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Old 09-22-2016, 03:02 PM   #1
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What's wrong with my Battery Bank?

This is a fork off of this thread: Using 24v to get better 12v load performance?

The Gist: I'm getting an abnormally large voltage drop when turning on devices on my 12v system and I suspect one of my 4 6v batteries (wired in series-parallel) is bad. My experience with running our 3-way fridge on DC was the first indicator of this, and further evident by my experience in this thread about low voltage alarms going off with a relatively light load.

The Test: I ordered a Battery Load Tester, but until I get that to do proper load testing I did some preliminary testing with a standard volt meter and the various electrical loads in our AS.

1st Case - No Load
12v system voltage with no load: 13.4v (just unplugged from shore power)
Battery 1 (Bottom Left): 6.70v
Battery 2 (Bottom Right): 6.75v
Battery 3 (Top Left): 6.68v
Battery 4 (Top Right): 6.51v

2nd Case - Some Load
I turned on 16 LED lights (varying wattage), a composting toilet circulation fan and the BMS display.

12v system voltage with this load: 11.4v
Battery 1 (Bottom Left): 5.23v
Battery 2 (Bottom Right): 5.78v
Battery 3 (Top Left): 6.12v
Battery 4 (Top Right): 6.5v

Picture of Battery Bank:

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Diagram of 12v Electrical System:

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THE QUESTION: Is it evident which, if any, of my batteries are bad?
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Old 09-22-2016, 03:07 PM   #2
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First reply!

Did you notice your voltages don't match. How about that!
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Old 09-22-2016, 03:11 PM   #3
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Quote:
Originally Posted by BoldAdventure View Post
First reply!

Did you notice your voltages don't match. How about that!
Yeah, how 'bout that? Interesting... did you notice that the battery that seems to be in the best condition is the one with the power lead coming off of the charger? Oh, and the battery that seems to be in the worst condition is "furthest away" from the charger power lead chain? INTERESTING indeed, very very interesting.

I try not to lead the conversation by talking about the conclusions I've already come to; I want to hear everyone else's input, not the voice in my head. I respect the 1st reply status nonetheless Mike!
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Old 09-22-2016, 03:14 PM   #4
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Looks like you have one marginal... Have each tested standalone. No juice no use.

If depending on DC extensively, might be time to replace all... Cost of batteries surely out weighs paying for RV hookup ...cost of living off the grid... So to speak...
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Old 09-22-2016, 03:23 PM   #5
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In the picture it looks like top left is in series with bottom right and top right is in series with bottom left, not what your diagram shows. When you add the voltages up that way they are pretty close, but not exact. You may have one or more cables with some excess resistance or corrosion.

Based on your voltages, and assuming you measured right at the battery terminals, both bottom batteries are bad and top left isn't great. With the load you applied there should be very little drop from the unloaded voltages.

Alternative is the weak ones aren't fully charged. Only a hydrometer can tell you accurate state of charge.

On the surface I'd replace at least three and probably all four batteries.

Al
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Old 09-22-2016, 03:32 PM   #6
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Quote:
Originally Posted by Al and Missy View Post
In the picture it looks like top left is in series with bottom right and top right is in series with bottom left, not what your diagram shows. When you add the voltages up that way they are pretty close, but not exact. You may have one or more cables with some excess resistance or corrosion.



Based on your voltages, and assuming you measured right at the battery terminals, both bottom batteries are bad and top left isn't great. With the load you applied there should be very little drop from the unloaded voltages.



Alternative is the weak ones aren't fully charged. Only a hydrometer can tell you accurate state of charge.



On the surface I'd replace at least three and probably all four batteries.



Al

These batteries we purchased in December of 2014. During that time they've always been on a charger (except once when I had someone doing some work and they drained them down to 8v, ugh). Hard to believe they are done already, yet the evidence is before me.

Well, I've been looking at going the lithium route anyway, maybe it's about that time...
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Old 09-22-2016, 03:35 PM   #7
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Quote:
Originally Posted by Al and Missy View Post
In the picture it looks like top left is in series with bottom right and top right is in series with bottom left, not what your diagram shows.

