to describe his interaction with the manufacturer. They advised him that his “batteries were sulfated due to insufficient charging.” Cruiser X expressed some surprise, “Apparently these batteries must be recharged to 100 percent of capacity after each discharge” to prevent this from occurring.
AGM batteries 101
AGM batteries have been around for nearly three decades, and they have hundreds of thousands of hours of operational use under their collective positive and negative posts. The telecom industry embraced this technology in the 1970s for powering remote switching and relay sites, and it continues to make extensive use of AGM batteries for this purpose. AGMs are now used extensively in military and civil aviation, electromotive applications, aids to navigation (marine and air), terrestrial-based alternative-energy generation (solar and wind), and uninterrupted power supplies. If you have a UPS unit for your computer, chances are it’s powered by an AGM battery. While the technology may be relatively new to recreational cruisers, its strengths and weaknesses are well understood, thanks to this extensive network of varied uses for AGM technology.
Although they exhibit different operating characteristics than conventional flooded batteries, AGMs have much in common with their ancestors. Conventional batteries operate using positive and negative plates made from lead dioxide and sponge lead respectively, immersed in an electrolyte — sulfuric acid in the case of nearly all flooded and AGM batteries (see Battery know-how Issue 135, Jan./Feb. 2004 for more on different battery chemistries and how they work).
Instead of using electrolyte sloshing around the case as a free liquid, in AGM batteries the electrolyte is “absorbed” and held within a fine mesh of fiberglass cloth or mat in what’s referred to as a “starved” state. This starved state means the cloth is nearly — between 90 and 95 percent — but not fully, saturated; a small portion remains gas- rather than liquid-filled. This gas void allows for the transfer of oxygen from the positive to negative plate in its molecular form during charging, which suppresses the production of hydrogen. Water is produced and remains within the battery. The entire recombinant process occurs within the battery’s sealed chamber, under slight pressure. Thus, AGM batteries never require the addition of water, making them maintenance free.
AGM drawbacks
As the axiom goes, however, there is no such thing as a free lunch. AGM batteries do have shortcomings. Chief among these is their price. Expect to pay two to three times as much for AGMs as for conventional flooded batteries. This difference is less when compared with exceptionally high quality, proprietary marine flooded batteries.
Perhaps the greatest shortcoming of AGM batteries — although less so now that it is well understood by most boat builders and professionals in the marine industry — is their sensitivity to chronic under- and overcharging, particularly in the float mode. The requirements vary from manufacturer to manufacturer. For example, Concorde Battery Corp., manufacturer of a complete line of AGM batteries, calls for 14.1 to 14.4 volts for bulk charging and 13.1 to 13.3 volts for float charging, at 77º F (25º C). East Penn Manufacturing, another maker of AGM batteries specifies a maximum charge voltage of 14.6 volts and a float of between 13.4 and 13.7 volts, again at 77º F. Thus, not only are the charge voltages different from conventional flooded batteries, they vary from manufacturer to manufacturer. Conventional flooded batteries call for a bulk-charge voltage of about 14.2 and float of 13.2, once again at 77º F. Not a huge difference, but an important one, particularly where multistage high-current chargers are used. It’s important to note that while AGMs will suffer from chronic under- or overcharging, so too will all lead-acid batteries.
The importance of using a fully regulated multistage temperature-compensated charger for AGM batteries cannot be overemphasized. Ferro-resonant (old-fashioned and unsophisticated) shore-powered chargers or internally regulated automotive alternator regulators (found on most stock marine engines) are inadequate for charging these and most other deep-cycle batteries. These basic, single-step chargers offer a constant voltage rather than what’s needed — a voltage-regulating or voltage-limiting charge profile.
Temperature compensation important
Where AGMs are concerned, temperature compensation factor is especially important. It ensures a full charge at the highest rate possible, which equates to the shortest charge time, while preventing damaging overcharging. Even if the battery isn’t driven to the venting stage, constant, chronic overcharging caused by a lack or failure of temperature compensation in the charge cycle will lead to galvanic corrosion of the positive grid and shedding of the active plate material, which is essentially the muscle within every battery. The structure that supports the lead in the positive plates will simply corrode and crumble like the zinc anodes on your propeller shaft.
