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ContinuousWave: Small Boat Electrical
AGM Battery Life Span
|Author||Topic: AGM Battery Life Span|
posted 10-12-2007 07:57 PM ET (US)
In my job I monitor two UPS systems which each contain 40 batteries. The batteries are valve regulated lead acid (VRLA) absorbent glass mat (AGM) construction. These batteries are kept under a constant floating charge current and are subject only to occasional discharges, in some cases only once a month during a short test period of a minute or two. The batteries are kept indoors at a constant temperature (70-degrees). In other words, the conditions for long battery life are just about ideal.
This morning we replaced an entire string of 40 of these batteries in one of the UPS systems. They were 48-months old. The string was replaced because more than 10-percent of the batteries in the string had degraded to an internal resistance which exceeded our alarm limit. Or, in simpler terms, we had decided that the batteries had become unreliable for their purpose of providing emergency standby power. We had already replaced four of the forty, and three more had recently reached an alarm threshold. Rather than further mix and match new and old batteries, we replaced all the batteries.
These batteries were quality batteries and each weighs about 70-lbs, or more than most automobile cranking batteries. They are 270-AH rated batteries, and the retail price is about $250.
To facilitate the replacement of the batteries, we contracted with the UPS manufacturer to perform the service. The service technician had more than 30 years experience with industrial batteries. I asked him about the life span of these AGM batteries as compared to flooded cell lead-acid batteries. Flooded cell lead-acid batteries are also common in emergency power back up equipment. I was quite surprised by the technician's answer. He told me that a flooded cell lead-acid battery in this type of service has a service life of 20 years. The AGM batteries typically have a service life of five years.
The sealed nature of the AGM battery is a nice feature, and it reduces the chance of exposure to acid that can occur with a flooded cell battery. However, the inability of the AGM to have any electrolyte added tends to limit their life span relative to a flooded cell lead-acid battery.
posted 10-12-2007 08:02 PM ET (US)
Some AGM batteries can have electrolyte added. Adm. Linda
had one for her motorcycle that shipped with a bottle of
acid, and had to be filled before use.
posted 10-12-2007 08:12 PM ET (US)
I should also disclose that we have a sophisticated monitoring system which measures the internal resistance of each battery. The lower the internal resistance, the better the battery condition. Of the 36 four-year-old batteries which we were discarding (and recycling), I did cull out the two batteries with the lowest internal resistance readings. I have one of them home with me tonight and plan to use it for powering my bench 12-volt power in conjunction with a float charger. This is, of course, for further research on AGM battery life. And it replaces an AGM battery from the previous replacement of the other UPS string, a seven-year-old battery which unfortunately I neglected to keep on a floating charge and suffered some degradation. That battery was sent in for re-cycling.
Another data point: the four-year-old AGM batteries were all made in the USA. The new AGM replacements were all made in China. The field service technician commented that all the AGM batteries he sees now are made in China. The flooded cell batteries are still made in the USA. I think the problems in shipping a flooded cell battery probably prevent economical shipping of them to the USA from China. But apparently there is no problem in shipping VRLA AGM batteries to the United States from China, even in spite of the very considerable weight of each battery.
posted 10-13-2007 09:56 AM ET (US)
This is very interesting real world experience. You mentioned that the batteries are kept under constant float charge. What is the voltage level for this? Do you think it is ideal to keep an AGM cell constantly powered when it may not be necessary because of their very low internal leakage? I am sure that the conditions are set by the UPS manufacturer, I am just curious about them.
posted 10-13-2007 10:12 AM ET (US)
During my Army days flooded cell replacement 12 volt batteries were shipped dry. Electrolyte was added just prior to use. This eliminated shelf life issues.
This could be done with batteries shipped from China if manufacturers wished to do so. Electrolyte could be added at regional distribution centers.
posted 10-13-2007 11:24 AM ET (US)
In the UPS system I am referring to, there are 40 batteries in series. The float charge is applied to the string of batteries, not to the individual cells. This may be less than the optimum. The voltage across each battery is dependent on its own internal resistance and chemistry. The float current is typically less than 0.05 ampere.
On an AGM battery the float voltage should be in the range of 13.5 to 13.8 volts to keep the battery in optimum condition. The target voltage for the string of 40 cells is 540-volts. The voltage across each cell is individually monitored and an alarm is raised if it falls outside the tolerance mentioned above.
Because of the many batteries in a series string, the integrity of the whole battery is only as good as the weakest individual battery. To insure the highest reliability, we are probably discarding some batteries which still have plenty of useful life in them. However, what we have seen is that over a period of four years about ten percent of the batteries will degrade to a point where we do not trust them. We could continue to replace individual batteries in the string, but the recommended practice is to maintain a more uniform age to the string of batteries, rather than mixing up batteries of many different ages.
The VRLA AGM battery is often cited as a good choice for use in marine applications. The sealed characteristic is a definite benefit in a marine application on a small boat which can be subjected to some rather strange maneuvers and attitudes. The resistance to damage from motion and vibration are also good characteristics of the AGM battery for marine applications. However, the anticipated life span of an AGM battery is not as great as a flooded cell battery, and this point may be worth considering in a marine application.
posted 10-13-2007 11:27 AM ET (US)
I should also mention that the post style of the new batteries was changed. The new batteries have a almost flush pad with a threaded boss to which the terminal lug is fastened. The old batteries had a post style connection. The service technician commented that the flush pad style connection is just about universal in VRLA AGM batteries these days, supplanting the older post style.
posted 10-13-2007 09:18 PM ET (US)
I'm not an expert here, but maybe there should be an artificial duty cycle imparted on those otherwise stagnant and unexercised batteries! Certainly the lead acid types like to be cycled , don't they?
posted 10-14-2007 10:19 AM ET (US)
Let's continue the metaphor of "exercise" which has been applied to a battery. A battery stores energy, like a human muscle stores energy. If you exercise your muscles frequently, and you are in good health, your muscles will grow and increase their capacity. Unfortunately, batteries are inanimate, and they do not grow greater capacity in response to their "exercise" of charge and discharge cycles. If the manufacturers of batteries are to be believed, each charge and discharge cycle removes capacity ("life") from the battery. Therefore a VRLA AGM battery will not have its life span improved by causing it to be discharged and recharged more frequently. In fact, my understanding is that frequent charge and discharge cycles will have the opposite effect.
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