madin88 wrote: ↑
Jun 28 2018 7:24am
I don't think the coulomb efficiency of the electrolyte has much todo with self balancing...
100% relevant for drifting, because it is a way to loose electrons in a series connection.
What about the internal resistance? It's known that it depends on the S.O.C.
100% irrelevant. Ri does not kill a single electron. Higher Ri means higher internal voltage loss at a given current and more heat, but doesn't change the mAh witin a cell the slightest bit.
So regarding mentioned example of Syonyk, if two cells one at 4,1V and the other at 4,2V are put in series and you charge them, the cell with 4,2V probabaly will become warmer as the cell with 4,1V.
This would mean each time the one cell becomes overcharged, a part of the excess energy is turned into HEAT wich means the voltage difference will become less and less each time you charge them.
Cells heat up because P thermal = I² * Ri.
But for changing the state of charge you need a loss of electrons in one of the cells. This is impossible in a series connection, where I is the same in every cell (basic physics), no matter the Ri of each sell.
If I is the same and time is the same (obviously), the charging in I*t (Coulomb) is exactly identical.
The only way for drifting is "killing" electrons. This is possible in nickel and lead based batteries because in those batteries electrons can be used for electrolysis of water based electrolyte and this is the reason why their Coulomb efficiency is way below 100%.
Li-cells do not have such reversible reactions.
Every "killed" electron in a series of Li-Ion cells is a small but irreversible destruction of that cell. (= coulomb efficiency below 100,00%)
There is one exception to this and this is (reversible) self discharge. If you want to have Li-Ion cells without drift you need to buy cells with identical self discharge. A self discharge of (practically) zero is also okay and this is what most modern Li-Ion cells have. This is the reason why there is no drift in modern Li-ion cells, except if you buy the Chinese junk.
And this is why there was huge voltage drifting in a series of NiMh cells, because those had huge and various self discharge rates.
Self balancing is completely impossible for Li-Ion cells. There is no mechanism to transfer electrons from one cell to the other if connected in series, this is against basic laws of physics.
Despite all that the Sony V3 cells used in BionX batteries without BMS have outstanding quality and consistency.
If you buy 50 cells and plan to use them without BMS, it does matter if they have differences in capacity and internal resistance of 2%, or only 0,5%.
Ri is not relevant for drift.
Differences in capacity are possible and so you get different voltages at low SOC, but this is NOT drifting.
Driftig is the loss of electrons in single cells within a series connection. This is impossible in Li-Ion batteries (except for self discharge).
I wouldn't recommend V3 cells for BionX batteries. There are much better performing cells available today.