Couple of Noob Questions

CFProject

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Hello all, I am new to this wonderfulforum and to battery projects. I am planning a small Power Wall/Portable pack with about 2kWh capacity for power backup and feasibility testing with thoughts of going bigger if this works out for me.That being said, I have obtained about 50 old laptop battery packs and have started processing the cells. I harvested about 320 18650 cells, separated by their voltage as I found them, and I am now charging and capacity testing them. It seems to be going much like the experiences of others from what I have seen andread so far. I do have a couple of noob type questions that I have not seen discussed in my research so hopefully you guys already know the answers.

I am capacity testing with three OPUS BT3100 chargers and using the default rateof 500mA for testing.

Q1: For charging I have been using two 2-cell chargers that I had laying around already, it turns out that one of them is charging to 4.3V . Will this have an effect on the capacity test? Does the Opus start measuring capacity after it discharges down to 4.2 or does it start measuring capacity at the starting voltage of 4.3? I tested a set of 4 cells as follows...Charged them to 4.3, put them in the Opus for capacity test and subsequent charge to 4.2V, record the result, let them sit for the time it takes to cycle another batch of 4, then re-cycle tested them in the same Opus. Three of the four cells capacity tested about 100mAh higher than previously and one tested about 50mAh lower. Does this fall within normal variation?, does the precharge to 4.3 actually help? does it make any difference at all? should I stop using the charger that goes to 4.3 or not worry about it? Note: I have 3 different DVMs I use for testing and they all significantly agree on the measurements.

Q2: When planning my pack arrangement (cells in P configuration) I see most people mixing capacity ranges to make each pack match the others onaverage and with the same # of cells. I have been thinking that, for example,a 20P pack of 2000mAh cells with 40,000mAh capacity would match nicely with a 25P pack of 1600mAh cells with 40,000mAh capacity. My rationale is that since all ofthe cells in the pack will be matched, they willbehave similarly and no single cellwill get dragged down to a low discharge statewhile it's pack mates are still producing. It seems more important to me to match the pack capacities this way than to match them in cell qty. However, I don't see anyone doing this in their threads orvideos. Then when I look at the pack builder tool on this website there is an option to do it this way so clearly people think aboutit. What are the Pros and Cons of this method vs the "averaging" method? I feel like I am missing something.

Thanks for any help that anyone can provide.

And by the way , I am looking for good sources of used packs in Central FL, not having much luck to date.

Chris
 
A1: No the Opus wont wait it will start directly. 4.3V is a bit high and can damage the cells in long term though since its only 1 test i doubt the effect is large. The Opus is not that much better. But why not let the Opus do all the work?

A2, Not true. In the pack with 20p you will have a higher current per cell and that would potentially drag them down faster than the pack with 24 cell in it. Since you have no way of how fast a cell will age and the only thing you know about is the actual current in total you should base your pack build on that. My advise is to make sure you:
* Have same amount of cells in each pack
* Same total capacity in each pack
* Same amount of low capacity and high capacity cells in each pack

Before assembly also make sure you have tested for seld-discharge since that will drag a pack out of balance!

Testing at only 500mA i also assume that you never will go above 500mA as max current per cell? This is rather important since if you go to 1A per cell you dont know how those 2nd hand cells will behave!
 
There are so called high voltage lithium cells out there, often called LiHV, which have a end of charge voltage of 4.35V instead of the usual 4.20V. Maybe that what the charger is for, or it is just a bit inaccurate. When charging normal lithium cells to 4.30V or 4.35V you will damage them eventually. Likewise, when charging LiHV cells to the usual 4.20V you won't get the full capacity.
 
DarkRaven said:
There are so called high voltage lithium cells out there, often called LiHV, which have a end of charge voltage of 4.35V instead of the usual 4.20V. Maybe that what the charger is for, or it is just a bit inaccurate. When charging normal lithium cells to 4.30V or 4.35V you will damage them eventually. Likewise, when charging LiHV cells to the usual 4.20V you won't get the full capacity.

If that's the case, the OPUS can be changed to 4.35V by toggling a switch inside the device.

Although I agree with Daromer, use the OPUS to do it all. Charge, Discharge, Capacity test. It's 1 test (Charge/Test).

I do that with my cells and it works very well. It will charge to 4.2V and then discharge to 3.0V or something close and that's how it checks capacity, during discharge. Great units.
 
The reason why many people, me included, don't use the discharger for charging is efficiency. When charging by some other means the discharger can discharge and test cells all the time and additional chargers are usually cheaper than achieving the same capacity (as in tested cells per time) with more dischargers that do charging 50% of the time.
 