Might just be the quality of the picture or that it's misleading because the connecting cables are under the red power leads, but the bank is wired exactly like the diagram.
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Old 09-22-2016, 03:41 PM   #8
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I'd say battery 1 & 2 are worn out and #3 is borderline but you will have to replace all of them if you want the bank to work right. All the batteries should read the same if all your connections are clean and tight. You could check each cell with a hydrometer to see if some are diluted too much for some reason but really it's better to start over with new batteries. They should all have the same manufacturing date and all cells should have the same specific gravity. You should check the cells with a hydrometer every 3 months or so and keep a log if you want to keep ahead of failures. Also I'd think running the refrigerator on battery would be a heavy load, 10 to 15 amps if it's an adsorption type gas/electric, and you would be lucky to get any battery charging done while driving with the fridge on 12V. Just my 2 cents worth!
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Old 09-22-2016, 08:40 PM   #9
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Hi Trekerboy. That drain to 8 volts by your dealer or whoever did that is concerning as you already know. I know you want to forget that but if it happened, well...it's hard to undo.
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Old 09-22-2016, 09:52 PM   #10
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Originally Posted by trekerboy View Post
Might just be the quality of the picture or that it's misleading because the connecting cables are under the red power leads, but the bank is wired exactly like the diagram.
I didn't look close enough, sorry. However, that may give us another clue. There is no way that the sum of the voltages in each of the two banks should be different if they were measured as connected under load. Did you separate the banks prior to making the measurements under load? If not you may have one or more bad cables in addition to bad batteries. Or bad cable(s) may be contributing to bad charging. If you can, put a 6V charger on each of the batteries independently and re-run your tests.

I meant to note that your wiring configuration should give you the best balance possible. Both of the parallel banks should have the same cable resistance for charging and discharging.

Al
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Old 09-23-2016, 07:04 AM   #11
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Fortunately, where I bought my batteries (Sam's club) has a 3 year replacement policy/warranty, so I'll just take them all back and get new ones.



Duracell® Golf Car Battery - Group Size 230

Now, I just have to figure out what caused this to happen in the 1st place so I don't have it happen again. Anyone see anything obviously wrong with my wiring? It's 4/0 gauge with fittings attached with solder.

What does it say about my setup that the battery closest to the charger is in the best shape, and the battery furthest away in the chain is in the worst shape? Charger issue? Cable issue?
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Old 09-23-2016, 07:17 AM   #12
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Your charger is not going to charge some batteries OK and the others not; it puts out voltage and current, period.
Who did your connections? 4/0 is usually crimped for solid connection. To solder you would need a torch to get all of the wire hot enough to hold rosin core solder and the lugs would be glowing red during that process. I did not see burned insulation so I wonder about those cables.
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Old 09-23-2016, 07:54 AM   #13
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Did you separate the banks prior to making the measurements under load?
Yeah, similar question here. If all those batteries are connected to each other via those nice fat 4/0 cables, how are you measuring the voltage on each battery? Given good connections, I'd expect them to all measure the same...unless you're disconnecting them before reading them. So, if they are giving you different readings while connected, I'd take a good close look at the cables connecting those batteries.

<edit> BTW, we are also using 4/0 cables to connect our two Group 27 batteries in parallel, and they are crimped not soldered. Used a huge crimping tool at the shop where I bought the cables and connectors, as it's their policy to let customers make cables at a special counter once the materials are bought.</edit>

Oh, and I'd replace the batteries with something else, as you planned to do anyway.

Last silly question: What are the water levels like in each cell of each battery, and what did you get for specific gravity readings in each cell? When we had a couple of Group 24 batteries die, it was because one cell in each battery had gone completely dead. This condition was *only* detectable via a specific gravity check.
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Old 09-23-2016, 02:50 PM   #14
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Looks like you have one marginal... Have each tested standalone. No juice no use.

If depending on DC extensively, might be time to replace all... Cost of batteries surely out weighs paying for RV hookup ...cost of living off the grid... So to speak...
I concur. Test each one. if you have no way to test, take it to a local automotive store to get it tested.
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