In the case of AGMs using envelope-type separators, this material will be retained within the envelope; however, its effectiveness is considerably diminished. The full enclosure of the positive plate within one of these electrolyte-permeable envelopes is a decided advantage for batteries of any chemistry. This shed material, if allowed to accumulate at the bottom of a battery cell, will eventually lead to shorting between the plates, a condition that results in the battery’s ultimate demise.
Temperature can be the nemesis of batteries. During charging cycles, higher ambient and battery case temperatures call for lower charge voltages. The charge figures quoted above, given at 77º F, must be decreased by approximately half a volt when the temperature rises to 100º F and increased by 0.8 volts when temperature falls to 40º F. Charging AGM batteries using anything other than temperature-compensated charge sources will almost guarantee early battery failure.
The other component temperature plays in battery operation affects the life span of the battery, AGM or otherwise. Any increase in the optimum 77° F operating temperature will decrease the battery’s life. For instance, an increase from 77° to 95° F will cut a battery’s life span in half. Raise the temperature to 113° F and you’ve halved it again. This is a function of the chemistry taking place within the battery — heat it up and it accelerates the process; cool it off and things slow down. This is also why your boat’s and automobile’s engines are more difficult to start when the weather is extremely cold; batteries deliver less power when the mercury falls. Conversely, warm batteries, while they do not last as long, deliver more power.
This is why batteries should be placed in the coolest environment possible (many AGM batteries used in telecommunication and solar arrays are stored below ground level to take advantage of cooler temperatures). In essence, higher temperatures kill batteries. Too high and they die a premature death and when the charging regimen isn’t linked to temperature, batteries tend to be chronically under- or overcharged.
Undercharging and sulfation
Chronic undercharging of AGM batteries (and all lead-acid batteries), a common scenario where automotive alternators are used, leads to plate sulfation. The production of lead sulfate is a normal component of battery use; it’s formed on plates as the battery is discharged. Initially, the sulfate is relatively soft and thus is easily converted back into active plate material during charging. If, however, an AGM or flooded battery remains in a discharged state for any length of time, the sulfate crystallizes, immobilizing active material in the process. Once crystallized, sulfate is difficult or impossible to remove, permanently diminishing the battery’s capacity.
Repeatedly operating or cycling a battery in an incompletely discharged state can also cause sulfation. This is, perhaps, the most common failure mode of AGM and other lead-acid batteries. For vessels that rely solely on the propulsion engine and alternator for charging, recharging a discharged AGM battery from 50 percent state to 85 or 90 percent state of charge is relatively quick and easy. Because AGM batteries are capable of accepting especially high charge currents, upwards of 100 percent of their amp-hour capacity, power can be replaced very quickly, until the 85 to 90 percent threshold is reached. Then the process slows down. And this roadblock exists for AGM as well as other lead-acid battery types.
Cruiser X expressed surprise when he came to the realization that AGM batteries “must be recharged to 100 percent capacity after each discharge, to avoid sulfation.” In fact, in order to maximize performance and longevity this is true of every lead-acid deep-cycle battery. According to the Lifeline/Concorde owner’s manual/warranty, “For maximum battery life, a battery must be recharged to 100 percent capacity. Recharging less than 100 percent may result in premature battery failure. Lifeline batteries are not covered under warranty if they are not recharged properly.” Similar language is used in literature from other manufacturers of AGM batteries. East Penn Manufacturing’s VRLA technical manual states, “Any battery will be damaged by continual under or overcharging.” Ed Mahoney, director of sales and marketing and an electrical engineer at Concorde Battery Corp., said, “Sulfation is the cause of most battery failures.” Trevor Bittner of East Penn Manufacturing agreed, “Chronic undercharging will shorten battery life considerably.” Xantrex, manufacturer of sophisticated charging equipment specifically profiled for AGM and other battery chemistries, takes a similar stand. Brian Ulrich, mobile market product manager, weighs in on this subject with a clear recommendation to users of any lead-acid battery, “Not charging up to 100 percent is indeed not good for lead-acid batteries.”
