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Old 04-18-2021, 09:31 PM   #21
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Originally Posted by Nic7320 View Post
I'll agree with your diagram, since that does balance the two battery cable resistances if it's wired that way.
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But if he decides to add a third battery, that's when matching cable sets to balance batteries won't work.
LMGTFY:

SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

Summary:
The pattern I showed is indeed not optimal for three batteries, but it is good enough FAPP. There are optimal solutions for this (and all other) cases, at the expense of some complexity. They become important with very large banks and/or very low internal resistance batteries.
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Old 04-19-2021, 06:48 AM   #22
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Someone else might have mentioned this, but you can always balance the batteries by using a cable that is about as long as the one that connects to your other battery but just keep it coiled up next to it. As long as the parallel runs are equal you will be fine.
Just snake the excess wire length of 2nd hookup, don't coil it.
This should mitigate the magnetic concerns.
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Old 04-19-2021, 04:13 PM   #23
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I was going to post a new question but reading this posting it appears that I may have done something not quite right but unsure how to fix it the correct way. I want to wire up THREE 12V AGM batteries in parallel but I don't have room for all three batteries near each other. The original battery compartment is on the passenger side of the RV. It is now holding ONE 12V AGM 115A battery (battery ONE) wired to the inverter nearby. Battery cables are 2 gauge. Length of the original cables are about 2FT to the inverter.

I have room to install 2 additional batteries on the opposite side of the RV in another compartment. The run from the passenger side battery (Battery ONE) to the Battery TWO on the opposite side would be 6FT and the the run from Battery TWO to Battery THREE would be 2FT.

Can I make this work? What should the battery cable run/diagram look like to make this work?

Thanks for your time and consideration. Bill
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Old 04-19-2021, 06:08 PM   #24
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I was going to post a new question but reading this posting it appears that I may have done something not quite right but unsure how to fix it the correct way. I want to wire up THREE 12V AGM batteries in parallel but I don't have room for all three batteries near each other. The original battery compartment is on the passenger side of the RV. It is now holding ONE 12V AGM 115A battery (battery ONE) wired to the inverter nearby. Battery cables are 2 gauge. Length of the original cables are about 2FT to the inverter.

I have room to install 2 additional batteries on the opposite side of the RV in another compartment. The run from the passenger side battery (Battery ONE) to the Battery TWO on the opposite side would be 6FT and the the run from Battery TWO to Battery THREE would be 2FT.

Can I make this work? What should the battery cable run/diagram look like to make this work?

Thanks for your time and consideration. Bill
Find the recent thread on "isolator" and go to post 12. It shows a battery cable junction lug. Get 2 of these: 1 for +, the other for -. Attach equal length battery cables from each battery to the lugs. Then run equal length cables from the lugs to the inverter. Make sure the cables from the lugs to the inverter are sized for the full inverter load.
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Old 04-19-2021, 06:16 PM   #25
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Originally Posted by Rockwood27 View Post
Find the recent thread on "isolator" and go to post 12. It shows a battery cable junction lug. Get 2 of these: 1 for +, the other for -. Attach equal length battery cables from each battery to the lugs. Then run equal length cables from the lugs to the inverter. Make sure the cables from the lugs to the inverter are sized for the full inverter load.
Yep. That is optimal for most any configuration or number of batteries.

Next best would be to do the three-battery version of the illustration I showed above. Unlike the two-battery case, this is slightly non-optimal, but good enough for most situations, and often easier.
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Old 04-20-2021, 03:23 AM   #26
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...

I have room to install 2 additional batteries on the opposite side of the RV in another compartment. The run from the passenger side battery (Battery ONE) to the Battery TWO on the opposite side would be 6FT and the the run from Battery TWO to Battery THREE would be 2FT.

Can I make this work? What should the battery cable run/diagram look like to make this work?

Bill
First question is, are you doing this for more hours of operation or more peak current to run a large appliance?

Any parallel configuration can work, but they won't be optimally balanced and can also limit your maximum current, like running a microwave. Here are some methods used:

The 'home run' configuration the OP illustrated is the lowest resistance solution and can make cable routing to separate batteries easier. Effectively, the battery's internal resistance and the cable resistance all get cut in half. For three batteries, it's even better, it's 1/3 of the original resistance. But it takes some math to balance the cable resistance (simple high school algebra) and uses more than one size of wire, if the batteries aren't all at the same distance.

Aventi's suggestion -- coming from industry -- is by far the simplest and most fool-proof for pairs of batteries, and can work with any even number. Use matched cables in sets and wire corner to corner all charger and load connections. And you have to use the opposite corners or it just becomes a parallel string.

Another way is a brute force parallel string. Use large enough cables and battery balancing issues become less. A lot of home builds and some pro upfitters do it this way, but large wire gets expensive and very hard to assemble.

