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Old 09-16-2019, 11:19 AM   #1
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Battery Bank Connection Advise

So this subject was brought to my attention and I need some advise.

I am looking at installing a 9 battery 12v 150Ah battery bank in my bus for the purpose of temporary Mini Split A/C operation. I know I cant run 100% of my A/C needs off of this bank but some time can be powered by the bank.

Ive got a 1950 watt array to charge these batteries and a 4000w generator and will be installing a 250 amp auxiliary alternator to charge the bank as I drive.

My question from those in the know is if my proposed battery bank connection is ok to use.

I will be connecting 3 batteries in parallel then connect the 3 groups of 3 in parallel.

I will be using 4/0 cable between batteries and then to the inverter, a 3000w inverter/charger.

Excuse my poor drawing but it might add some clarity.
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Old 09-16-2019, 12:25 PM   #2
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Well....

With a 12v 150a/h battery bank your a/c runtime is going to be VERY limited. I would not be surprised if it fails to start the mini split.

There are some great stickies and sharp solar folks over on Forums - Solar Panels - Solar Panels Forum

Good luck.
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Old 09-16-2019, 02:22 PM   #3
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Originally Posted by FamilyCircus View Post
So this subject was brought to my attention and I need some advise.

I am looking at installing a 9 battery 12v 150Ah battery bank in my bus for the purpose of temporary Mini Split A/C operation. I know I cant run 100% of my A/C needs off of this bank but some time can be powered by the bank.

Ive got a 1950 watt array to charge these batteries and a 4000w generator and will be installing a 250 amp auxiliary alternator to charge the bank as I drive.

My question from those in the know is if my proposed battery bank connection is ok to use.

I will be connecting 3 batteries in parallel then connect the 3 groups of 3 in parallel.

I will be using 4/0 cable between batteries and then to the inverter, a 3000w inverter/charger.

Excuse my poor drawing but it might add some clarity.
When you connect large banks in parallel you should connect all + to a single charge wire and all - to a single charge wire, for example if you have 5 positives you should run 5 positive wires to one single point and connect the charge line, same with the negative. This provides the most balanced charge of the bank you can get. This may seem overly complicated but if you get one battery out of balance with the rest you're going to ruin all the batteries. Batteries prefer to be run in series, but obviously that's not practical for most installs.
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Old 09-16-2019, 02:30 PM   #4
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I don't think your idea will work too well! Any time you have an odd number of batteries it makes their interconnection difficult or impossible. Whenever you have more than 2 batteries in parallel per string you may run into issues of uneven charging due to slightly different resistance between the cables - SmartGauge explains it best. Paralleling 9 batteries will not work!

I have about 2kW of panels split into two entirely separate systems, each system consisting of 4 panels powering a Morningstar TS-MPPT-60 charge controller and then charging 4 golfcart batteries in series and parallel. There is no one single charge controller that can send all the current from almost 2kW of panels to your 1350aH of batteries if you're using a 12V system. With a 24V system it's possible, but not for 12V. You need to think through your plan again, and it's wise to do an energy audit first to determine your actual needs which will dictate what size battery bank will be sufficient. Once you know how big the battery bank needs to be, you can then calculate how many amps is needed to charge it at between 5 and 13% of its 20-hour rate (for regular FLA batteries). That in turn tells you what charge controller(s) will be needed.

As mentioned, the NAWS forum is a great resource, as is SmartGauge's wealth of info. Use them!

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Old 09-16-2019, 03:11 PM   #5
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Great Info

Well this has been a great help, looks like 3 series strings of 6 volt batteries is the way to go. Ill abandon the 9 battery set up and just use 6 batteries that are 6 volt each.

Thank you group. saved me some headache.
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Old 09-16-2019, 03:38 PM   #6
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I thought maybe I had misread so I went back and reread the op. I read it right.

Do you have a 12v 150a/h battery bank or a bank made up of 12v 150a/h batteries?
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Old 09-16-2019, 04:15 PM   #7
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BAttery bank Change

I was planning of 9 12 Volt batteries in 3 strings of 3 batteries set up in parallel but that is not a good idea so im going to switch 3 strings of 6 volt batteries in series for a total of 6 batteries.

should be 6 batteries of 6 volt and about 360 Ah to produce 12 volts and 1080 Ah's
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Old 09-17-2019, 03:12 AM   #8
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Quote:
Originally Posted by FamilyCircus View Post
I was planning of 9 12 Volt batteries in 3 strings of 3 batteries set up in parallel but that is not a good idea so im going to switch 3 strings of 6 volt batteries in series for a total of 6 batteries.

should be 6 batteries of 6 volt and about 360 Ah to produce 12 volts and 1080 Ah's

Are the 6V batteries each 120Ah?