Thanks everyone for the responses any info is helpful.

DarkRaven said:
The reason why many people, me included, don't use the discharger for charging is efficiency. When charging by some other means the discharger can discharge and test cells all the time and additional chargers are usually cheaper than achieving the same capacity (as in tested cells per time) with more dischargers that do charging 50% of the time.

This is correct, at least for me. It keeps the Opus testing. I like to have them pre charged as a time saver.

It has nothing to do with HV cells. it's just that the charger stops at 4.3 instead of 4.2.


daromer said:
A1: No the Opus wont wait it will start directly. 4.3V is a bit high and can damage the cells in long term though since its only 1 test i doubt the effect is large. The Opus is not that much better. But why not let the Opus do all the work?

I actually like the idea of letting the Opus do all of the work and will likely start doing exactly that. It seems strange to me that if the Opus starts counting discharge right away that the susbsequent test which should have had lower capacity has a higher capacity. Thoughts?

daromer said:
A2, Not true. In the pack with 20p you will have a higher current per cell and that would potentially drag them down faster than the pack with 24 cell in it. Since you have no way of how fast a cell will age and the only thing you know about is the actual current in total you should base your pack build on that. My advise is to make sure you:
* Have same amount of cells in each pack
* Same total capacity in each pack
* Same amount of low capacity and high capacity cells in each pack

I don't understand how the higher current drain per cell is an issue. It would still be the same % of charge capacity per cell in both packs. I still actually believe this to be a preferable situation. Help me understand why having the same # of cells per pack is more important than the cells within the pack being matched capacity wise ( I don't think it is). My goal is to have the same total capacity per pack, hence the variation in # of cells in a pack of different cell capacities. It seems to me that mixing capacities is hard on the lower capacity cells, if they ever get low, the rest keep on going and drag them along.

daromer said:
Before assembly also make sure you have tested for seld-discharge since that will drag a pack out of balance!

Yes, I have specific plans for this as well. Not building the packs imminently so no problem getting this part correct.

daromer said:
Testing at only 500mA i also assume that you never will go above 500mA as max current per cell? This is rather important since if you go to 1A per cell you dont know how those 2nd hand cells will behave!

I am thinking as a powerpack/powerwall to not drain them hard and fast ( I hope) compared to say an ebike or something like that. I like to test as close to real world use for an accurate picture.
 
I'm just gonna jump in here real quick and give kudos to CFProject for a job well done on replying to multiple responses! :cool: We need to use your layout for a HowTo make a proper reply :p There's lots of people who don't know how to use properly use the quote tags. Well done! :claps and cheers:
 
That's true, I guess many people aren't familiar common forum technologies, especially when it starts to vaguely look like source code :D
 
Thanks Korishan, I try....I am new to this forum, but not new to forums.

I want to keep this thread easy to follow. I know I need it that way. Hopefully it increases the likelihood of me finding answers to my questions from the SME's ( Subject Matter Experts).
 
Yeap kudos to proper quoting :)

CFProject
I don't understand how the higher current drain per cell is an issue. It would still be the same % of charge capacity per cell in both packs. I still actually believe this to be a preferable situation. Help me understand why having the same # of cells per pack is more important than the cells within the pack being matched capacity wise ( I don't think it is). My goal is to have the same total capacity per pack, hence the variation in # of cells in a pack of different cell capacities. It seems to me that mixing capacities is hard on the lower capacity cells, if they ever get low, the rest keep on going and drag them along.
Because you want equal wear on the packs. The only way to get 100% equal wear based on our rather low knowledge about the cells life cycle and remaining cycles is to have same amount of each type of cell in each pack . If you don't have this you actually cant say that they will equal at same rate unless you have done your tests.

A cell will just give out the current it can based on what it got and due to the Ir. This works find on LiIon based cells where the voltage is very linear to the SoC. Wouldn't work as well with LiFe that have a different layout.

Note that the current drain on a cell is also affecting the total number of cycles. So you can potentially wear out the lower number packs faster than the other packs. Note the "potentially". Since its 2nd hand cells its hard predict what will happen but if you look at new cells you will see that this is the case. Also the actual available capacity per cell is different. A cell that you drain with 1A could potentially give you 2A meanwhile at 500mA it gives you 2.3Ah. So doing it with different number of cells also includes that you need to test them differently to be able in theory make the call of what pack they should be in.