It’s worth noting that the difference between a fully charged and dead (dead is defined in the industry as a battery whose terminal voltage is 11.8, which is 100 percent discharged) battery is only a few tenths of a volt. Because AGM batteries are sealed and thus incapable of undergoing a specific-gravity test, this becomes even more important for determining their state of charge (SOC). A fully charged AGM will read 12.8 volts or higher under no load or charge, resting for at least eight hours, while a conventional flooded battery’s full SOC voltage is 12.6 volts. An unloaded, rested, 90 percent charged AGM would register approximately 12.7 volts. The same battery will register 12.35 volts at a 50 percent state of charge.
It is easy to see, therefore, that a scant few tenths of a volt separate a fully and partially charged AGM or flooded battery. That meager difference, between 85 or 90 percent and 100 percent, can have a dramatic difference on the overall life of the battery. This also is the case for float voltages; just a few tenths of a volt can mean the difference between a well maintained battery and positive grid corrosion or sulfation. This is true, however, for all batteries — AGMs as well as other lead-acid varieties. There is little evidence to prove that conventional flooded batteries endure chronic undercharging any better than AGMs. Sulfation occurs in either chemistry. Flooded nonsealed batteries may better tolerate overcharging because water loss can be mitigated with replacement. However, grid corrosion and shedding of plate material will occur in both battery types. AGM, flooded and gel cells will all suffer from chronic under- or overcharging.
Building AGM battery banks from multiple cells is an acceptable and safe practice. However, using batteries of differing case designs, chemistries or manufacture dates is not recommended. The internal resistance of batteries may differ with capacity, and older batteries may exhibit higher resistance than newer batteries. This imbalance could lead to premature failure. Different battery chemistries have markedly different charge profiles and as a result, they must never be interconnected.
Equalization controversial
There is a measure of controversy concerning the practice of equalization of AGM batteries, a process that mitigates the effects of acid stratification. At one time, it was considered unnecessary and even forbidden by the manufacturer. Times have changed since these batteries were introduced, and Concorde/Lifeline recommends the practice when “the battery is showing symptoms of capacity loss.” East Penn Manufacturing, on the other hand, indicates that equalization is unnecessary in their AGMs because “stratification cannot occur in batteries of [this] size.”
Where permitted, carefully follow the recommendations of the manufacturer concerning voltage requirements and duration of the equalization charge. In my experience, AGMs appear to be particularly susceptible to sulfation as a result of being left in a discharged state for an extended period (more than 30 days). Equalization under these circumstances, if permitted by the manufacturer, usually restores the battery to full or nearly full capacity. If the battery is suffering from chronic overcharging and the positive plates are corroded as a result, equalization will have no effect and may actually exacerbate the problem.
Regardless of the fact that AGMs do not gas under normal circumstances, they still require ventilation. Never install AGM or any other lead-acid batteries, regardless of design, in a sealed or poorly ventilated compartment. AGM batteries, like all other batteries, should be well secured in a dedicated battery locker or box where they cannot be damaged or short circuited by shifting gear.
Gel cells fall somewhere in between these two philosophies. As they do not charge as quickly as AGMs, they are more expensive than flooded batteries but cost about the same as AGMs. They also typically offer more cycles than AGMs but fewer than flooded batteries, and they cannot be equalized. Both AGMs and gels offer superior storage or shelf life because their self-discharge rates are exceptionally low, roughly 3 percent per month. Even gels, however, will be permanently damaged by chronic under- or overcharging. Under some circumstances, gel batteries are subject to explosion as a result of overcharging, resulting in the issuance of a U.S. Coast Guard consumer advisory in August 1996.
The advisory indicates that gel batteries may be subject to explosion if they “are recharged with battery chargers which are not automatic temperature-sensing voltage regulated chargers.” I suspect the likelihood of explosion is rare, particularly with the increased popularity of sophisticated chargers; however, according to the advisory, it has happened and thus is a factor worth considering. While overcharging may damage AGM batteries, they are particularly safe and not subject to the above advisory.
