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Old 05-03-2010, 19:23   #1
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Simple Voltage Question

I apologize ahead of time for my lack of electrical knowledge, this simple mind was meant to sail and not to tinker!

Have an 800ah 4D (4 batteries) AGM battery bank (Deka batteries) with a Xantrex 40amp battery charger (I know, too small). When 20 amp load placed on battery bank (charger off) voltage will drop to 12.0V. 30 amp load placed and it drops to 11.8V. When all loads off the voltage rises to 12.7 volts suggesting that batteries are full.

Question: Why do AGM's let voltage drop to 12.0-11.8V under a 20-30 amp load? Too rapid a discharge? Faulty batteries?

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Old 05-03-2010, 20:14   #2
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The voltage drop is due to the internal resistance of the battery (~0.035 Ohms). AGM batteries are supposed to have lower internal resistance than flooded cells, and 0.035 Ohms does seem a bit high. It could indicate end of life condition.

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Old 05-03-2010, 20:44   #3
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You have to measure the voltage with a good multimeter (like a Fluke) directly on the battery terminals to check if the reading you get is correct. Panel meters often measure at the panel instead of the battery terminals, in which case you measure the loss over the cabling.

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Old 07-03-2010, 08:37   #4
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Yes, by all means measure the voltage with a good multimeter directly at the battery terminals as Jedi recommends.

However, you should know that with an 800AH AGM battery bank a 20A load (representing 2.5% of total capacity) should make virtually no discernable difference in voltage at the batteries.

My bet is that those Dekas are shot. Beg, borrow or steal a good battery analyzer (like one of the Midtronics testers with AGM capability) and see about their condition.

The only other reasonable conclusion -- even if someone wired the meter to the panel instead of the batteries -- is that you have some very bad (i.e., high-resistance) connections in the battery cables, connectors, switch, wiring, etc.

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Old 15-05-2011, 18:29   #5
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Re: Simple Voltage Question

i know old thread but fact is as batteries get older the internal resistance increases over time indicating that the batteries are getting closer to need of replacement. If you could have a batt with zero internal resistance, you would have a perfect batt and it would last forever.....
This is true with your AA AAA C D 9VOLT batts also. To make them last longer in storage put them in an airtight container, tupperware etc and put them in the fridge. Do not freeze them!!!!just keep them fridged until you need them. They will last much longer in storage and yes they do start to "go away" simply due to age and this is helped by ambient heat. The fridge slows the internal chemical activity thus makes them last longer. I've been storing my spares like this for several years and when i take them out for use they are just like new...(hot and fresh)
Let me see if I have this right. "the water is below and outside, and the air is above and inside? sounds good to me
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Old 15-05-2011, 19:52   #6

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Re: Simple Voltage Question

Todo del previoso es exactamundo correcto.

Sus batterias estan basura, problamente.

El Senormechanico.

I agree, they're probably better left at the recycling center.
Mine are getting there (220 ah/r) but not quite that much drop yet, but they're over ten years old.
Eagle eyed readers may want to argue with my previous posts about battery age, as I've been GIVEN these latest batteries.
They're basically free and work 90% of new. Good enough for my sailing style.

As for yours, assuming they're lead acid, and not gel or AGM,give 'em a good equalizing charge.
Do an internet or CF search for the particulars, I used a 19V. laptop power supply! It worked fine.
Now I have a MPPT solar controller which does it once a month.
Check the water afterwards!

BTW, I'm Norwegian !
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Old 15-05-2011, 19:58   #7
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Originally Posted by senormechanico
BTW, I'm Norwegian !
We won't hold that against you

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