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My Open Source Solar BMS and Digital MPPT
#41
(12-16-2018, 04:43 AM)Cherry67 Wrote: My Idea from the beginning was, if IR (increase ) would have ANY significance in determining the Spent or remaining lifetime, it could be used, in the right way, to ientify bad cells more quickly instead of processing them through the capacity test. IR would be quick, for sure.

So, after this long story, the simple question is- am i on the wrong track, or do you confirm something of this ?
Thanks so much.


It is not feasible to use IR because you have cells from different manufacturers with different characteristics and you may not even know what the manufacturer of the cells is or exact model if they are not labeled.
Then there is also the internal resistance based on SOC and temperature so it will just be impossible to get any meaningful result from just measuring the internal resistance of cells with different initial specs and different unknown SOC when you do the test



Notice the large impact of State of Charge on internal resistance (that is the red line the DC internal resistance not impedance in blue).
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#42
(12-16-2018, 11:53 PM)electrodacus Wrote:
(12-16-2018, 04:43 AM)Cherry67 Wrote: My Idea from the beginning was, if IR (increase ) would have ANY significance in determining the Spent or remaining lifetime, it could be used, in the right way, to ientify bad cells more quickly instead of processing them through the capacity test. IR would be quick, for sure.

So, after this long story, the simple question is- am i on the wrong track, or do you confirm something of this ?
Thanks so much.


It is not feasible to use IR because you have cells from different manufacturers with different characteristics and you may not even know what the manufacturer of the cells is or exact model if they are not labeled.
Then there is also the internal resistance based on SOC and temperature so it will just be impossible to get any meaningful result from just measuring the internal resistance of cells with different initial specs and different unknown SOC when you do the test



Notice the large impact of State of Charge on internal resistance (that is the red line the DC internal resistance not impedance in blue).
PooF! Goes a good part of my idea..... Big Grin . Thanks for your answer.

Still, i would like to consider the following issues:
- Yes it is obvious that there may be maaany different cells and even chemistries. What i want to point out that i dont want to determine an absolute value of IR to spezify a good cell. What i still want to give a chance for is taking a quite higher ( doubling, say, >80mOhm) IR to be a malus sign, if it proves to be such ind the majority of cases.
So far most of my bad cells match this definition.

- It might even be necessary to make some slight special handling of cells or manufactures, as i expect names could be (red) Sanyos or the NCR high cap cells.

- I do not even want to "qualify" cells, just pre-sorting them to modify the testing time management.

- If it would be necessary to avoid the upper and lower end of SOC it would still not be a big problem. I am currently measuring two cells IR over SOC with 50 Hz AC and can quite confirm your data, which has for sure better grounds than my hobby approach.
It was logical for me from the beginning that Lower and higher end of SOC would have some increasing or IR. Sure i didnt know how much. Question was, if that makes such a difference.

I am as well cycling two cells, one with a medium IR, one with a bad IR, and both with high CapLoss%. Its to early to say anything except it develops as can be expected.
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#43
There seems to be 2 main tracks of thought when it comes to IR measurements when dealing with secondhand cells. I’ll stick to the most commonly used cell that is used for what this community is building.
The first thought is don’t bother testing. We are dealing with secondhand cells after all. After the full testing process, if the test above our cut off limits then the IR should be low enough to work. This was my original thought.
The second train of thought is that IR needs to be tested. Then you need to set yourself a cut off limit of what is acceptable with the IR. Personally I use 100mOhm. A bunch of times the question has been asked when to test for IR? My problem is with the charge/dischargers testing IR is that it is proven that they are inaccurate and I believe that they test the IR when the cell is placed in the charger. So this could be at any stage of charge. This will effect the outcome of this test and not give you a solid benchmark.
Personally I feel using a seperate device to test IR is fairly slow. So I will only take that step when I know I have a cell that is above my minimum capacity and does not self discharge after a month of sitting at full charge. As for testing IR at different voltages I place all my cells into storage voltage of 3.7v before testing IR. This ensures that all of my testing is running off of the same benchmark. Out of my 240p packs this weeds out only 2/3 more cells which can make a difference when trying to balance a pack.
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#44
(12-17-2018, 09:52 PM)Minatugar Wrote: There seems to be 2 main tracks of thought when it comes to IR measurements when dealing with secondhand cells. I’ll stick to the most commonly used cell that is used for what this community is building.
The first thought is don’t bother testing. We are dealing with secondhand cells after all. After the full testing process, if the test above our cut off limits then the IR should be low enough to work. This was my original thought.
The second train of thought is that IR needs to be tested. Then you need to set yourself a cut off limit of what is acceptable with the IR. Personally I use 100mOhm. A bunch of times the question has been asked when to test for IR? My problem is with the charge/dischargers testing IR is that it is proven that they are inaccurate and I believe that they test the IR when the cell is placed in the charger. So this could be at any stage of charge. This will effect the outcome of this test and not give you a solid benchmark.
Personally I feel using a seperate device to test IR is fairly slow. So I will only take that step when I know I have a cell that is above my minimum capacity and does not self discharge after a month of sitting at full charge. As for testing IR at different voltages I place all my cells into storage voltage of 3.7v before testing IR. This ensures that all of my testing is running off of the same benchmark. Out of my 240p packs this weeds out only 2/3 more cells which can make a difference when trying to balance a pack.

