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How important is cell voltage during PCB population?
#21
(07-23-2020, 04:38 AM)aventeren Wrote:
(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...

Im buillding my batt with a 14S PCB and my recommendation is start with a small load, and closely monitor behavior of the cell with the BMS so you can sense your system and know if its ok to add more load to it.... and read a lot here in the forum FB groups.

Cheers from Panam
CG
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Based in Panama

Off-Grid
PV = 3 x 200W
Bat = GEL 250Ah
Feeding Fans / CCTV / Modem-Router / Android Boxes

Going on-grid
** 345 ** batteries harvested / getting GTILv2 2Kw auto sense 45-20V 240V
Lots of items coming from china - usa

...Updating as progress flow...
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#22
Since the OP's cells are new, not used, agree the need for testing is reduced but still a good idea in case there are "less than A grade" cells in there.

The >0.1V difference can cause high current flows.
Knowing the cell mAhr numbers lets you build packs with nearly equal mAhr capacities, which makes for a more stable battery system & balancing easier.
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Running off solar, DIY & electronics fan :-)
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#23
(07-23-2020, 07:38 PM)dhanushx012 Wrote: 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
I do have a 29.2V 10a charger that I was going to use to charge the 7s48p battery--but I have been reluctant to use it because of the voltage difference amongst the individual 18650 cells at this point (vary between 3-4.2V). My initial thought was just to populate a 7s16p battery bank with an integrated 50A BMS and then hook it up to my 29.2V 10a EV charger--but then I've learned that I cannot have a voltage difference in the cells. 

So I've now pivoted to populating a single 7s PCB with pretty similar voltage cells, and then using my BattGo to balance them--and then hooking that 7s to the 29.2V 10a EV charger to bring it up to 29.2V...and then combining that 7s cell in parallel with other 7s cells that have also been charged up to 29.2V...and slowly building the pack out to 7s16p.

I guess I am just really surprised how long it is going to take to actually assemble each 7s16p bank at this rate. I definitely underestimated this part of the build.
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#24
(07-21-2020, 10:06 PM)aventeren Wrote: 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.

Instead of a bunch of dummy charger... go TP4056

https://blog.seidel-philipp.de/download/diy-project-reverse-protected-18650-lithium-ion-charging-unit/

a
 a large number of users here use this charger module

Cheers from Panama
CG
aventeren likes this post
**************************************************************
Based in Panama

Off-Grid
PV = 3 x 200W
Bat = GEL 250Ah
Feeding Fans / CCTV / Modem-Router / Android Boxes

Going on-grid
** 345 ** batteries harvested / getting GTILv2 2Kw auto sense 45-20V 240V
Lots of items coming from china - usa

...Updating as progress flow...
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