Know the facts, and read all the available literature from the manufacturer, including the warranty, before investing in AGM batteries or batteries of any chemistry, regardless of the manufacturer. Had Cruiser X done this, he may not be complaining bitterly about his experiences with AGM batteries. In my opinion, AGM batteries perform as advertised, provided they are used and maintained in accordance with their manufacturer’s recommendations. Just like anchors, there’s no one battery type that will be all things to all users.
A letter from the front
As the manager of an active repair, refit and custom boatbuilding yard, I've seen and worked with AGM batteries basically since their introduction in the early 1990s. My experience with these batteries has been generally positive; however, a few months ago I received a letter from a reader whose experience with his AGM batteries was disappointing at best. This sailor, we'll call him Cruiser X, described how their capacity was woefully inadequate and considerably less than their rated values. He was convinced they were inferior. Upon testing, Cruiser X's 360 amp-hour bank showed a capacity of just 230 amp-hours after less than a year of use. Furthermore, Cruiser X went on to describe his interaction with the manufacturer. They advised him that his "batteries were sulfated due to insufficient charging." Cruiser X expressed some surprise, "Apparently these batteries must be recharged to 100 percent of capacity after each discharge" to prevent this from occurring.
What went wrong for Cruiser X? Is he or his vessel's charging system at fault, or as he suspects, are these batteries inferior? In his closing remarks he indicated that his "next battery purchase will be gel cells." Is this the solution for Cruiser X, or will he face similar battery problems in the future? Although I've not been deluged with letters like Cruiser X's, I have received others. Clearly, a problem exists.
AGM batteries 101
 |
AGM batteries have been around for nearly three decades, and they have hundreds of thousands of hours of operational use under their collective positive and negative posts. The telecom industry embraced this technology in the 1970s for powering remote switching and relay sites, and it continues to make extensive use of AGM batteries for this purpose. AGMs are now used extensively in military and civil aviation, electromotive applications, aids to navigation (marine and air), terrestrial-based alternative-energy generation (solar and wind), and uninterrupted power supplies. If you have a UPS unit for your computer, chances are it's powered by an AGM battery. While the technology may be relatively new to recreational cruisers, its strengths and weaknesses are well understood, thanks to this extensive network of varied uses for AGM technology.
Although they exhibit different operating characteristics than conventional flooded batteries, AGMs have much in common with their ancestors. Conventional batteries operate using positive and negative plates made from lead dioxide and sponge lead respectively, immersed in an electrolyte &mdash sulfuric acid in the case of nearly all flooded and AGM batteries (see Battery know-how Issue 135, Jan./Feb. 2004 for more on different battery chemistries and how they work).
Instead of using electrolyte sloshing around the case as a free liquid, in AGM batteries the electrolyte is "absorbed" and held within a fine mesh of fiberglass cloth or mat in what's referred to as a "starved" state. This starved state means the cloth is nearly &mdash between 90 and 95 percent &mdash but not fully, saturated; a small portion remains gas- rather than liquid-filled. This gas void allows for the transfer of oxygen from the positive to negative plate in its molecular form during charging, which suppresses the production of hydrogen. Water is produced and remains within the battery. The entire recombinant process occurs within the battery's sealed chamber, under slight pressure. Thus, AGM batteries never require the addition of water, making them maintenance free.
Additionally, the glass mat material is compressed as it's packed into each battery cell, making for an exceptionally sturdy, vibration-resistant design (the U.S. Navy thinks so; they use AGM batteries in combat aircraft). This high-density packing and other unique cell features give the AGM battery particularly low internal resistance, resulting in what is one of the AGM's greatest attributes &mdash an exceptionally fast recharge time. They will accept the highest charge rate of any of the three popular marine battery chemistries. Additionally, this low resistance means AGM batteries maintain a higher terminal voltage under heavy loads such as those imparted by inverters, windlasses, starters and bow thrusters. The low resistance also means AGM batteries tend to stay cooler during heavy charging and discharging applications.