That's my 10 cents :)
 
Thanks again everyone for the replies. I am grateful for the assistance in understanding this better. However, I am stilling grappling with matching cell quantity(pack to pack) being more important than matching cell capacity (within a pack) .

daromer said:
Because you want equal wear on the packs.

Yes, this is what I am trying to accomplish.

daromer said:
The only way to get 100% equal wear based on our rather low knowledge about the cells life cycle and remaining cycles is to have same amount of each type of cell in each pack .If you don't have this you actually cant say that they will equal at same rate unless you have done your tests.

Is this accurate? The averaging method seems to me to cause more uneven wear than the grouping method that I am considering. Let me expand my example from above to clarify my question. Lets say I am building a 24V battery bank from 18650 LiIon cells. To keep the match simple FOR ME lets say the setup is 7S20P and all of the cells in all of the packs are 2000mAh capacity for 40A total, so I have a 960Wh bank. If I draw 2000mAh from the bank it takes 2000mAh from each of the 7 packs which is 100mAh per each of the 20 cells in each pack (5% of each cell's capacity). If we draw a not quite random 34A from the bank (and each pack) then all of the cells in each pack lose 1700mAh and reach 15% of charge around the same time. We recharge the bank back to full. Now lets say that we remove 1 of the packs in the series and replace it with a 25P pack of 1600mAh cells and then perform the same task which draws 2000mAh from the bank in the same way. 2000mAh from the 25P pack of 1600mAh cells would draw 80mAh per each of the 25 cells (5% of each cell's capacity). And if we again draw 34A from the bank (and each pack) then all of the cells in each pack again reach 15% of charge around the same time (1700mAh from each cell in the 2000mAh packs and 1360mAh from each cell in the 1600mAh pack) .This seems like a well balanced scenario vs. the averaging concept. In the averaging concept all of the packs have the same # of cells (20) and total capacity (40A) but the cells within a pack have varying capacities. For example, we redistribute the same collection of cells in order to average the packs. Each of the 7 packs has 17 2000mAh cells and 3 1600mAh cells ( 4 left over 1600mAh cells and 1 left over 2000mAh cell) . What happens when we draw 2000mAh from the bank (and each pack) as we did before? We are drawing 100mAh from each cell within the pack which is discharging the 2000mAh cells by 5% but discharging the 1600mAh cells by 6.25% so we are putting greater stress on the lower capacity cells. If we draw 34A from the Bank (and each pack) then we draw 1700mAh per cell within the pack. The 17 2000mAh cells can handle this and drop to 300mAh (15% of charge) but the 3 1600mAh cells in each pack are now overdischarged and being pulled down by the other cells. Why would this be preferable? Or do I have it wrong?

daromer said:
Note that the current drain on a cell is also affecting the total number of cycles. So you can potentially wear out the lower number packs faster than the other packs. Note the "potentially". Since its 2nd hand cells its hard predict what will happen but if you look at new cells you will see that this is the case. Also the actual available capacity per cell is different. A cell that you drain with 1A could potentially give you 2A meanwhile at 500mA it gives you 2.3Ah. So doing it with different number of cells also includes that you need to test them differently to be able in theory make the call of what pack they should be in.

This makes sense to me. I feel like with the method I am considering, I could expect the whole pack to go bad a the same time rather than some of the individual cells within the pack.

daromer said:
That's my 10 cents :)

Inflation and the exchange rate are brutal :0
 
:)

Note that the IR on a pack will make sure that the cells that can deliver the current will do so meanwhile the others will balance out after a time.
Instead of calculating the mAh you use out of a cell base it on % instead. If you take out 10% out of the pack you will have 10% less in each cell. Personally the mAh is a vague way to measure due to the fact most people for instance test with an Opus and doesn't even temperature compensate.

I have no scientific tests if even spread packs in numbers in total and numbers of each type of cell (Range of capacity) is the best way or doing it with only 2Ah and 3Ah and 1.5Ah cells in each pack and then vary the number of cell. But in my mind the first is better since the second would yield a very uneven wear due to different currents going in and out of each cell.

The balance of cells mixed is not a problem on a LiIon pack since they SoC vs Voltage is very very consistent. On LiFe i would NOT do the above and on that I would only mix cells in each pack with same type of capacity and numbers due to that LiFe can have same voltage during 70-80% od the DoD.

Would love to see a proper test with lets say 100 strings of each and atleast 1000 full mixed cycles to see which one wears out first :)
 
Thanks so much for the discussion, Daromer. I tjunk I just needed to talk through it. Matching %soc coming out of each parallel cell was how I needed to think about it.
 
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