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How important is cell voltage during PCB population?
#11
(07-22-2020, 05:43 PM)gauss163 Wrote:
(07-22-2020, 05:33 PM)Crimp Daddy Wrote: It's very important...  all cells should be at the same voltage when populating the boards.

Why do you believe that?  Did you read the (different) answers above?

I did not read the above answers... I am providing a general response to a to a general question "How important is cell voltage during PCB population?"

The larger the voltage delta, the larger the inrush current when being put in parallel with cells of different voltage.  Ir may or may not exceed the current path.

Voltage delta and cell IR will play a roll in the actual current flow during equalization.
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#12
^^^ This was already explained in prior answers, where it is was also explained that being "the same voltage" is a needless restriction at low SOC (unless you have a very general definition of "same voltage" - which is not at all clear).
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#13
(07-22-2020, 03:54 PM)OffGridInTheCity Wrote: >I now see why people have a whole wall of Lii-500 (or similar) units. This takes forever!
It sure does.   I have 3 OPUSs... and I'm retired...  and I hang out in my office a lot of the day...    For me, I just sit with OPUSs on my desk and do 12 at a time inbetween youtubes for the last 2.5yrs and I'm nearing 10,000 cells processed and 84 packs built to date (e.g. 80kwh battery bank) .   

I agree - if you want to make rapid progress you either need a lot of smaller chargers or go EV (larger batteries + maybe a larger charger) and/or focus on if the sheer quantity of work is OK for you're situation / time-frame Smile
Yeah that was kinda my question. Could I just drop all of these tested cells into a 7s16p battery with a BMS, and then just hook up my 29.2V charger, and let the charger bulk charge all of the cells for a week. I would just do this one time--but that would then allow me to re-charge all of the cells back to 4.2V using the bulk charger in the 7s16p battery rather than having to re-charge them all (4) at a time in my "dumb" charger or with my Lii-500 chargers after all of the cells have been tested. I suppose I could then let the charged 7s16p battery rest for 30 days, and then test all of the 7s packs using my BattGo Meter to find any self discharge cells within the 7s packs. 

Do you think that the above would be an acceptable way to do a one-time bulk re-charge these various cells--instead of having to re-charge them all after my testing using my one "dumb" charger and my (2) Lii-500 units after the testing is complete?

(07-22-2020, 03:55 PM)Wolf Wrote:
(07-22-2020, 03:15 PM)aventeren Wrote: I now see why people have a whole wall of Lii-500 (or similar) units. This takes forever!
Yes we have all kinds of testers.


Several things to mention.The LiitoKala only charges at 1000mA and its max discharge rate is 500mA.
That is one reason it takes a long time. Especially if you have high capacity cells.
OffGridInTheCity mentioned this and you should really do it. That is to finish the C/D/C sequence so the cells you remove are at ~4.2 V.You can finish this in your "dumb" charger if you wish but you really should finish them so that after 30 day rest you can check for any substantial V drop. This is necessary to find any SD cells you will not want in your pack. After that if you want you can discharge to ~3.6 storage V if you need to store them for an extended period of time.
It wouldn't be a post by me if I didn't mention IR The LiitoKala is not your optimum IR tester by any stretch of the imagination consistency is very lacking.
There are many posts by others and me to discuss this so look for them. Here is a good starter. https://secondlifestorage.com/showthread...5#pid42065

Wolf
Fantastic link. Thank you! I'm seeing my mR readings using my Lii-500 tests somewhere in the high 20s to 50s. I'm don't think I've ran across one yet over 50 mR. FYI, these are all new 18650 Panasonic NCR18650BD cells.
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#14
(07-22-2020, 08:33 PM)aventeren Wrote: [...] Could I just drop all of these tested cells into a 7s16p battery with a BMS, and then just hook up my 29.2V charger, and let the charger bulk charge all of the cells for a week [...]

You could do something like that to bulk-charge all the discharged cells. Then add the other already-charged cells to the newly charged (sub)pack and do a final charge (with low termination) to help top-balance them.  Or, you could first discharge the high ones, then put all the discharged cells in a pack (generally it is safer to work with lower voltages), but then you might need some extra work to ensure they get top (vs. bottom) balanced (esp. necessary of the capacities are not closely matched).
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#15
(07-22-2020, 08:33 PM)aventeren Wrote:  FYI, these are all new 18650 Panasonic NCR18650BD cells.
Ok if they are brand new cells is there a reason you are testing them?
If I had a lot of new cells I would check them for storage/shipping V when they arrive and if that is OK build the pack.
Charge the pack test and end of story.