When you connect batteries together in series, the voltage adds up. The current rating and capacity does not.


Using 120Ah @ 6V batteries, each string will have 120Ah @ 12V. Three such strings wired in parallel will produce a battery bank of 360Ah @ 12V.


If the individual batteries are 360Ah @ 6V, then your battery bank (3 parallel strings of 2 series wired batteries per string) will be 1080Ah @ 12V.
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Old 09-18-2019, 11:08 PM   #9
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If you haven't purchased anything yet, I would choose to run the bank at a higher voltage and sacrifice the charging from the alternator. Especially with that many watts and that large of an inverter.

Also better to buy bigger cells and run less in parallel.
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Old 09-19-2019, 10:23 AM   #10
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Credit to Forums - Solar Panels - Solar Panels Forum a great source of solar info.


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Parallel Battery Connections
11-16-2014, 11:57 AM
There are three ways to parallel batteries for Cycle Use. One is the Conventional Method seen below, two is Diagonally as seen in the link , three seen below as Alternate if you are using single cells. Example 2-Volt Lead Acid or 3.2 volt Lithium cells

First a little background. When you parallel batteries, it is almost impossible to balance Battery Internal Resistances, Cable and Connector Resistances. This forces one string to do most of the work. It weakens the strings and results in early failure.

There are only two reasons to parallel batteries.

1. To achieve a desired AH capacity. Problem is a lot of folks who come here fail to do homework. Rather they go and buy a bunch of 12 volt 100 AH batteries. Example 16 of them to make a 48 volt 200 AH battery. Parallel batteries should be avoided. Folks are stuck in 12 volt boxes. In this example all they had to do is buy 8 6-Volt 200 AH batteries resulting in a single 48 volt battery string. Well here is a fact unless you you need more than 4000 AH there is no reason to parallel battery strings as they make 2 volt cells up to 4000 AH

2. Mission Critical Applications to allow you to take one string off-line for maintenance. In this approach, no more than two parallel strings. Note it takes a special disconnect switch and a extra charger to do this. You must equalize the string voltage before connecting. Otherwise if you have more than 1/4 volt difference between strings, you will weld the disconnect contacts together.


Edit Note:

The Alternative method below I stumbled onto about 10 years ago when I started building my own racing golf carts and helping friends build EV's. Every Commercial EV manufacture and DIY EV guys if they Parallel Batteries use the Alternative method below. Caution it can only be done with Single Cell Batteries like 2-Volt Lead Acid Cells, and Lithium Cells. It will not work 4,6,8 0r 12 volt batteries made from more than 1-cell. So any of you with Lithium should be wiring as below for many reasons.

OK the PROs are many, and few CONs: On the Pro side it gives you full redundancy as conventional. If any one cell fails, you can still make it home or keep the lights on until repairs can be made. Internal Resistance is Automatically Balanced at the cell level, so there is no fancy cable arrangements, or buss bars to fuss with to balance current and resistance. If you happen to using Lithium batteries with BMS would require only 4 Circuit Boards as opposed to 8 circuit boards in this example. The only Con I have come up with is maintenance could be more labor intensive if you have to replace a cell with the extra wires paralleling the cells. Myself I would use buss-bars for all Inter-cell connections.
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Old 09-19-2019, 11:02 AM   #11
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It doesnt matter how many batteries. It matters that you are always drawing/inputting current through all of them. Your design has "hot spots" that will cause certain batteries to die before others.



In these areas you have no current running though the batteries on the end points so the current in them must will always be back filling the first one in line and then charge after the first one tops off.

A better design would be



That way all current must flow though all batteries evenly.
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Old 09-19-2019, 11:42 AM   #12
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My bank is only about 2/3 the capacity of what I am working on. I found a fairly good fit for my energy budget with four L-16's in series. No parallel batteries. No equalization troubles. No premature battery death...

I suspect that you could find a solution similar. Just a bit bigger battery.
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