I've seen comments on the forum suggesting that, given a string with load connected in the diagonal manner, there might be a benefit to making the load connections more toward the middle of the string rather than at the very corners. For the 4 battery parallel string that would be done like...
Hi, everybody! I'm a retired EE. I didn't want my EE skills to fade away after retirment, so I exercise them when I can. Whenever I see a non-trivial circuit analysis problem on one of various forums I frequent, I challenge myself to solve it. Recently I watched Will's video about current...
Remember that the full diagonal connection gives more current in 1 and 4, less in 2 and 3. This new connection adds more resistance for 1 and 4 which reduces their current, and it add less resistance for 2 and 3, which leads to more current in 2 and 3. Thus, the currents are balanced for the...
Readers should understand that the results I'm going to post are theoretical values obtained with mathematical circuit analysis. In the real world
things will be different since battery IR is quite variable due to temperature, age, SOC, etc., whereas in the mathematical analysis the parameters...
Now I'll obtain some results for the same 4 battery string, but with the load connected in the well known diagonal manner.
The battery IR is reset to 5 milliohms, with link resistance still at 1 milliohm, and 100 amp load current. The calculations show (I'm showing 6 digits for a reason...
Here is a comparison of two hookups of 4 batteries in parallel, with an image from Victron to illustrate:
The hookup on the left is the well-known "Halfway" connection which gives theoretical perfect balance. It depends on the IR of the 4 batteries remaining identical, and, of course, in the...
For the "Posts" configuration, yes, the goal is to have the same cable resistance from each battery to the load points; also you need the batteries to be nominally identical.
But for the diagonal connection of 4 batteries in parallel, as in this image, the cable resistance from each battery...
Having seen that simply moving the load connection to the 4 battery parallel string slightly away from the far corners of the diagonal, more toward the center of the string, can give a theoretical perfect balance, I can't help but wonder if this would work with parallel strings with other...
I said in the beginning I didn't want this thread to be a propellor head thread. The community is not all EE's and I don't to scare anyone away from reading this thread all the way through.
It ocurred to me recently that there is a hookup that will give perfect balance to 6 or 8 battery parallel strings. Consider the diagonal connection to two batteries in parallel with identical link cables. This connection has perfect balance:
Now imagine that each of the two batteries is...
First read this https://diysolarforum.com/threads/calculation-of-parallel-string-battery-currents.37937/ until at least post #61.
This post will show the theoretical calculated battery currents in parallel strings of 5, 6, 7 and 8 batteries with load currents of 100 amps times the number of...
Have a look at post #25 (also see post #4). The right hand part of the image shows 4 batteries connected to a common busbar with equal length cables. Whether or not it gives a balanced charge/discharge depends on how the load is connected to the busbars. If the load is connected as shown by...
I don't mind, but I wonder where to put such a thread; it will be full-on propellorhead.
Perhaps @Will Prowse or one of the moderators has a preference.
You are apparently not using the diagonal connection like this:
Also see: https://diysolarforum.com/threads/no-busbars-needed-with-two-batteries.66691/
I've recently posted more results for longer parallel strings: https://diysolarforum.com/threads/calculation-of-parallel-string-currents-addendum.94178/
I recently received a YR1035+ meter and I wanted to see just how good it is. I have a Hioki LCR meter that was recently calibrated to compare to. One of the things I did was to see if the YR could measure the ESR of an electrolytic capacitor. First I measured the capacitor on the Hioki and...
A string of 5 batteries in parallel with the load/charger connected diagonally will experience substantial battery current unbalance.
See: https://diysolarforum.com/threads/calculation-of-parallel-string-currents-addendum.94178/
I've been asked by @chrisski and @MisterSandals to show the details of the circuit analysis math being used in another thread https://diysolarforum.com/threads/calculation-of-parallel-string-battery-currents.37937/
The method I've used is familiar to EE's as nodal analysis, formulated using...
The chart shows how bad the unbalance becomes for 8 batteries compared to 5 batteries. Ten batteries will be just that much worse than eight for the total diagonal connection. Connecting more toward the middle would help a lot but I haven't done the math for that case so I can't say where best...
Finally, here are some results for a parallel string of 6 batteries. I probably won't go beyond this number of batteries.
