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Old 18-08-2009, 16:06   #1
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Question About Float Charge for AGM Batteries

Hi,

I am puzzled here and need some advice. We recently installed new AGM batteries for starting (engine and genset). These batteries are charged by the standard alternators on the engine and genset.

But now, I am installing a new inverter/charger and the charger has a second 4A output that I could use to charge them. This output is not very intelligent, I think it follows the main output voltage (steps) at just a little lower voltage (echo charge).

Is it wise to connect this to the AGM cells? Or, in other words, are AGM's happy when they are kept at 13V or so for long periods, like months? We used gel batteries for this before and they were only charged by alternators. They kept their charge for long times, 6 months once, and no trouble starting the engine. I tend to feel like not connecting this "echo-charge" output to the new AGM's either but would like your opinion!

thanks,
Nick.
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Old 18-08-2009, 16:40   #2
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Many of the AGMs are used in Telco applications where they stay at float voltage 24/7. You should be able to get a correct, temperature adjusted, float voltage from the manufacturer of your battery. The different AGMs are a few tenths of v different from each other.

Paul L
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Old 18-08-2009, 17:56   #3
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Paul,

Of course, that info is available and I used to have such a battery backup system myself. But I don't know if the batteries last as long when under continues float charge as compared to charging when engine/genset runs and doing nothing in between. If there is no difference, I would say why not? But is there is a difference, I don't need the maximum runtime the telco needs, I can start with an 80% charge state of the battery.

Also note, that the echo-output of the charger follows the main output of the charger, so it will cycle through other stages as well.

thanks,
Nick.
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Old 19-08-2009, 11:46   #4
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Nick,
As I understand the AGMs the key to their life on a cruising boat is re-charging to 100% as often as possible. Generally you don't go to 100% when charging with a generator, as the last 10-15% takes too long.

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