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Old 08-01-2015, 04:29 PM   #1
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Default 50 percent agm voltage

hey smart people- i have a question about AGM batteries.

what voltage is considered 50 percent -just wondering
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Old 08-01-2015, 05:05 PM   #2
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Would it be different from other batteries? (which would be about 12.0) Or is that your question that I can't answer...
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Old 08-01-2015, 07:16 PM   #3
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Old 08-01-2015, 07:53 PM   #4
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Where did you get that voltage chart? It does not match the data from Lifeline AGM that I have. The 100% voltage on Lifelines is 12.8V.
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Old 08-01-2015, 07:59 PM   #5
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There are alot of variations on this chart online. Just google "battery voltage chart" and you'll see tons. I consider them a rough approximation at best.
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Old 08-01-2015, 08:17 PM   #6
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Here's a chart I found on Phillips Industries site. It was part of some load testing instructions which wanted to ensure that you recharged the battery if below 75% state of charge before load testing the battery.

Phillips State of Charge chart flooded and agm.JPG
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Old 08-01-2015, 11:02 PM   #7
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how low is an agm battery if an inverter sounds alarm at 10.5 and shuts off at 10 volts
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Old 08-01-2015, 11:45 PM   #8
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how low is an agm battery if an inverter sounds alarm at 10.5 and shuts off at 10 volts
It all depends on how much current the inverter and loads are taking out of the battery at that time.
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Old 08-01-2015, 11:59 PM   #9
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Here is a chart from Lifeline that shows what the voltage will read at a given state of charge, when being discharged at various rates.

This is a very useful chart for setting low voltage cutouts and alarms.

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File Type: jpg Lifeline volts vs discharge rate.jpg (87.4 KB, 48 views)
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Old 08-02-2015, 12:03 AM   #10
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how low is an agm battery if an inverter sounds alarm at 10.5 and shuts off at 10 volts
Low enough to destroy your battery I would think.
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Old 08-02-2015, 12:26 AM   #11
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Low enough to destroy your battery I would think.
I think that would depend on the battery type and brand (10.5v or 10.0 shutoff)

From the lifeline chart, it sure looks like it would hurt them, but I know I could drag down our wet cell Trojans to 10 volts when at even 50% state of charge, by pulling lots of current, so those would probably survive just fine.
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Old 08-02-2015, 12:51 AM   #12
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I know this will be stupid is the low voltage cutoff in an inverter there to save the battery or the inverter. if it's there to save the battery how come it's set at 10 volts.

is it actually 10 volts or is 10 volts what it predicts the rate of inverter use will cause.
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Old 08-02-2015, 12:56 AM   #13
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The voltage for the cutoff would normally be what the predicted voltage would be with the inverter running it's biggest load and the battery at the lowest SOC you want to be at and still be able to run the inverter.
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Old 08-02-2015, 01:13 AM   #14
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The voltage for the cutoff would normally be what the predicted voltage would be with the inverter running it's biggest load and the battery at the lowest SOC you want to be at and still be able to run the inverter.
so it's predicted voltage not actual voltage?
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Old 08-02-2015, 01:27 AM   #15
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It is predicted in the sense that the inverter makers guess what batteries and loads you will have.

For best accuracy for your system you just need to run a test to find out what the real life voltage would be.

1. Decide what the minimum SOC you want to have and still be able to run the inverter.

2. Discharge your batteries to that SOC

3. Run you biggest, or typical, load through the inverter, with all your regular 12v loads on at the same time

4. See what the voltage reads under the those inverter running conditions.

5. Set the voltage you saw as your low level cutoff.

Done
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Old 08-02-2015, 01:30 AM   #16
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Actually, what he's saying is that it's set to account for the voltage drop attributed to your loads with the inverter running. In my recent tests, all my heavy loads totaled 1/2 a volt in voltage drop.

I still say that is too low. Even 11 volts is too low for an AGM. I'd set your cutoff above 11.5 for your AGMs. When you get your tandem, I have no clue, but maybe you can go lower.
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Old 08-02-2015, 01:40 AM   #17
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i have seen many people with e-trek and e-trek packages complaining that the inverter turns off and battery still shows good charge-i know idiiot lights are not perfect.

what i was trying to determine is if the inverter is shutting off at 50 percent.

the inverter specs say low voltage alarm at 10.5 volts and shut off at 10 volts

yet it seems to be happening a lot sooner
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Old 08-02-2015, 01:45 AM   #18
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Quote:
Originally Posted by gerrym51 View Post
i have seen many people with e-trek and e-trek packages complaining that the inverter turns off and battery still shows good charge-i know idiiot lights are not perfect.

what i was trying to determine is if the inverter is shutting off at 50 percent.

the inverter specs say low voltage alarm at 10.5 volts and shut off at 10 volts

yet it seems to be happening a lot sooner
You just need to run the test I listed and you will know for sure.
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