Then there's a final option. Just wire them up and not worry about balancing. Your battery life won't be quite as much, but for your smaller loads, you still get the benefit of more hours and more battery capacity. The maximum current will be less and your charger might not be able to fully charge the last battery in your string; so it might be coasting while the others do all the heavy lifting. But it can still work.


I did a three battery configuration (using three Interstate 31MHD's) in my Pleasureway, with one in the lower tray and two next to the inverter. To connect the upper pair, I took copper tubing, cut it and flattened the ends, drilled a hole in each end, and used them for battery cables (no crimping lugs or wire needed). It provided a solid mechanical and electrical connection between the pair. For the lower battery, I used an existing cable and kept it as short as possible. It ran my microwave and that was the goal. No generator!

As a last note, It's best not to mix lead acid battery sizes or ages, since parallel configurations rely on the battery's internal resistance to share current equally. It's not 100% necessary, but if you want the most from each battery, they should all work in concert and do the 'heavy lifting' together. But also, don't ever mix lithium and lead acid. Those are two completely different chemistries and different charge profiles.

My current rig uses 74 batteries in parallel, stacked with 5 more sets of 74, for a total of 444 batteries. It's a Li-ion module from a Tesla using 18650 batteries. Weighs 55 lbs and has the usable capacity of 8 or 9 car batteries. Fortunately, Tesla wired the module and there's only two big wires to connect, plus a small balancing cable. Their balancing is actually stack to stack voltage balancing, and not the wire resistance balancing we are discussing here.
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Old 04-20-2021, 01:17 PM   #27
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Aventi's suggestion -- coming from industry -- is by far the simplest and most fool-proof for pairs of batteries, and can work with any even number.
Why do you say "...can work with any even number"?
I don't see any difference between even and odd numbers.

As far as I know, the ONLY reason that this pattern is slightly less than optimal with more than two batteries (it IS optimal with two) is because the buss wires on the outer batteries carry more current than the ones near the center, which causes the center battery or batteries to deliver slightly fewer amps. If even vs odd number of batteries matters, I can't see it.
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Old 04-20-2021, 02:15 PM   #28
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Wow, we need to rename this discussion "how to make the simple ridiculously confusing". There seems to be a lot of that lately.

For two batteries Avanti's drawing is the go to, and for the OP the best way to go, IMO.

All you need to do is measure current path lengths for the batteries and keep them as close the same for each battery, no matter how many you have of them. If using a buss it has to be quite large capacity if even if you have equal lengths to it. That problem doesn't happen with binding post combining, which isn't as clean as a buss sometimes but simpler. Upsizing the cable from the binding post to the inverter or loads is normal to do because on increased current unless quite short.

The most clever use of binding post method that I saw was the use of a single binding post for 3 batteries that were spread out around the RV. Centrally located in an easy accessible spot in the middle, under the van in this case, with one big cable to loads/inverter which was in a fourth different location. Negatives for each battery and the inverter went to chassis at each source. Perfect balance in a system with all the parts spread out, and still a clean installation.

If the OP has an accessible point to easily put a binding post midway between the two batteries, they could do similar if it made the wiring easier than going all the way between them with a second cable. Take the original cable and use it to connect the two battery positives together, then cut it and put a binding post in the it at the midpoint. Run a size or two larger cable from the loads/inverter to the binding post. Grounds all to chassis. done.....

The above would also make adding a third battery much easier in the future, I think, if that is a consideration.
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Old 04-20-2021, 04:59 PM   #29
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Why do you say "...can work with any even number"?
I don't see any difference between even and odd numbers.
See method 4 of your LMGTFY. It's two pairs of two cross connected batteries for a total of four batteries. Similarly, you can connect two pairs of three batteries (using a star point connection on three batteries) to achieve the same balance.

But you always have only TWO corner connections and two feed wires, so to achieve balance you can only achieve balance with EVEN numbers.

I do like it, it's a simple and elegant solution, but it adds resistance to the circuit. Your total loop resistance for each battery is:
Positive feed wire + Internal battery resistance + Cross connect wire + Negative feed wire.
This works great when the batteries are close to each other, because your cross connects are kept short.

But the OPs method eliminates the cross connect wire resistance from the loop. PLUS: every time you add another battery, it effectively parallels more wires back to the inverter/charger, achieving even lower resistance of the feed wires.

So let's look at the math to see if the OPs method can balance. The resistance per foot for copper wire you get from a lookup table online.
Side note: Make sure you're actually using copper wire and not something else. I bought some cheap jumper cables thinking they were a good source for large gauge copper wire, but they were copper plated aluminum, so they didn't follow the tables for copper. Similarly, you'll find jumper cables that only look like big wire but it's actually a small wire with lots of insulation there to fool you.
Resistance per foot x feet of feed wire = total wire resistance.

