AGM Batteries

Alternatives to Lead-Acid Batteries:

Most of us are familiar with the basic lead-acid battery that's in our car or truck. Some of these are referred to as "sealed" batteries but in reality they are not totally sealed. Sealed lead-acid (or flooded) batteries still require venting to the atmosphere. While sealed batteries tend to minimize venting, this venting still does allow water to escape via "outgassing". So while reduced, it's not eliminated. Plus the drawback is that it is very hard, if not impossible, to add water to a battery that needs it. Fortunately, there are a number of other variations in battery technology that offer either advantages or disadvantages, depending upon their application. In the following paragraphs we'll discuss these other types of batteries.

Gel Cells:

Gelled batteries or "Gel Cells" contain acid that has been "gelled" by the addition of Silica Gel, turning the acid into a solid mass that looks like gooey Jell-O. The advantage of these batteries is that it is impossible to spill acid even if they are broken. However, there are several disadvantages. One is that they must be charged at a slower rate to prevent excess gas from damaging the cells. They cannot be fast charged on a conventional automotive charger or they may be permanently damaged. This is not usually a problem with solar electric systems, but if an auxiliary generator or inverter bulk charger is used, current must be limited to the manufacturer’s specifications. Better inverters commonly used in solar electric systems can be set to limit charging current to the batteries. Some other disadvantages of gel cells are that they must be charged at a lower voltage (2/10th's less) than flooded or AGM batteries. If overcharged, voids can develop in the gel which will never heal, causing a loss in battery capacity. In hot climates, water loss can be enough over 2-4 years to cause premature battery death. It is for this and other reasons that gel cells are not suitable for use in an RV environment. The newer AGM (absorbed glass mat) batteries have all the advantages (and then some) of gelled, with none of the disadvantages.

AGM, or Absorbed Glass Mat Batteries:

A newer type of sealed battery uses "Absorbed Glass Mats” or AGM between the plates. This is a very fine fiber Boron-Silicate glass mat. These type of batteries have all the advantages of gelled, but can take much more abuse. Concorde (and Lifeline, made by Concorde) are a popular brand of AGM batteries. These are also called "starved electrolyte", as the mat is about 95% saturated rather than fully soaked. That also means that they will not leak acid even if broken.

AGM batteries have several advantages over both gelled and flooded, at about the same cost as gelled:

Since all the electrolyte (acid) is contained in the glass mats, they cannot spill, even if broken. This also means that since they are non-hazardous, the shipping costs are lower. In addition, since there is no liquid to freeze and expand, they are practically immune from freezing damage.

Nearly all AGM batteries are "recombinant" - what that means is that the Oxygen and Hydrogen recombine INSIDE the battery. These use gas phase transfer of oxygen to the negative plates to recombine them back into water while charging and prevent the loss of water through electrolysis. The recombining is typically 99+% efficient, so almost no water is lost.

The charging voltages are the same as for any standard battery - no need for any special adjustments or problems with incompatible chargers or charge controls. And, since the internal resistance is extremely low, there is almost no heating of the battery even under heavy charge and discharge currents. The Concorde (and most AGM) batteries have no charge or discharge current limits.

AGMs have a very low self-discharge - from 1% to 3% per month is usual. This means that they can sit in storage for much longer periods without charging than standard batteries. The Concorde batteries can be almost fully recharged (95% or better) even after 30 days of being totally discharged.

AGMs do not have any liquid to spill, and even under severe overcharge conditions hydrogen emission is far below the 4% max specified for aircraft and enclosed spaces. The plates in AGMs are tightly packed and rigidly mounted, and will withstand shock and vibration better than any standard battery. AGMs are also lighter in weight than comparable flooded batteries.

Even with all the advantages listed above, there is still a place for the standard flooded deep cycle battery. AGMs will cost 2 to 3 times as much as flooded batteries of the same capacity. In many installations, where the batteries are set in an area where you don't have to worry about fumes or leakage, a standard or industrial deep cycle is a more economical choice. AGM batteries main advantages are no maintenance, completely sealed against fumes, Hydrogen, or leakage, non-spilling even if they are broken, and can survive most freezes. Not everyone needs these features.

Submitted by Mike Sundberg - 2/07/06
 

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