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Old 09-21-2020, 09:55 AM   #1
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AGM battery voltage level

Does this look like an accurate voltage chart for 3 new agm batteries?

Mine don't hold a charge like this. Mine seem to stay closer to 12 volt.



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Old 09-21-2020, 10:36 AM   #2
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That chart may be for flooded batteries, AGM have about .1 volt more at 100% charge.

You will need to have no draw on the batteries, for at least a few hours, before measuring the battery voltage and comparing it to the charts.

Yes, an AGM fully charged should show 12.7 volts.
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Old 09-21-2020, 03:43 PM   #3
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maybe that's it...I have never measured them having not had a load on them for a couple of hours.


Do folks keep their inverters on while plugged into shore power or turn it off?
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Old 09-21-2020, 04:20 PM   #4
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Quote:
Originally Posted by kongmen View Post
maybe that's it...I have never measured them having not had a load on them for a couple of hours.


Do folks keep their inverters on while plugged into shore power or turn it off?
In your rig it won't matter. While on shore power, the inverter/charger becomes a charger only. The shore power is passing thru the inverter to whatever it powers.

If it senses a loss of shore power it will come on to power stuff. If you shut it off, it won't.
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Old 09-25-2020, 08:25 AM   #5
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When I look at the 3 different options for checking voltage,onnan energy command, inverter, toggle switch for coach they all read within 2 or 3 tenths of a volt. After 4 days of 50 amp shore power they all read 13.5 before unplugging from 50 amp power. Right after unplugging they now read 12.1 - 12.3 volts. The chassis batteries read 12.5 volts. I don't understand the drop in voltage. I am leaving the refrigerator on at 425 watts maybe that's it don't know.
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Old 09-25-2020, 08:16 PM   #6
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Something is wrong with These batteries. 13.5 volts after 4 days on 50 amp shore power (inverter showing float mode so batteries were fully charged) 12.2 volts 20 minutes after unplugging. Now 11.8 volts after driving 7 hours and running a norcold refrigerator at 425 watts.

Batteries are only 2 months old. Duracell 31 AGM from Sam's club.

Any ideas?
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Old 09-26-2020, 01:53 AM   #7
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7 hours of 425 watts equals 236 AH from your batteries 300 AH of batteries. Your voltage readings make sense.

425 wats at 12.6 volts is 34 Amps. 34 amps for 1 hour is 34 AH. 7 hours X 34 AH = 234 AH from your 300 AH battery bank.

If your engine charging system was connecting to your house batteries, like it should, you would be ok after the trip. Big chance its not connecting.

Now you need to check your isolation/charging solenoid system.

The thing that fails most often, is the solenoid. You can locate it by pushing on and off the boost or emergency start button, while listening for it to clunk.

Find it, check it, and if bad replace it.
With engine running, engine battery voltage should read on both large terminals of it.

Why are you running a gas/electric fridge on your batteries while driving ?
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Old 09-26-2020, 04:53 AM   #8
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Battery voltage / SoC chart for estimating status of lead acid battery type. Click image for larger version

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Old 09-26-2020, 07:50 PM   #9
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twinboat thanks for the info.

I will check the solenoid.

I was running the fridge on battery power because I thought as long as I was driving why not use the engine generated power?

I'm new at rving. It looks like I will be using gas for the fridge when not plugged into shore power.

How do people with residential refrigerators power their fridges while not plugged into shore power?
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Old 09-26-2020, 07:55 PM   #10
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They run them with inverters like you tried but the isolation solenoid needs to be working. You can run yours on battery but you need to fix the charging.

Residential fridges use about 1/3 the power of a gas/electric model, so even without chassis charging, they can go for much longer.
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Old 09-26-2020, 07:55 PM   #11
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Generator, or a large battery bank with an inverter. What many people fail to understand is that when the 120VAC draw is 400 watts, or about 4 amps, when you are drawing from a 12VDC battery bank, the amperage draw goes up by times 10. So the 4 amps becomes 40 from the battery.
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Old 09-27-2020, 07:08 AM   #12
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Thanks again fellas.

I think I'll just keep it on gas while not plugged in. I'm going to see if the batteries charge at all while driving. If they charge during a drive without the fridge on then I can assume the solenoid is working?
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Old 09-27-2020, 07:32 AM   #13
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Like I said, they should have charged even with the fridge running.

It will work your alternator pretty hard but it should be rated at 160 amps.

Run it on gas and see if your getting charging.
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