Round trip resistance = 2 x wire resistance (using same gauge wire, and ignoring the battery's internal resistance)


For the longer run it's the same equation, but with more feet of wire:


To balance two legs, you chose a larger gauge wire that matches the resistance of shorter wire. Three, four, or more batteries, just continue to match the resistance:


6 AWG @ 2 feet = 1.58 mOhms round trip.

0 AWG @ 8 feet = 1.57 mOhms round trip.

This now makes sense, since the ratio of the 8 foot and 2 foot legs is 4:1, so the wire gauge has to be the inverse: 1:4 (#6 to #4 to #2 to #1 to #0 is also four steps of increasing diameter).

Normally, 6 AWG would be considered too small for a battery, but this is an example where a really short wire doesn't need to be as large as a big battery cable. "Ampacity" tables are available in two forms, one for longer runs, and one for interchassis wiring. A short wire doesn't have to follow the long run ampacity table because its resistance is kept small.

Now compare that with cross-connect method, if you need to span a 10 foot long cross connect between two batteries, the wire will add more resistance to the loop. To make this anywhere comparable to the OPs home-run method, you'd need really, really (really!) large cross connects. I'm not saying this can't be done, but it is a big trade-off. Actual cable routing pathways and big things in the way often determine what wiring method you CAN use, so his routing will be ultimately dictated by his vehicle layout.

A 0 AWG cross connect that is 10 feet long adds 0.98 mOhms to the loop.
That's really a really big number compared to 1.57 mOhms. So let's try even bigger wire:

00 gauge is 0.78 mOhms,
000 gauge is 0.62 mOhms, and
0000 gauge is 0.48 mOhms -- which is still a large number compared to 1.57 mOhms and huge 0000 gauge wire is nothing you want to deal with.

So it can't be practically done without a big penalty. But it achieves balance -- without doing ANY math. That's why it's recommended.

From what I see here, the cross connect method has a serious flaw when the batteries are far apart, but a real benefit when they're close to each other.

There's more to this if the OP provides any specific constraints of his rig, so I'm not ready to say quod erat demonstrandum.
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Old 04-20-2021, 05:49 PM   #30
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But you always have only TWO corner connections and two feed wires, so to achieve balance you can only achieve balance with EVEN numbers.
Huh?

I am out of this one.

Here's my parting shot:
If there is anyone left following this thread who actually wants practical advice on this topic, my recommendation is the same as it was in the beginning: The pattern shown in message #17 is just fine with 2, 3, or 4 batteries. If they are far apart, you might want to go up one size wire for the interconnects.
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Old 04-20-2021, 08:18 PM   #31
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Huh?

I am out of this one.

Here's my parting shot:
If there is anyone left following this thread who actually wants practical advice on this topic, my recommendation is the same as it was in the beginning: The pattern shown in message #17 is just fine with 2, 3, or 4 batteries. If they are far apart, you might want to go up one size wire for the interconnects.

I couldn't agree more....
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Old 04-23-2021, 01:25 AM   #32
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Default retraction, and apology.

Quote:
Originally Posted by avanti View Post

LMGTFY:
SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

Method 3 is what the OP illustrated. He just doesn't have matched lengths.

Summary:
The pattern I showed is indeed not optimal for three batteries, but it is good enough FAPP.
Actually, I was wrong and your conclusion from that is also wrong; apparently when I made you question it, you saw it incorrectly as I did. Three batteries do work (and they work optimally). Any number works.

I did some math. Then I redrew the diagram vertically as a ladder.

EVERY battery in a ladder sees the same distance (and therefore the same resistance) from the upper corner to the lower opposite corner, regardless what rung it traverses. And there can be missing rungs (i.e. any distance between batteries) and they all add up to the same distance (and resistance).

Quote:
Originally Posted by avanti View Post
As far as I know, the ONLY reason that this pattern is slightly less than optimal with more than two batteries (it IS optimal with two) is because the buss wires on the outer batteries carry more current than the ones near the center, which causes the center battery or batteries to deliver slightly fewer amps. If even vs odd number of batteries matters, I can't see it.
Since they all see the same resistance corner to corner, they all put out the same current, regardless of position. We were just looking at it wrong.

In engineering school, this is all done with Kirchoff's current and voltage laws. Isc divided by Voc give you the resistance, and that all worked out the same all the way up and down your ladder.

The only problem is the extra resistance of the cross connects to do it this way. But those can obviously be larger.

If my contributions here are not appreciated, delete all my posts. There are other places that both want and need engineering guidance.
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Old 04-23-2021, 01:32 PM   #33
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There are other places that both want and need engineering guidance.