AGM drawbacks
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As the axiom goes, however, there is no such thing as a free lunch. AGM batteries do have shortcomings. Chief among these is their price. Expect to pay two to three times as much for AGMs as for conventional flooded batteries. This difference is less when compared with exceptionally high quality, proprietary marine flooded batteries.
AGM batteries tend to weigh more than their conventional flooded brethren, amp-hour for amp-hour. On a voyaging vessel, this weight is negligible and most folks equate battery weight with quality. While this is generally true, more weight often means thicker and, therefore, longer-lasting plate structures. Nevertheless, weight is always a factor, particularly on lighter-displacement vessels.
Perhaps the greatest shortcoming of AGM batteries &mdash although less so now that it is well understood by most boat builders and professionals in the marine industry &mdash is their sensitivity to chronic under- and overcharging, particularly in the float mode. The requirements vary from manufacturer to manufacturer. For example, Concorde Battery Corp., manufacturer of a complete line of AGM batteries, calls for 14.1 to 14.4 volts for bulk charging and 13.1 to 13.3 volts for float charging, at 77� F (25� C). East Penn Manufacturing, another maker of AGM batteries specifies a maximum charge voltage of 14.6 volts and a float of between 13.4 and 13.7 volts, again at 77� F. Thus, not only are the charge voltages different from conventional flooded batteries, they vary from manufacturer to manufacturer. Conventional flooded batteries call for a bulk-charge voltage of about 14.2 and float of 13.2, once again at 77� F. Not a huge difference, but an important one, particularly where multistage high-current chargers are used. It's important to note that while AGMs will suffer from chronic under- or overcharging, so too will all lead-acid batteries.
The importance of using a fully regulated multistage temperature-compensated charger for AGM batteries cannot be overemphasized. Ferro-resonant (old-fashioned and unsophisticated) shore-powered chargers or internally regulated automotive alternator regulators (found on most stock marine engines) are inadequate for charging these and most other deep-cycle batteries. These basic, single-step chargers offer a constant voltage rather than what's needed &mdash a voltage-regulating or voltage-limiting charge profile.
Because of the recombinant technology used in AGMs, they are entirely intolerant of venting. That is, if seriously overcharged, their regulating valves will open to prevent over-pressurization of the case, resulting in a permanent loss of hydrogen and oxygen or water. If allowed to continue, this process, tantamount to never adding water to a conventional flooded cell, will eventually destroy the battery.
Temperature compensation important
Where AGMs are concerned, temperature compensation factor is especially important. It ensures a full charge at the highest rate possible, which equates to the shortest charge time, while preventing damaging overcharging. Even if the battery isn't driven to the venting stage, constant, chronic overcharging caused by a lack or failure of temperature compensation in the charge cycle will lead to galvanic corrosion of the positive grid and shedding of the active plate material, which is essentially the muscle within every battery. The structure that supports the lead in the positive plates will simply corrode and crumble like the zinc anodes on your propeller shaft.
In the case of AGMs using envelope-type separators, this material will be retained within the envelope; however, its effectiveness is considerably diminished. The full enclosure of the positive plate within one of these electrolyte-permeable envelopes is a decided advantage for batteries of any chemistry. This shed material, if allowed to accumulate at the bottom of a battery cell, will eventually lead to shorting between the plates, a condition that results in the battery's ultimate demise.
Temperature can be the nemesis of batteries. During charging cycles, higher ambient and battery case temperatures call for lower charge voltages. The charge figures quoted above, given at 77� F, must be decreased by approximately half a volt when the temperature rises to 100� F and increased by 0.8 volts when temperature falls to 40� F. Charging AGM batteries using anything other than temperature-compensated charge sources will almost guarantee early battery failure.
The other component temperature plays in battery operation affects the life span of the battery, AGM or otherwise. Any increase in the optimum 77� F operating temperature will decrease the battery's life. For instance, an increase from 77� to 95� F will cut a battery's life span in half. Raise the temperature to 113� F and you've halved it again. This is a function of the chemistry taking place within the battery &mdash heat it up and it accelerates the process; cool it off and things slow down. This is also why your boat's and automobile's engines are more difficult to start when the weather is extremely cold; batteries deliver less power when the mercury falls. Conversely, warm batteries, while they do not last as long, deliver more power.
