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Old 16-12-2008, 07:43   #1
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AGM's only reading 12.4 v

Got to the boat after six months on the hard. My house bank 2x4D AGM's (two years old) was down to 12.4 v. Everything was off during the six months. I put them on charge for a couple of days. 100 amp smart charger only putting in about 20 amps. Let them rest for 24 hours, now reading 12.6 v. I noticed at the end of last season they weren't lasting as long as when new. Had some problems last season when the alternator went crazy (actually a regulator problem) and put out over 16v for a while. Those are the facts. Are the batteries shot??
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Old 16-12-2008, 07:51   #2
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What the makers of AGMs don't tell you is that the batteries need to be charged 100% after use. And that is very hard to do on a boat. If you can't accomplish this the life of the battery is seriously decreased and some of them will give you a hassle about the warranty. 12.4 after sitting for 6 months is actually pretty good if no charger was applied.
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Old 16-12-2008, 08:17   #3
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12.4 after 6 months self discharge sounds pretty good to me as well. If you want too it is easy to do a check of their capacity, but it is a bit brutal on the batteries and I would not suggest it unless there is serious reason to believe they shot, or you were sailing to places where replacing them was difficult.
To check capacity just put a set load on the battery say 8A and measure how long it takes to get to a low voltage say 11v (be careful the voltage will drop quickly towards the end). The capacity will vary depending in discharge current (lower with more current) and termination voltage. Look up how the manufacturer of your batteries defines these and match them. You can then compare your results with the batteries when new. The only thing to watch is getting 100% charge into the battery as a starting point is difficult. So a result of 90% or so on a new battery would be typical.
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Old 16-12-2008, 08:36   #4
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Quote:
Originally Posted by noelex 77 View Post
The only thing to watch is getting 100% charge into the battery as a starting point is difficult. So a result of 90% or so on a new battery would be typical.

And that is my point but the manufacturers want this to honor warranties and the don't tell you this up front nor do the retailers. But unless you can accomplish this the longevity of the batteries decrease, another thing the manufacturer or retailers leave out so that perhaps you don't buy their very expensive batteries. But it is something that buyers and owners should be aware of. BTW, this is not suggested as a starting point but is supposedly required with each charge after use.
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Old 16-12-2008, 09:10   #5
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Quote:
Originally Posted by Vasco
... Had some problems last season when the alternator went crazy (actually a regulator problem) and put out over 16v for a while.
Increasing the charging voltage of “Lifeline” AGM batteries 1.0V above the recommended charging voltage results in a 23% reduction in the cycle life.

http://www.lifelinebatteries.com/manual.pdf
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Old 16-12-2008, 09:27   #6
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Gord, Does the increase affect the batteries if it is applied for five seconds, five minutes, five days? That is what they often fail to tell you.
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Old 16-12-2008, 11:12   #7
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Gord, Does the increase affect the batteries if it is applied for five seconds, five minutes, five days ...
Actually, the Lifeline AGM Battery Manual, that I previously linked, provides a considerable amount of technical information, for those who chose to read it.
It may be assumed, that the full reduction would occur over a complete charge cycle.
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Old 16-12-2008, 14:26   #8
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One thing that bothered me was that although it was showing 12.4v it wouldn't take more than 20 amps in the bulk phase and went to accept and float in a couple of hours. And after this I let it rest for 24 hours ( luckily we're not living on the boat yet and can do this) and it only showed 12.6v. My concern is that I seem to have lost about half my capacity.
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Old 16-12-2008, 14:38   #9
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Rick, how do you determine that you have lost half of your capacity? I don't understand that. 12.6 for a battery at rest, being off charge for some time, after being charged for only a couple of hours, again sounds about normal.
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Old 16-12-2008, 15:24   #10
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I have a 100 amp charger. Why won't it take more than 20 amps in the bulk phase when it was reading 12.4. This is a 400 amp bank. Forgot to say I let it charge for 24 hours before letting it rest for 24.
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Old 16-12-2008, 15:27   #11
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So your concern is your battery charger output and not your battery capacity?
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Old 16-12-2008, 15:35   #12
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"Had some problems last season when the alternator went crazy (actually a regulator problem) and put out over 16v for a while. "
16V will boil out some electrolyte from most AGMs (Concorde/Lifeline is, I think, the only maker to even recommend a 15V+ equalizing charge for AGMs) and once it has boiled out--it is gone forever.
12.4V is still not bad, 12.6 is typically considered 100% and 12.4 would be an 80% charge. Not bad after six months on the hard without attention.

Checking the instruments (Link system, multimeter, whatever) for calibration is also a good idea--when you are talking about tenths of a volt, they often drift and need to be verified with a calibration source or routine from time to time.
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Old 16-12-2008, 15:55   #13
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Thanks everyone, my brain's gone, must be the rum and I haven't even got the boat in the water yet. I thought a full charge was over 13v. Just saved myself $800. Will spend it on rum.
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Old 16-12-2008, 16:16   #14
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Glad you figured it out before spending the dollars.
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Old 16-12-2008, 17:21   #15
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Rick,
I would take the time to read the lifeline battery link that Gord supplied. An excerpt from that manual follows:
"The most efficient method of charging Lifeline® AGM batteries is to use a 3 stage charging profile. In the first stage, a constant current is applied until the voltage reaches a pre-set limit.


The first stage is often called the
Bulk charging stage. In the second stage, the voltage is held constant at the same pre-set limit until the charging current tapers to a very low value, at which point the battery is fully charged.
The second stage is often called the Absorption charging stage. A voltage limit of 14.40 volts ± 0.06 volts (7.20 ± 0.03 volt for a 6 volt battery) should be used when the battery temperature is 77°F (25°C). The battery is fully charged when the current drops below 0.5% of the battery’s rated capacity (0.5A for a 100Ah battery).
In the third stage, the charging voltage is reduced to a lower value that minimizes the amount of overcharge, while maintaining the battery at 100% state of charge. This third stage is often called the Float charging stage. A float voltage of 13.26 ± 0.06 volts (6.63 ± 0.03 volts for a 6 volt battery) should be used when the battery temperature is 77°F (25°C). The charging voltages at other temperatures can be determined from the following table"

The voltages stated above are far higher (a full 2 volts) than those reported by you. They give a procedure for equalizing the AGMs. If they were my batteries, I would be planning that now.
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