HA! A wonderful write-up to describe (the core of) my idea, which has developed after all that work i have done to clear the facts in my mind.

1) when to test for IR?
in my Kevin Thread I describe a DIY tester in the "Swiper" configuration.
https://secondlifestorage.com/t-Kelvin-m...nce?page=2

(The swiper idea came from somebody else, i took the idea to make a 4-wire contact version. For the hardware there will soon come an updated version)
With this you can test at any time, as quick as just measuring the Voltage you will get both, Voltage and IR.

Quick and at any time.

With the data from a degree thesis (Peter Keil 2017 TH Munic, Germany), with electrodacus input and a test on two cells i did by myself i dare to say you can get acceptable IR results at any voltage between 3.6 and 4.0 Volts, rising to a +20% result on goning to 4.2 or lower to 3.0. This is good enough to be taken in account just in mind.

2) why test IR first ?
Now, that is easy. If IR is high, it ill stay high. And you can save all other testing time for that cell....... and do others which have low IR (and better prospects by it).
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4 kWh battery target - plus Mobile Home battery
Ultra low cost
Electronics ? No clue. Am machinery engineer.
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#45
Now were drifting away from the original thread. The IR discussions should be continued in the other threads please.
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#46
(12-18-2018, 08:48 PM)daromer Wrote: Now were drifting away from the original thread. The IR discussions should be continued in the other threads please.
I am sorry. Can you remove my questions and Electrodacus answer, the last part of the thread, to one of my mains threads ? Kelvin or harvest 50% ?
I would appreciate.
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#47
Hi, I am working on a 24v modular battery project that will be fed by 4 250w solar panels and a "1000w" wind turbine (rarely makes more than 400w). the plan to start is to build 3 7s40p batteries with 2.4kwh each, but to be able to add more batteries in parallel in the future as more cells are acquired. I currently have one SBMS40 with the plans to use it as the main "brain" in battery number 1, and to parallel the other batteries into battery 1 when extra capacity is needed. I plan on buying an SBMS0 for each of the batteries, but I first wanted to ask your opinion on this battery system. I understand the voltage will need to be the same between packs when they are plugged in parallel to each other.
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#48
@Sunrise: Are you asking a question about the Open Source BMS project or are you asking something new? If something new please make a new thread and not hijack someone else's thread.
Thank you
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#49
@korishan this is a question regarding the SBMS0. I want to make sure I have the correct understanding on this system. After the battery system is finished I will make a new thread on the system as I'm working on the build.
-Rex
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#50
Hi electrodacus. I really like your work on your solar bms and thanks for providing data from your off-grid home. It helps alot when planning new installations.

I live in sweden, and unfortunately the sun only outputs around 10% during the darkest winter months (200kW during december with 14kw array) when compared to the summer months (>2000kw).

Because of this, it is very hard to build an off-grid solution using only solar. I thought about this and a fireplace or similar is most likely necessary to heat the home during winter, when the power is not sufficient.

Also, I thought about installing a wind turbine, as the wind blows a-lot here (average wind is 3.5-4.0 m/s during december), and it will help a-lot during the winter months. So, I was wondering if your Solar BMS could support wind as well? or does the MPPT work differently for wind and solar? 


Also, I want to add that there are successful installations of off-grid homes in Sweden, but most of them have to rely on gas or a fireplace to heat the house during winter. There are some installations which utilizes fuel cells, with >10 000kWh of storage. But those installations are not cost-effective, and probably not very safe for DIY enthusiasts.
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