I find that a very interesting comment, as this forum (from what I have seen anyway) has much more of a reputation for having high tech, engineering level if you choose that term, information in comparison to most other RV based forums. Discussions here have ranged into the heavily data driven range and theoretical and we have learned a lot over the years. This forum is much more likely to be criticized for being too technically oriented than not enough.


Engineering or tech information is only as good and well received as it is presented and backed up with data and quality references. As soon as relatively simple things are infused with confusing and overly complex information, be it right or wrong, it gets common to question if the writer really understands the topic or is just throwing out "stuff".


I can say from personal experience that non mainstream ideas certainly will get highly (and rightly) questioned by others on here and that is the way is should be if we want to move forward and all learn.


The less tech oriented who come here to get the information they can't get elsewhere deserve to be given that information clearly and concisely in understandable terminology for them, so filling up threads of that type with other stuff will just send them away. We need to strive for good, usable, correct information.



This is a all off topic and if the mods choose to delete or move it, fine with me.
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Old 04-23-2021, 02:36 PM   #34
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I am not going to delete anything, but if there are any more off-topic comments, I will lock the thread.

That said, (and despite my claim that I am "out"), there is a need to correct the technical record:

Quote:
Originally Posted by Nic7320 View Post
Actually, I was wrong and your conclusion from that is also wrong; apparently when I made you question it, you saw it incorrectly as I did. Three batteries do work (and they work optimally). Any number works.

I did some math. Then I redrew the diagram vertically as a ladder.

EVERY battery in a ladder sees the same distance (and therefore the same resistance) from the upper corner to the lower opposite corner, regardless what rung it traverses.

Since they all see the same resistance corner to corner, they all put out the same current, regardless of position. We were just looking at it wrong.
This is simply incorrect.

If you just "count links" you can convince yourself that all the battery positions are equivalent. But, this amounts to analyzing each battery as if it were alone in the circuit. It fails to account for the interactions among the batteries when they are all operating together. As I explained earlier, the links between the batteries carry different currents, which means that they have different voltage drops, even though the lengths are identical. If you were to run a simulation, or do a proper analysis of the entire circuit at once, this would become obvious. With two batteries it is optimal, with more than two, it is not (although often close enough).

Here is a link that provides a quantitative analysis of this effect:
SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank

My original advice stands.


Rant:
This kind of thing really points out the dangers of trying to reason from general principles when you are going against best practice in a mature industry. MAYBE you will discover something that nobody has ever noticed before, but the odds are very, very much against you. Before taking out your calculator, it is always best to start by fully-understanding what the professionals believe.
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Old 04-23-2021, 03:43 PM   #35
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Here is a link that provides a quantitative analysis of this effect:
SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank
And this is one reason why I love this forum. Here is an explanation from a pro that is understandable and easy to apply. I would never have found this on my own.

Countless times I have obtained such knowledge from links provided and from various forum members.
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Old 04-23-2021, 04:07 PM   #36
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The nice thing about method 3 in the referenced material is that the "terminal posts" need only be bolts through ring terminals. No physical landing-point needed. Just bolt them up and wrap with several layers of electrical tape.
Have done this on several 5th whl gone past.
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Old 04-23-2021, 04:13 PM   #37
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The nice thing about method 3 in the referenced material is that the "terminal posts" need only be bolts through ring terminals. No physical landing-point needed. Just bolt them up and wrap with several layers of electrical tape.
Have done this on several 5th whl gone past.
Agree.

Also, method 4 is pretty easy if you use battery-mounted MRBF terminal fuse blocks such as these:





https://www.bluesea.com/products/215...k_-_30_to_300A
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Old 04-26-2021, 02:48 PM   #38
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The SmartGauge link was very helpful. I have only two batteries, but for convenience was tapping my inverter from a different positive post then the rest of the coach connection and my negative cross-connect wire was shorter than my positive cross-connect wire.

The article made clear that the difference in current balance created by even small differences in resistance can be quite substantial. Since I was already working on the batteries this weekend I went ahead and cut a new cross-connection wire that is exactly the same length as the other one and made some adjustments to the other cables so that I tap the same positive post for all connections.
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Old 05-03-2021, 11:34 PM   #39
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The SmartGauge link was very helpful. I have only two batteries, but for convenience was tapping my inverter from a different positive post then the rest of the coach connection and my negative cross-connect wire was shorter than my positive cross-connect wire.

The article made clear that the difference in current balance created by even small differences in resistance can be quite substantial. Since I was already working on the batteries this weekend I went ahead and cut a new cross-connection wire that is exactly the same length as the other one and made some adjustments to the other cables so that I tap the same positive post for all connections.
Should work nicely.
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