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Old 06-19-2021, 10:48 AM   #1
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Can someone explain battery discharge

I think I have at least one bad batter in my bank. I can only go about 1.5-2 hours before I hit my LBCO which is set at 11.7, I did have it lower and it was about the same It looks like I am drawing about 18 amp (240 watts) with just the basic on, refrigerator, DirecTV receiver and pepwave router and 28 amps when the refrigerator compressor kick on.

When the batteries got below 11 volts I checked them to see if I could find a bad battery but got some odd reading. Look at picture below, all 8 batteries had 10.7 volts each 12 volt bank had ~10.7 volts and ever other battery was ethier ~6.3volts or ~4.3volts. Why would they discharge that way? Is this normal?
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Old 06-19-2021, 11:04 AM   #2
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You may have to measure them with at least one side of each battery disconnected to get a better picture of what is actually happening.
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Old 06-19-2021, 11:39 AM   #3
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You may have to measure them with at least one side of each battery disconnected to get a better picture of what is actually happening.

Correct. Fully charge and disconnect to load test them.


If wet cell, fully charge and use an inexpensive BATTERY HYDROMETER (any auto parts house-- under $10). Lots less disconnecting to check.
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Old 06-19-2021, 12:28 PM   #4
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I should have said they are Harris AGM batteries in a 2018 DSDP
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Old 06-19-2021, 12:35 PM   #5
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Then, after charging, disconnect and load test them individually.
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Old 06-19-2021, 01:05 PM   #6
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I should have said they are Harris AGM batteries in a 2018 DSDP
Some may make way more technical arguments, but I will put it in plain English. Your entire battery bank will only be as good as the worst battery .

The P.O. replaced one battery between the time I purchased it and picked it up. I donít know why he felt it failed but he did. I have 3 12v flooded wet batteries.

I was having trouble going overnight without the voltage going low enough to shut the inverter down. Tv, fridge and a laptop. 480 watts total. Since I only had 2 nights without shore power I left it alone until I got home this spring.

I pulled all 3 out, charged and tested them with my computerized tester. The new one checked out at 100%. The other 2 20%. I put the one new one back in and I had more amps/time with one good one than with 2 bad and 1 good.

I replaced all 3. Bought 3 identical 120 ah flooded at Samís on sale for $68 each.

The only proper way to test batteries is to disconnect each, put a charger on them and test with a good battery tester. Checking voltages even across one battery will give wired readings. You may test one at 6 or 12 volts, but if connected to one with a bad cell one connection may only show 4 v

And just because you have voltage doesnít mean you have amps. Wit 8 6v batteries like you have, if they are all good and fully charged you should get 2-3 days drawing 200 watts.

I know a lot of coaches and a lot of people like using 6v golf cart batteries, I have found 12v deep cycle battery to work as well if not better. Remember, if you buy 6v 120ah batteries and put them in series you still only have 120ah, at 12v I find golf cart batteries just as much as a similar size 12v battery. So why buy 8 when you get the same power out of 4. Plus a lot less cables and less chance to make a mistake.

Find an old 12 v battery, wire just 1 positive and one negative, hook to shore power, so your converter has something to keep 12v on while you charge and test all your batteries. If you find one dead, itís time to replace them all.
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Old 06-19-2021, 01:12 PM   #7
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As stated, you can't test dead batteries, once fully charged, disconnect them from the others and load test.

Here is a low cost load tester that is good enough to tell you if you have a bad battery.

https://www.harborfreight.com/100a-6..._q=load+tester
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Old 06-19-2021, 01:42 PM   #8
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Some may make way more technical arguments, but I will put it in plain English. Your entire battery bank will only be as good as the worst battery .

The P.O. replaced one battery between the time I purchased it and picked it up. I don’t know why he felt it failed but he did. I have 3 12v flooded wet batteries.

I was having trouble going overnight without the voltage going low enough to shut the inverter down. Tv, fridge and a laptop. 480 watts total. Since I only had 2 nights without shore power I left it alone until I got home this spring.

I pulled all 3 out, charged and tested them with my computerized tester. The new one checked out at 100%. The other 2 20%. I put the one new one back in and I had more amps/time with one good one than with 2 bad and 1 good.

I replaced all 3. Bought 3 identical 120 ah flooded at Sam’s on sale for $68 each.

The only proper way to test batteries is to disconnect each, put a charger on them and test with a good battery tester. Checking voltages even across one battery will give wired readings. You may test one at 6 or 12 volts, but if connected to one with a bad cell one connection may only show 4 v

And just because you have voltage doesn’t mean you have amps. Wit 8 6v batteries like you have, if they are all good and fully charged you should get 2-3 days drawing 200 watts.

I know a lot of coaches and a lot of people like using 6v golf cart batteries, I have found 12v deep cycle battery to work as well if not better. Remember, if you buy 6v 120ah batteries and put them in series you still only have 120ah, at 12v I find golf cart batteries just as much as a similar size 12v battery. So why buy 8 when you get the same power out of 4. Plus a lot less cables and less chance to make a mistake.

Find an old 12 v battery, wire just 1 positive and one negative, hook to shore power, so your converter has something to keep 12v on while you charge and test all your batteries. If you find one dead, it’s time to replace them all.
6v flooded golf cart batteries are 215ah or more. You’ll get near double the AH with two 6v batteries than you will with one 12v one.
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Old 06-19-2021, 02:20 PM   #9
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6v flooded golf cart batteries are 215ah or more. Youíll get near double the AH with two 6v batteries than you will with one 12v one.
Lets not start this again. Battery capacity is a function of LEAD...weight, things like that. It is correct that one nice 6 volt deep cycle Trojan will have more AH than a deep cycle 12 volt...but only at 6 volts!

Think of two deep cycle 6 volt units as one big 12 volt battery with 3 cells in each case. Two 6 volt deep cycle units will have a little more capacity than 2 deep cycle 12 volt units of the same case size.
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Old 06-19-2021, 03:05 PM   #10
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The Harris batteries probably sat dead, on the lot, for months, replace them all with another brand.

800 AH of battery should run 28 amps for over 24 hours.
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Old 06-19-2021, 10:22 PM   #11
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Lets not start this again. Battery capacity is a function of LEAD...weight, things like that. It is correct that one nice 6 volt deep cycle Trojan will have more AH than a deep cycle 12 volt...but only at 6 volts!

Think of two deep cycle 6 volt units as one big 12 volt battery with 3 cells in each case. Two 6 volt deep cycle units will have a little more capacity than 2 deep cycle 12 volt units of the same case size.
Yes youíre right. 6 volt batteries have more capacity than 12 volt batteries.
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