That way you are building the pack with a lower V usually ~3.6V don't have to worry about SD's charge up the parallel pack to 4.2 run a capacity test and your done. I wouldn't even worry about IR (well I would measure it cause I am anal).

Build your series pack and enjoy.

Wolf
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#16
(07-22-2020, 11:13 PM)Wolf Wrote:
(07-22-2020, 08:33 PM)aventeren Wrote:  FYI, these are all new 18650 Panasonic NCR18650BD cells.
Ok if they are brand new cells is there a reason you are testing them?
If I had a lot of new cells I would check them for storage/shipping V when they arrive and if that is OK build the pack.
Charge the pack test and end of story.

That way you are building the pack with a lower V usually ~3.6V don't have to worry about SD's charge up the parallel pack to 4.2 run a capacity test and your done. I wouldn't even worry about IR (well I would measure it cause I am anal).

Build your series pack and enjoy.

Wolf

I guess I'm testing them because everything that I've watched or read says that I need to be populating each PCB with cells that are within 100 Ah of each other--and I'm seeing some that are outside of 100 Ah in my testing. I guess I don't understand the impacts well enough to know what populating one 7s PCB with a bad cell will do to a 7s or 7s16p or 7s48p battery. It's that uncertainty with not wanting to burn down my house that is leading me to test all of these.

But then that has created this voltage differences in the cells, as the ones that I test are somewhere between 3-4.2-ish volts...and this has created a balancing issue where I am now being told not to populate a PCB with cells that aren't within 0.1V of each other because I'll damage the PCB. So candidly, I'm getting myself wrapped around the axel here a bit.

For the most part, IR seems within 30-60 mR, so that's probably good.

I just remembered that I bought a BattGo BS-8G battery tester & balancer. It maxes out at 2A. So maybe I just populate these PCBs one at a time, and then set the BattGo unit to balance mode, which will then balance the voltage out....and then once that is done, add that 7s unit in parallel to the other balanced 7s units to populate the 7s16p banks...and then use the BMS and a 29.2V charger to charge up that bank to full capacity (the charger only delivers 10A at 29.2V, so on a per 18650 cell basis, the charing current will be a trickle). But it's the uncertainty of this whole thing that I'm trying to understand so that I don't mess something up.

I'll look back at this thread in 6 months and laugh--but I'm definitely on the steep part of the learning curve now.

Thanks a ton for your help! This is fun!
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#17
Re assembling low SoC cells, then charging a whole pack, sounds OK, just don't include any "full" cells.
Also somewhere in your testing you need find SD cells & they have to be separate for this.
Like Wolf suggests, testing IR is a key tool to weed out old/damaged cells too, so keep doing that.
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#18
(07-22-2020, 11:57 PM)aventeren Wrote: I guess I'm testing them because everything that I've watched or read says that I need to be populating each PCB with cells that are within 100 Ah of each other--and I'm seeing some that are outside of 100 Ah in my testing [...]

Presumably you mean 100mAh. Don't expect much better than around 5% capacity accuracy with low-end analyzing chargers like the Lii-500. In particular, for your 3Ah cells the reported capacity could be off by 150mAh, so the 100mAh differences you are seeing are well within the noise. Keep in mind there will also be wide variances between slots, and between different units. You can't expect better at such a bargain-basement price point.

A google search will turn up much evidence to support such, including a thread on this site, and also some sources of dubious competence,  e.g. on the page of the featured google snippet is the following nonsensical remark, which raises serious doubts that the author has the required knowledge to properly perform Lii-500 accuracy tests

the Battery doctor Wrote:Notice that NO capacity tester is 100% accurate that is because the battery cells "suck up"  different amount of ions every time! compare this to a machine filling up a glass of water to exactly 100.00% of every time, no matter how good a machine is it will never be the exact same amount if microlitres in the glass [,,,]

In fact, with higher end equipment one can get accuracy very close to 100% (and charging alone suffices since Li-ion batteries have close to 100% Coulombic efficiency). That's how Tesla's guru Jeff Dahn manages to efficiently empirically optimize Li-ion chemistry - by making various chemistry tweaks and observing infinitesimal capacity changes during cycling. But this is of little import for the matter at hand.
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#19
(07-22-2020, 11:57 PM)aventeren Wrote: I'm curious to know how a 0.1 V difference will damge the pcb ?
CG.
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#20
(07-23-2020, 04:18 AM)507PowerWall Wrote:
(07-22-2020, 11:57 PM)aventeren Wrote: I'm curious to know how a 0.1 V difference will damge the pcb ?
CG.
That was an earlier comment in this thread...
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