I'm using a value of 1 milliohm for the battery IR, and .025 milliohm for the link resistances which I think is more representative of what one gets with heavy busbars...
Here's how you could implement the half-way connection from post #4:
The orange connections would be made by short cables.
It would probably work best if the busbars were all one piece with 4 holes connecting the 4 terminals of one 4P string. Drilling a fifth hole would give the place to...
Consider an 8 battery stack, diagonally connected. With a 400 A load the currents in the batteries would be:
#1 65
#2 52
#3 43.4
#4 39.5
#5 39.5
#6 43.4
#7 52
#8 65
Batteries #1 and #8 will reach low voltage cutoff first, at which point your string is effectively only a 6 battery string...
Quite true. Conversely, as the batteries are being charged, the IR will go down leading to worse balance.
The remarkable property of these new connections I've discovered is that the balance doesn't change when the IR changes, as long as all the batteries' IR tracks under all conditions...
See this post: https://diysolarforum.com/threads/calculation-of-parallel-string-battery-currents.37937/page-3#post-574248
Details in post #4 of that thread.
Full tilt boogie here: https://diysolarforum.com/threads/propellorhead-circuit-analysis-math.38972/
Consider the "posts" configuration:
The batteries don't have to be arranged in a straight line as shown in the image. Is your proposed configuration a version of posts but with the batteries in a circle? The copper disc in the center could be like this...
You need to connect one of the main cables to one side and the other main cable to the other side.. This is the diagonal connection like the image in post #9.
I've done some calculations on a 5 battery parallel string and here are my results.
I'm using a value of 1 milliohm for the battery IR, and .1 milliohm for the link resistances which I think is more representative of what one gets with busbars rather than cable links. The load current is 100...
That's better than 4/0 gauge cable! A 72mm link of that would have a resistance of .001 milliohm; that's 1 microohm! In such a case I suspect that the clamping resistance will dominate the overall link resistance.
If I set the link resistance to .081 milliohms battery IR to .50 milliohms and...
Are all these batteries LiFePo4 chemistry? What is the internal resistance of the 100 Ah batteries? What is the internal resistance of the cells making up the DIY 280 Ah batteries; these are made up of 16 cells I assume.
The ideal situation would be to have the 280 Ah batteries deliver 2.8...
The very first paragraph in my post #1 says:
"Readers should understand that the results I'm going to post are theoretical values obtained with mathematical circuit analysis. In the real world things will be different since battery IR is quite variable due to temperature, age, SOC, etc...
Have you measured the currents of the individual batteries under load? It's possible to work the problem backwards and calculate what the battery parameters such as IR must be to give the measured currents.
If you stack the two busbars you'll have a cross sectional area of 111 mm^2, which will have a resistance of .16 microohms/mm. It looks like the distance between terminals will be 36 mm, so your resistance between terminals will be about 5.8 microohms. The clamping resistance will probably be...
Your diagram shows only one wire connecting to the red busbar where I've shown a blue dot. Surely you have 4 wires connected to 4 studs on the busbar I've shown as 4 big black dots; is that correct? Could you also show where your load cable is connected to the red busbar?
If each 4 battery series string is considered to be a single 48 volt battery, then it can be wired as I described here: https://diysolarforum.com/threads/calculation-of-parallel-string-battery-currents.37937/post-487633
All you would have to do to achieve (theoretically) perfect balance is to...
Are you using the total diagonal connection to the 8 parallel batteries?
The effect of moving the load connection from total diagonal to a point more toward the middle is shown here for 5 batteries...
I have to use actual resistance values to make calculations, so if I attribute .06 milliohms to the clamping resistance, doubling because there are 2 of them, that's .12 milliohms. Add .05 milliohms for the bar resistance, I get .17 milliohms for the total link resistance. If the battery...
The choice of 100A per battery is just a mathematical convenience. When a calculated current is not 100A but, say, 113A you can see in an instant that the error is 13%.
Balance is not affected by how much draw except as second order effects.
It's linear. If you're drawing 20A per battery just divide the values in the chart by 5.
The values in the chart are calculated from the resistances in the stack; link resistance, battery IR. If those don't change...
This post: https://diysolarforum.com/threads/calculation-of-parallel-string-battery-currents.37937/page-3#post-488325
discusses what can be done to improve a 5 battery parallel string.
