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Old 05-28-2013, 10:17 PM   #1
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Bad battery bank connection?

My house battery bank has six batteries: 6 volts each, wired with three PARALLEL sets, each set consisting of two batteries in SERIES (I.e. three parallel 12 volt sources). I noticed this last weekend that both batteries in one ( and only one) of the parallel sets are showing more signs of usage than all of the other batteries (meaning more water loss and "grungier" appearing surface of the electrolyte). I am thinking that this might be an indication of some bad connections in the wiring causing reduced current to the other two parallel sets of cells. Does anyone have some similar experience or alternate thoughts???
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Old 05-28-2013, 10:30 PM   #2
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I have noticed that on my batteries also. Every spring I take all the cables off and dip the ends in a baking soda solution then put baking soda solution on the battery posts then wash it all off and wire brush all. I then reconnect all and spray the connections with battery terminal protection spray It maybe time to do that on yours. I take a picture with my cellphone before just to make sure I hook all cables back the same way. You might want to also move those batteries to a different position
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Old 05-28-2013, 10:38 PM   #3
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When we purchased our MH I found that the original house batteries had similar issues and we decided to replace all four. Inspection of the cables showed significant acid intrusion that made the terminal connection poor at best. Soaking them in baking soda did not help. I had new cables made and the new batteries are performing well. Also make sure the 6v are connected correctly as the proper connection for the lowest IR loss through the cables is not a simple series connection of 3 batteries and then paralleling the sets. Plenty of info out there for the correct cable pattern.
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Old 05-28-2013, 10:42 PM   #4
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I don't have experience in your particular scenario, but I'm an electrical engineer and deal with something similar on smaller scale batteries.

Are all the batteries the same age/vintage? Same model?

The concept that batteries "age" in different conditions is called "balance". As the sets age, various things make some batteries perform better than others.

So two big factors that could make your batteries get out of balance:
1) Different ages or models of batteries.
2) How the batteries are connected. I understand you've got 3 parallels of 2 x 6v (series) batteries, but how are the loads distributed across the sets? Are all 3 grounds and all 3 positives bonded together? Are the cables between the battery sets of the same length and material?
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Old 05-28-2013, 11:23 PM   #5
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No experience. But what I know is, if you have bad batteries in the same bank, they will make the good ones overcharge. The basic rule is, all the batteries in the same bank must be of the same type and age and basically be identical.

I suppose if you have bad connections on a set, that will cause problems. I have no experience beyond 2 6v in series. If you have unanswed questions about your bank, take them to a local battery shop and have them all load tested.
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Old 05-29-2013, 09:13 AM   #6
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Quote:
Originally Posted by cb1000rider View Post
I don't have experience in your particular scenario, but I'm an electrical engineer and deal with something similar on smaller scale batteries.

Are all the batteries the same age/vintage? Same model?

The concept that batteries "age" in different conditions is called "balance". As the sets age, various things make some batteries perform better than others.

So two big factors that could make your batteries get out of balance:
1) Different ages or models of batteries.
2) How the batteries are connected. I understand you've got 3 parallels of 2 x 6v (series) batteries, but how are the loads distributed across the sets? Are all 3 grounds and all 3 positives bonded together? Are the cables between the battery sets of the same length and material?
That is the kind of weird thing here: the batteries that are showing the most "exercise" are the ones with the longest physical cable distance between the batteries and the load (with a connection made to the bus via an intermediate battery terminal). This is what makes me wonder more about the health of the contacts to the other batteries versus the cable-to-cable connections....

I think I just need to clean and re-connect everything. Don't know how I am going to find the time to do that though: might need to leave it up to my local RV dealership/shop.....
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Old 05-29-2013, 12:13 PM   #7
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... the proper connection for the lowest IR loss through the cables is not a simple series connection of 3 batteries and then paralleling the sets. Plenty of info out there for the correct cable pattern.
ALAN24601: are you able to clarify this please (or supply links to these discussions - I can not find them). Please see diagram (actually attached file) below, which is standard in the discussion I do find, and is the way my bank is wired. Is there a better way to wire to reduce IR loss??
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Old 05-30-2013, 12:20 AM   #8
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Just looking at this diagram connecting the load at the center connection would reduce the IR loss by half. I am not sure where I saw the diagram but I believe in my MH there is an extra cable connected to the load end from the + and - terminals at each end of the series stings. I believe that will reduce loss by a factor of four, I square x R. Gotta love the iPad when you need to write something out of the ordinary.
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Old 05-30-2013, 12:56 AM   #9
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Check to see where the main negative cable and positive cable that leads to your coach is connected. The positive cable should be connected to the first battery in the system. The negative battery cable should be connected to the last battery in the system. This way, everything is drawn through all six batteries. It will work by picking up a negative and positive connection in other places, but it works best as described above, drawing power through all batteries.
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Old 05-30-2013, 01:40 AM   #10
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Do a specific gravity check and load test on each battery to ensure not a poor one in the bunch. Clean all cables and terminals. Use terminal protector spray.

Do the above yearly, more often if a performance or other issue suspected...
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Old 05-30-2013, 03:13 AM   #11
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'nuther electrical engineer here. I would think a good idea would be to go straight to a battery shop and have them do a load test. You *could* have a bad cell in one of the batteries.

But if it is in 2 batteries in the same pair, that seems to suggest something other than a bad cell(s) as it would be odd for the cells to go bad in each of the 2 pairs of batteries at the same time.

If your batteries are only from 2013, then I think I would suspect poor connections more than a bad battery(s).

But I have to say that sometimes figuring out electrical issues like this can be one of the hardest things there is and you are better off to just go to a reputable shop that has the right diagnostic equipment and expertise. Sometimes it's just not worth trying to figure out by yourself what those pesky little electrons are doing.....
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Old 05-30-2013, 06:36 PM   #12
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Quote:
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'...sometimes figuring out electrical issues like this can be one of the hardest things there is and you are better off to just go to a reputable shop.....
But it is so much FUN trying to figure out what is going on

Ok ... I will admit I am an engineer also (Chemical Engineer - but have pretty extensive background in EE, at least the basic stuff) ... so I have a question for ALAN24601: if you connect directly only to the central bank, would'nt you then be biasing the LOWEST IR drop to that central bank, and thus over-exercise it (relative to the other two banks)?? Isn't that the idea of the diagram above, where the AVERAGE IR drop through the combined + and - legs becomes equal for all banks???

The above is mostly just a "fun" question: I tend to agree with MYREDRACER in that given that I DO have TWO cells in the same bank behaving the same, I strongly suspect my solution lies in a wire brush, chemicals, and a little elbow grease
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Old 05-31-2013, 11:57 AM   #13
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If you connect the load to the center of the 3 battery string you do get the lowest average IR drop for that cable connection solution. One big battery would have the lowest cable loss so connecting multiple batteries to emulate as closely as possible a single battery should be the best solution.
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Old 05-31-2013, 12:24 PM   #14
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As suggested. Cleaning until shiny and bright and measure the specific gravity. You could have one of the 6 volt batteries very sick. I too like a load test if you have the equipment.

What is the age of the batteries. We don't really buy them, we just tend to rent them for a time and that varies. Most batteries in RV's are murdered by poor maintenance and abuse.

I'm not an engineer but a Certified ET with 40 yrs working on vehicle electrical systems. We do high tech equipment installations in every kind of vehicle you can imagine.

From U2 chase cars to helicopters.
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