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Old 28-05-2013, 07:01   #1
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AGM Batteries

We are out cruising for 6 weeks now and I find my agm batteries don't do what they should do
If we motor for hrs the batteries should get a good charge . I have a smart regulator that bulk charges at 14.4 volt and float charges at 13.4 volt
As soon as we turn the motor off the batteries drop to about 12.8 volt and go down to 12.4 volt within a few hrs. They will sit on 12.4 volt for about 8 hrs before dropping further
Our power consumption is about 160ahrs per day and the house bank is 480ahr
The batteries are about 12 month old
I have a bad feeling that the batteries are faulty.
Any ideas?

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Old 28-05-2013, 07:21   #2
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Re: AGM Batteries

They should drop to 12.8V when the charging source is removed. Dropping to 12.4V during the next few hours and staying there for 8hrs may be normal depending on how much current you are pulling from them during this time.

If the behavior you describe is happening with no loads at all, then this is not normal. A capacity test would tell you much.

Mark
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Old 28-05-2013, 08:28   #3
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Re: AGM Batteries

It sounds perfectly normal.

When you turn the engine off, if you are using 160 AH a day, then you are pulling an average of 7 amps from the batteries sometimes much higher or lower as refrigerator and other loads cycle on and off. That will do exactly what you are describing.

And FWIW, measuring voltage while pulling amps from a battery tells you very little about their state of charge. You have to let them rest for hours, maybe a half a day with no current coming or going to effectively use voltage as an indication of state of charge.

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Old 28-05-2013, 09:30   #4
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Re: AGM Batteries

What is the voltage after a full charge and then waiting a few hours with no load on the battery bank?
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Old 28-05-2013, 11:43   #5
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Re: AGM Batteries

I'd have thought the charger (set for AGMs or lead acid) would float at 13.8v vice 13.4...

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Old 28-05-2013, 11:55   #6
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A friend of mine has a similar setup and his batterie sit on 13.1 volt for hrs before dropping to 12.8volt
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Old 28-05-2013, 11:57   #7
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Quote:
Originally Posted by David M View Post
What is the voltage after a full charge and then waiting a few hours with no load on the battery bank?
Good point, I will test that today
Thanks
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Old 28-05-2013, 15:58   #8
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Re: AGM Batteries

Quote:
Originally Posted by reiner View Post
A friend of mine has a similar setup and his batterie sit on 13.1 volt for hrs before dropping to 12.8volt
With a 7A load on them during this time?

You haven't been clear on whether your battery behavior is with or without loads being applied. This will be meaningful.

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Old 30-05-2013, 03:24   #9
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Re: AGM Batteries

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Originally Posted by colemj View Post
With a 7A load on them during this time?

You haven't been clear on whether your battery behavior is with or without loads being applied. This will be meaningful.

Mark
Sorry Marc, yes that is while supplying about 8 to 12 amp
I keep looking at the voltmeters to use them like a fuel gauge but I am starting to understand that the voltage goes up and down depending how big the load is
I saw earlier the voltage drops by .2 volt when a fridge cuts in
I still feel that my battery capacity should be higher
My plan is to disconnect one battery (240ah) for a day or two and then swap to the other battery and see if they behave in the same way, maybe one of them is not working right
Thanks
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Old 30-05-2013, 08:06   #10
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Re: AGM Batteries

If you are drawing 8-12A, then the behavior you describe sounds normal to me.

Mark
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Old 30-05-2013, 15:56   #11
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Re: AGM Batteries

Something else has changed
When I used to start the motor to charge the batteries the voltmeter jumped from 12.2 volt to about 13 volt
From there it took about 60 minutes for the voltage to go slowly up to 14.2 volt
Yesterday when I started the engine the voltage went to 14.3 volt within 10min
My alternator charges at 63 A
About 4 hrs after I turned the motor off the batteries were back at about 12.4 volt
To me those numbers don't make sense
The batteries are 240Ahr each( X2)

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Old 30-05-2013, 16:56   #12
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Re: AGM Batteries

Sounds very much like the batteries have lost capacity, probably thru undercharging and sulfation.

AGM batteries need to be brought to FULL charge frequently, or they will sulfate.

Exercising them and charging at 14.6VDC or so may help a bit and, if they're Lifeline's, you could even try equalizing them.

But, their behavior (going to 14.3VDC very quickly, and dropping to 12.4VDC very quickly) sounds like much reduced capacity.

Bill
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Old 02-06-2013, 03:55   #13
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Re: AGM Batteries

install a battery monitor

the best way to test the batteries is a c20 discharge test.
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Old 02-06-2013, 04:01   #14
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Re: AGM Batteries

Quote:
Originally Posted by reiner View Post
A friend of mine has a similar setup and his batterie sit on 13.1 volt for hrs before dropping to 12.8volt
where are you measuring voltage? an analog gauge on the dash? they are normally way off. you need to stick a digital multimeter on the batteries. or as above read it off the battery monitor after you put one in.
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