I can't answer your questions about where to get bus bars, but there are many posts on these topics. Do a...
Are you referring to one of the calculated Examples? If so, cite which one. In post #3 I gave 3 examples of the calculated out of balance for 3 different values of battery IR. Could you be more specific please. I could do a special calculation if you can give the battery IR and link resistance.
Perhaps you're asking about sensitivity to a failure of all the battery IR's to track. Let me run an example. Set the battery IR's to 1 milliohm, typical for a somewhat high IR of modern LFP batteries, set the link resistance to 1 milliohm, higher than what a good busbar setup would give you...
I haven't found a perfect balance connection for 5 or more batteries in parallel, but the method does give improvements for those cases. I'll post some of my findings about those higher number strings before too long.
A couple of interesting articles about measuring DCIR and ACIR by a Keysight engineer:
https://www.electronicdesign.com/technologies/test-measurement/article/21128843/keysight-technologies-measuring-dcir-of-lithiumion-cells...
Here's a document put out by Hioki about their BT4560: https://www.hioki.com/download/32951
It talks about Cole-Cole plots and shows an example of one on page 2 of the document. The red line which is the Cole-Cole plot crosses the horizontal axis on the left at 1 kHz and that point corresponds...
OffGridForGood:
What is the maximum current you expect your battery bank to supply?
What is the length of the individual 1AWG cables?
It would be good to apply a fairly large load for 15 minutes, measure the individual battery currents and post those values here. Then disconnect the load and...
I'm not the only EE on the forum. Why don't you post your questions in the General Discussion forum and then everyone can benefit from the question/answer dialog resulting?
I have checked the 3 parallel battery string with the 1/3 offset of the two links giving perfect theoretical balance. This hookup has the same very worthwhile property the 4 battery hookup had, that if the IR of the 3 batteries changes any amount (within reason, of course), the perfect...
That article I referenced with calculations made 32 years ago set the battery IR at 20 milliohms; really? The LFP batteries everyone on this forum are using have IR typically less than 1 milliohm, and those numbers I posted for the 1/2 milliohm IR in various hookups like Example 6 are terrible...
Have a look at posts #36, #37, #38, and #39. No matter how beefy the busbars, the clamping resistance remains a factor. Would your beefy busbars be copper or aluminum? What is the manufacturer's spec for the IR of your cells?
Are your battteries paralleled with busbar or cable? Is the load connected diagonally? Can you measure the individual battery currents under load? What are the 8 currents?
I've done some further research and I found another way to achieve perfect balance for parallel battery strings of 3 and 4 batteries.
In post #4 I showed a method to achieve perfect balance that is most practical in a setup with busbars because the method requires making the load connection...
Break up the string of thirty into 5 strings of 6. Use the optimum connection for a string of 6
shown here: https://diysolarforum.com/threads/calculation-of-parallel-string-currents-addendum.94178/
Just clamp the output cables temporarily to the busbars at the optimum locations rather than...
The links labeled "3R" must have 3 times the resistance of the links labeled "R". Stacking 3 busbars will not multiply the resistance of that connection by 3; rather, the resistance of the 3 stacked busbars will be divided by 3. To do it as I described, you would have to stack 3 busbars...
You have 4 100ah cells in parallel so you could discharge as much as 800 amps and still be within the max average discharge rate for the individual cells! As I mentioned one needs to watch out for overheating the busbars.
You have the right idea about where to connect the BMS leads. As far as...
What is the manufacturer's specification for the internal resistance of the batteries?
The link cables between the batteries look like 4/0 cable, but the cables running off to your load don't look that big; are they also 4/0?
My calculations in this thread have shown that just where the load connection is made to the busbars can have an effect on balance, but if the busbars are very thick the effect will be quite small. I suppose that there might be a particular connection of load to your busbars, with which you...
Post #4 in this long thread shows the best way to achieve (theoretical) perfect balance when busbars are being used with a parallel string of 4 batteries.. You just drill a couple of holes at the halfway point in the appropriate busbars and make the load connections there.
OK. I have analyzed the 3 battery parallel string in another thread: https://diysolarforum.com/threads/calculation-of-parallel-string-battery-currents.37937/post-481671
To analyze your proposed connection of 4 batteries, I'll start with the 3 battery parallel string configuration. I assume...