lithium batteries never seem to fully self balance while in parrallel?

jimmyhackers

10 kW
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May 11, 2015
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I use two different sets of 3x 6s lithium batts in series for 72v on my bikes (an old 11ah rc lipo set and a newer 20ah eig cell set). i connect all 3 (per set) in parrallel for charging on my 6s isdt charger.

i set the max charge per cell to 4.15v

i check the individual voltage of each cell before connecting them in parrallel on the chargers balance board. They're never more that 0.05-0.03v out from each other.

after a charge they will all be at 4.15v (stilll in parrallel)

however on the older rc lipo cells, when I disconnect them all and re-check voltages there is some discrepancy between, say cell 1 in one battery is 4.18v, and cell 1 in another battery is 4.14v, and the final cell 1 is like 4.13v.
its like this for every cell (1 - 6).....

this discrepancy is way less pronounced/apparent on the newer eig cells.

im just wondering what exactly causes this, as i thought cells connected in parrallel should self balance?
 
im just wondering what exactly causes this, as i thought cells connected in parrallel should self balance?
Yeah, they do, but then you took them out of parallel, so they drift based on the minute discrepancies between them. Various IR, age, individual chemistry. Mostly age.

Secondly, check out a graph of the discharge curve of Li-ion. There's almost no capacity difference between 4.20v and 4.15v, and very little capacity difference between 4.15v and 4.1v. Meaning, just because some of your cells dropped from 4.15 to 4.13, that doesn't mean they actually lost a ton of energy. Just means they drifted a bit, small movements like that are perfectly normal for even moderately aging cells. Which is why:
this discrepancy is way less pronounced/apparent on the newer eig cells.
Since they're new, they're not drifting their resting voltage very much.
 
I use two different sets of 3x 6s lithium batts in series for 72v on my bikes (an old 11ah rc lipo set and a newer 20ah eig cell set). i connect all 3 (per set) in parrallel for charging on my 6s isdt charger.

i set the max charge per cell to 4.15v

i check the individual voltage of each cell before connecting them in parrallel on the chargers balance board. They're never more that 0.05-0.03v out from each other.

after a charge they will all be at 4.15v (stilll in parrallel)

however on the older rc lipo cells, when I disconnect them all and re-check voltages there is some discrepancy between, say cell 1 in one battery is 4.18v, and cell 1 in another battery is 4.14v, and the final cell 1 is like 4.13v.
its like this for every cell (1 - 6).....

this discrepancy is way less pronounced/apparent on the newer eig cells.

im just wondering what exactly causes this, as i thought cells connected in parrallel should self balance?
You’re also using an 11Ah pack in series with a 20Ah pack, so when you discharge the smaller pack will be being used up considerably quicker, and will be degrading its life considerably faster than the other pack. The differences in drop under load in this Frankenpack will destroy the smaller pack quite quickly.
 
i knew people would mis read that. (read it again)

i have 3 x6s 11ah, and 3x 6s 20ah......i do not swap or mix and match between the different 11ah and 20ah sets.....

i.e. i only use/charge the 11ah with its other 11ah batts in its set of 3,
and in only use/charge the 20ah with its other 20ah batts in its set of 3

the discrepancy is as harrison said....just different ir in batts as they are older...
 
Parallel charging two or more bricks of LiPoly on a RC toy charger will not bring all the cells into perfect balance, even when they are new as the charger sees the sum of each cell group. Also, as stated above, as the cells cycle w/ service, they will likely develop different internal resistances.
That said, imbalances of less than 0.05V are really not a problem. If you looked, you would see they quickly come into balance as the pack discharges. Problems arise if the starting imbalance is greater and they are pushed deep ito the dischage cycle. If they don't "self-balance" and if any are weaker than the others, those cells will "dive" off the deep end.
I get around this issue by two approches;
For my 12S/10Ah pack, In use BIG 6S/10Ah bricks and "break the string" and charge one brick at a time on my 6S cheap toy charger.
For my 14S/5.2Ah pack, I leave them together and bulk charge both 7S bricks at the same time w/ a MeanWell and ck and balance (if needed) w/ Battery Medics.
Also, I very rarely discharge below 3.80 V/cell., which really helps extent their service life.
 
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ive been ummng and arring over doning exacly that (more chargers) for a while

my current isdt is great. 600w max, but i only have a 12v power supply for it so its limited to 300w. which is still a decent 10a charge rate at 24v 6s.

finding two more isdts and a reasonable power supply for each.....will be pricey. theyve gone up a lot since i got mine.

i have a turnigy accucel but its only 80w max (3.3 amps at 24v 6s) and two more of those would be pretty expensive too.

the cheapest option is a 24v 600-700w psu for my current isdt so i can charge at 20a instead of 10a.... but it doesnt solve the problem im moaning about in this thread :p
 
ive been ummng and arring over doning exacly that (more chargers) for a while

my current isdt is great. 600w max, but i only have a 12v power supply for it so its limited to 300w. which is still a decent 10a charge rate at 24v 6s.

finding two more isdts and a reasonable power supply for each.....will be pricey. theyve gone up a lot since i got mine.

i have a turnigy accucel but its only 80w max (3.3 amps at 24v 6s) and two more of those would be pretty expensive too.

the cheapest option is a 24v 600-700w psu for my current isdt so i can charge at 20a instead of 10a.... but it doesnt solve the problem im moaning about in this thread :p
 
<<<< ....but it doesnt solve the problem im moaning about in this thread :p>>>>
Battery Medics will.
They just take some time.
 
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