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Old 06-30-2016, 08:49 AM   #1
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Expanding battery bank - will this properly work in the long term?

I've been researching how to expand my battery bank. My new RV came with 2 100Ahr wet cell batteries with a 1000 watt inverter. The inverter is too small for our needs so am adding a 2000 watt inverter with a built-in 70amp charger. Existing converter includes a 55amp 3 stage charger. From other postings I've confirmed having multiple chargers will work so long as I adhere to an upper limit on amps relative to the battery bank size.

I've tried to read as much as I can find on the web related to batteries and battery banks but it seems there is as much "art" in this field as there is science. General consensus is not to mix battery types, this seems self evident because of the different charging voltages. Since my existing 2 batteries (wired parallel) are only 2-3 months old I don't want to simply toss them away. That restricts me to adding new wet cell batteries. I've read conflicting opinions (???) on adding different size batteries or even different voltage (say 2 6 volt batteries in series). Even same age, size and voltage batteries from different manufacturers may not play together nicely so this leaves me all confused and wondering if I am just chasing my tail.

Where I am at now is adding two similar voltage/size batteries wired up as shown in the attached diagram. To avoid imbalances between the batteries I will keep all interconnecting cables the same gauge/length and run all loads off a buss bar. Hopefully this will alleviate any imbalances developing between the batteries.

Ok all you battery gurus, will my proposed approach work in the long term? I know it will work but will over the long term my batteries become imbalanced and experience a shortened life?
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Old 06-30-2016, 09:13 AM   #2
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I would not toss the batteries that are only a few months old. Adding another pair if same type / capacity / etc. is ok. IMHO.

If adding a new charger, why not disable the existing charger? While they should work cooperatively it seems redundant and unneeded complexity (albeit not much so).
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Old 06-30-2016, 09:23 AM   #3
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Not a battery expert and every time I try to read and study to get an understanding I get more confused. All the experts and novices alike have different viewpoints. I like the way you put it "Art/Science".

Me personally, I've never had a 100% positive outcome mixing batteries, I've done it and nothing catastrophic has ever happened, but not as efficient as all new and same batteries at one time. I'd either stay with what I have and get some use out of the current batteries and perhaps more research in the project and maybe a few bench test before install, or suck it up and change out the lot at one time. Perhaps a local battery shop would engage in some horse trading on your current batteries.

Either way, good luck and post end results, so I can add it to my battery confusion..
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Old 06-30-2016, 09:55 AM   #4
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Add the new batteries to the old, they aren't that old.

Move the old and new connections, in your drawing, to the bus bar instead of each other. Then your good to go.

I don't think a small imbalance is critical to life. You could rotate your batteries once a year to combat it.

People mix old and new batteries all the time. Anytime they replace house batteries and not start batteries the old and new are together.

The BIRD makes one big, uneven, sometimes oddly spaced, battery bank, while plugged in, running the generator or driving down the road.
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Old 07-07-2016, 06:43 PM   #5
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Thanks for all the inputs. I've completed the project and can provide this update which hopefully will be useful for others considering a similar project. See the attached diagram which captures the basic elements of the projects. A few points to make are:

- I had intended to keep all 6 cables connecting the 4 batteries together the same gauge and length but due to tight quarters this wasn't possible. Hopefully the 1.5 feet of additional length on the 2 cables connecting batteries 1/2 and 3/4 will not cause too big of an imbalance amongst the batteries.
- I had intended to connect the negative from battery 3 to the bussbar but due to tight quarters I needed to connect to battery 1. Again hopefully this will not cause to big of an imbalance.
- The new 2000 watt inverter was initially installed using AWG 1/0 cables which was what the manufacturer recommended for runs up to 10 feet. Since I was only about 5 feet I figured I'd be alright. My 1500 watt microwave would pop the 250A breaker despite the inverter only showing a 75% load. I'm guessing the resistance was too much and it was getting hot. I doubled up the wires (used AWG 2/0 for positive as that was all my local auto parts had in stock) and now the inverter handles the microwave with no problem.

I know 420Ahr isn't a huge battery bank but just need to be able to run the 1500 watt microwave for 2-3 minutes occasionally and my 1250 watt coffee maker 1-2 times per day. This setup should provide me 2-3 days of battery capacity based on my expected usage patterns. Hopefully this overview will be helpful to others considering a similar project.
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Old 07-09-2016, 10:29 AM   #6
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Rather than start a new thread I thought it better to add to this thread.

We are moving from a fifth wheel to a MH. The MH has two house 12V batteries with a 1000 watt inverter, batteries 1 yr old.

Month or so ago I purchased a stand alone unit from a tradesman's estate consisting of 4 six volt golf cart batteries, 3000 watt inverter, and huge charger w/battery minder etc. We prefer dry camping and that's why I grabbed the package. The deceased was a car detailer and would plug the system in overnight to charge and then use it for power when working. I also got a small very good air compressor with air hose reel along with a very good extension cord on reel.

I'm a welder so I have the cables and lugs to make any cable combination I want. If I hook up the six volt batteries so that I have two 12 volt banks instead of four six volt the charger will only see 12 volt load, right?

That could allow me to disengage the existing charger and inverter without losing the existing 12 volt batteries, right?
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Old 07-09-2016, 10:50 AM   #7
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Quote:
Originally Posted by Wroughtnharv View Post
If I hook up the six volt batteries so that I have two 12 volt banks instead of four six volt the charger will only see 12 volt load, right?

That could allow me to disengage the existing charger and inverter without losing the existing 12 volt batteries, right?
I'm not exactly sure , what your asking here , but I'll study your question , while scratching my head. In the mean time info on this web site may be of some help to you.

The 12volt Side of Life (Part 1)
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Old 07-09-2016, 09:50 PM   #8
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Thanks, that link will have to be printed out for those days in the woods where we don't have cellular or internet service. Thanks again
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Old 07-09-2016, 10:27 PM   #9
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Quote:
Originally Posted by Wroughtnharv View Post
If I hook up the six volt batteries so that I have two 12 volt banks instead of four six volt the charger will only see 12 volt load, right?
Yes that is the way many MH's are set up. I have 8 6V batteries, essentially 4 12V banks wire parallel. The inverter/charger sees only 12V.
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