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How important is cell voltage during PCB population?
#1
Howdy; I am currently in the process of testing (360) 18650 cells using the LiitoKala Lii-500, which can do (4) cells at a time--and I have two units. I also have another "dumb" charger only that does (4) cells at 1A per cell.

My capacity tests are taking 6-8 hrs each at 1A ea--and that is using cells that have been charged to 4.2V first using my "dumb" charger so that the cells don't have to first be charged up to 4.2V by the Lii-500 units...plus, I am also pulling the cells out after the END of the capacity test while the cells are being charged, which means that my tested cells have Ah capacities and mR resistance figures--but they have not been charged back up to 4.2V, which means that the tested cells are in various voltage states (2.9-4.2V).

I am building (3) 7s16p banks into a 7s48p battery using Jehu's PCBs (plus a BMS per bank). I plan on using rePackr to determine where to put the cells to create balanced 7s16p banks. 

My question is how big of a deal is it that the tested cells be populated in the PCBs be voltage matched? For instance, is it a big deal if I load (7) cells onto a given board--and then add that 7s cell into a larger 16p bank--with all of the individual 18650 cells in various states of voltage levels?

The reason I ask is that to charge the tested cells back up to 4.2V so that all cells have a similar voltage level is taking 2-4 hrs using my "dumb" charger...and given that I have so many 18650 cells to test, re-charging all the cells back to 4.2V is adding quite a bit of charging time. So I'm just trying to figure out if I can let my BMS handle the voltage balancing once my 18650 cells are populated into my 7s16p banks. From what understand, the BMS should balance all of the cells within the 7s16p bank. Is there a reason that I can just let the BMS equalize all of the voltages during the first charge of the 7s16p banks?

Thanks for your help--I REALLY appreciate it.
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#2
Can damage the boards, Charge the cells to the same voltage.
later floyd
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#3
If you connect cells together where the voltage is off by more than about 0.1V (100mV) large currents will flow while one "fast charges" the other.
This could easily blow fuses, melt contacts, melt PCB tracks & damage the cells.
It's pretty important to have them reasonably close - half a volt (empty to full) is going to cause trouble.
If you put say all cells at 3.6V in one bank & then say all cells at 4.0V in another & joined the two in parallel, you'd have the high current thing above.
If you put those two banks in series & tried to use the system, the lower pack would hit cell volt minimum or higher one to maximum way earlier & your system "should" know this & shutdown.
BMS's aren't designed to use currents large enough to equalize like this, it could take weeks.
You could charge the lower pack (separately, disconnected from system unless charger is isolated) up to closer to the higher pack....
Patience is needed :-)
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Running off solar, DIY & electronics fan :-)
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#4
Agree with above and would add - one of the standard/recommended steps in the process is to charge up to 4.2 and let the cells sit for a while - to find/weed-out self discharging cells.   This has the side affect of leaving good cells at very similar voltage (e.g. 4.13-4.17'ish) and you don't have to worry when building/assembling a pack.  

If you want to lower the voltage for longer term storage - you can discharge the whole pack or whole battery down to 3.6'ish (or whatever) in mass Smile
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#5
(07-21-2020, 10:06 PM)aventeren Wrote: [...] but they have not been charged back up to 4.2V, which means that the tested cells are in various voltage states (2.9-4.2V)

Is 4.2V a typo? (in "2-9-4.2V") After a full discharge they should be resting far below 4.2V. If they have all been fully discharged to very low SOC then it is generally safe to parallel them (they needn't be within 0.1V) because IR (internal resistance) is very high at low SOC - which inhibits large current when one charges the other(s). 

Further, generally it is true that Li-ion batteries can safely handle higher currents (than rated) at low SOC (that's the property that is exploited to the hilt by modern fast charging algorithms, such as Philips boostcharging algorithm, or multistage CC/CV algorithms, such as Tesla's fast charging algorithms, and recent fast-charging algorithms used in some phones).

Caution  The above does not apply to cells that are over discharged. They are generally revived via a timed low current "precharge" stage to bring them back up around 3.0V. Only after such a precharge should you apply your standard charging algorithm (be it fast or normal). 

Generally manufacturers do not recommend charging any cells that are below 1.5V (though some go to 1.0V) because this can lead to safety problems (copper can dissolve at low voltages and then during charge replate into dendrites, possibly leading to dangerous internal shorts at some future time - without any warning). For some very old cells (lacking modern safety improvements) it may not even be safe to charge them when below 2.0V.
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#6
(07-22-2020, 12:40 AM)gauss163 Wrote:
(07-21-2020, 10:06 PM)aventeren Wrote: [...] but they have not been charged back up to 4.2V, which means that the tested cells are in various voltage states (2.9-4.2V)

Is 4.2V a typo? (in "2-9-4.2V") After a full discharge they should be resting far below 4.2V. If they have all been fully discharged to very low SOC then it is generally safe to parallel them (they needn't be within 0.1V) because IR (internal resistance) is very high at low SOC - which inhibits large current when one charges the other(s). 

The cells are various voltage states because I have charged some back up to 4.2V after the capacity test with the "dumb" charger, whereas with others, I just pull the cells out at the end of the capacity test (and those cells are somewhere between the min discharge voltage of 2.8-9V if I catch them right at the end of the capacity test...but more realistically they hit the test end and then start re-charging back up to 4.2V...but I usually catch them in the mid 3V range before I swap them out with new cells to test), and then load new cells into the Lii-500 units to start the testing process. I can charge (8) cells using my one "dumb" charger in the time it takes my (2) Lii-500 units to test (8) cells--so re-charging with my (1) dumb charger makes more sense. I even use my "dumb" charger to do the initial charge before I start my capacity tests so that the test cycles don't have to first charge the cells to 4.2V. Instead I start the test with the cells in the 4.15-4.2V range to speed up the tests.

I now see why people have a whole wall of Lii-500 (or similar) units. This takes forever!
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#7
>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
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#8
(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
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#9
It's very important... all cells should be at the same voltage when populating the boards.
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#10
(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?
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