This is why batteries should be placed in the coolest environment possible (many AGM batteries used in telecommunication and solar arrays are stored below ground level to take advantage of cooler temperatures). In essence, higher temperatures kill batteries. Too high and they die a premature death and when the charging regimen isn't linked to temperature, batteries tend to be chronically under- or overcharged.
Undercharging and sulfation
Chronic undercharging of AGM batteries (and all lead-acid batteries), a common scenario where automotive alternators are used, leads to plate sulfation. The production of lead sulfate is a normal component of battery use; it's formed on plates as the battery is discharged. Initially, the sulfate is relatively soft and thus is easily converted back into active plate material during charging. If, however, an AGM or flooded battery remains in a discharged state for any length of time, the sulfate crystallizes, immobilizing active material in the process. Once crystallized, sulfate is difficult or impossible to remove, permanently diminishing the battery's capacity.
Repeatedly operating or cycling a battery in an incompletely discharged state can also cause sulfation. This is, perhaps, the most common failure mode of AGM and other lead-acid batteries. For vessels that rely solely on the propulsion engine and alternator for charging, recharging a discharged AGM battery from 50 percent state to 85 or 90 percent state of charge is relatively quick and easy. Because AGM batteries are capable of accepting especially high charge currents, upwards of 100 percent of their amp-hour capacity, power can be replaced very quickly, until the 85 to 90 percent threshold is reached. Then the process slows down. And this roadblock exists for AGM as well as other lead-acid battery types.
At this point, the battery's acceptance rate falls dramatically, making the final amp-hour replacement a slow process at best. Recharging from 90 to 100 percent of charge could occupy 40 percent of the overall battery charge time for a completely discharged battery. Additionally, it is typical, because of charge inefficiencies, to replace more charge than was used. Recharging to 100 percent often requires replacement of 105 to 115 percent of amp-hours removed for AGMs and more for flooded cells.
Cruiser X expressed surprise when he came to the realization that AGM batteries "must be recharged to 100 percent capacity after each discharge, to avoid sulfation." In fact, in order to maximize performance and longevity this is true of every lead-acid deep-cycle battery. According to the Lifeline/Concorde owner's manual/warranty, "For maximum battery life, a battery must be recharged to 100 percent capacity. Recharging less than 100 percent may result in premature battery failure. Lifeline batteries are not covered under warranty if they are not recharged properly." Similar language is used in literature from other manufacturers of AGM batteries. East Penn Manufacturing's VRLA technical manual states, "Any battery will be damaged by continual under or overcharging." Ed Mahoney, director of sales and marketing and an electrical engineer at Concorde Battery Corp., said, "Sulfation is the cause of most battery failures." Trevor Bittner of East Penn Manufacturing agreed, "Chronic undercharging will shorten battery life considerably." Xantrex, manufacturer of sophisticated charging equipment specifically profiled for AGM and other battery chemistries, takes a similar stand. Brian Ulrich, mobile market product manager, weighs in on this subject with a clear recommendation to users of any lead-acid battery, "Not charging up to 100 percent is indeed not good for lead-acid batteries."
It's worth noting that the difference between a fully charged and dead (dead is defined in the industry as a battery whose terminal voltage is 11.8, which is 100 percent discharged) battery is only a few tenths of a volt. Because AGM batteries are sealed and thus incapable of undergoing a specific-gravity test, this becomes even more important for determining their state of charge (SOC). A fully charged AGM will read 12.8 volts or higher under no load or charge, resting for at least eight hours, while a conventional flooded battery's full SOC voltage is 12.6 volts. An unloaded, rested, 90 percent charged AGM would register approximately 12.7 volts. The same battery will register 12.35 volts at a 50 percent state of charge.
It is easy to see, therefore, that a scant few tenths of a volt separate a fully and partially charged AGM or flooded battery. That meager difference, between 85 or 90 percent and 100 percent, can have a dramatic difference on the overall life of the battery. This also is the case for float voltages; just a few tenths of a volt can mean the difference between a well maintained battery and positive grid corrosion or sulfation. This is true, however, for all batteries &mdash AGMs as well as other lead-acid varieties. There is little evidence to prove that conventional flooded batteries endure chronic undercharging any better than AGMs. Sulfation occurs in either chemistry. Flooded nonsealed batteries may better tolerate overcharging because water loss can be mitigated with replacement. However, grid corrosion and shedding of plate material will occur in both battery types. AGM, flooded and gel cells will all suffer from chronic under- or overcharging.
Building AGM battery banks from multiple cells is an acceptable and safe practice. However, using batteries of differing case designs, chemistries or manufacture dates is not recommended. The internal resistance of batteries may differ with capacity, and older batteries may exhibit higher resistance than newer batteries. This imbalance could lead to premature failure. Different battery chemistries have markedly different charge profiles and as a result, they must never be interconnected.
Cruiser X mentioned that his "house bank has a claimed capacity of 360 amp-hours." The only way one could arrive at this amp-hour capacity would be to mix an 8D and a group 31, which provide 255 and 105 amp-hours, respectively. This is a less-than-ideal scenario that should be avoided during battery bank design, regardless of the chemistry involved. Additionally, batteries used to make up a bank should all be manufactured in the same month and year.
Equalization controversial
There is a measure of controversy concerning the practice of equalization of AGM batteries, a process that mitigates the effects of acid stratification. At one time, it was considered unnecessary and even forbidden by the manufacturer. Times have changed since these batteries were introduced, and Concorde/Lifeline recommends the practice when "the battery is showing symptoms of capacity loss." East Penn Manufacturing, on the other hand, indicates that equalization is unnecessary in their AGMs because "stratification cannot occur in batteries of [this] size."
Where permitted, carefully follow the recommendations of the manufacturer concerning voltage requirements and duration of the equalization charge. In my experience, AGMs appear to be particularly susceptible to sulfation as a result of being left in a discharged state for an extended period (more than 30 days). Equalization under these circumstances, if permitted by the manufacturer, usually restores the battery to full or nearly full capacity. If the battery is suffering from chronic overcharging and the positive plates are corroded as a result, equalization will have no effect and may actually exacerbate the problem.
Regardless of the fact that AGMs do not gas under normal circumstances, they still require ventilation. Never install AGM or any other lead-acid batteries, regardless of design, in a sealed or poorly ventilated compartment. AGM batteries, like all other batteries, should be well secured in a dedicated battery locker or box where they cannot be damaged or short circuited by shifting gear.
Gel cells fall somewhere in between these two philosophies. As they do not charge as quickly as AGMs, they are more expensive than flooded batteries but cost about the same as AGMs. They also typically offer more cycles than AGMs but fewer than flooded batteries, and they cannot be equalized. Both AGMs and gels offer superior storage or shelf life because their self-discharge rates are exceptionally low, roughly 3 percent per month. Even gels, however, will be permanently damaged by chronic under- or overcharging. Under some circumstances, gel batteries are subject to explosion as a result of overcharging, resulting in the issuance of a U.S. Coast Guard consumer advisory in August 1996.
The advisory indicates that gel batteries may be subject to explosion if they "are recharged with battery chargers which are not automatic temperature-sensing voltage regulated chargers." I suspect the likelihood of explosion is rare, particularly with the increased popularity of sophisticated chargers; however, according to the advisory, it has happened and thus is a factor worth considering. While overcharging may damage AGM batteries, they are particularly safe and not subject to the above advisory.
Know the facts, and read all the available literature from the manufacturer, including the warranty, before investing in AGM batteries or batteries of any chemistry, regardless of the manufacturer. Had Cruiser X done this, he may not be complaining bitterly about his experiences with AGM batteries. In my opinion, AGM batteries perform as advertised, provided they are used and maintained in accordance with their manufacturer's recommendations. Just like anchors, there's no one battery type that will be all things to all users.
