Mixing USED and NEW battery cells

[Just]

Hello All,

I want to understand why mixing the Used and New battery cells is not recommended.

Actually I have 48V battery pack, which is build from sixteen 10Ah LiFePo4 cells, which are connected in serial. Some of the cells are almost dead. The question is: "Why cannot I replace just deeply discharged cells and stay with the rest good cells? Why I need to replace the whole battery pack if only several cells in the pack are damaged?" The cells are connected in serial (not in parallel), so they cannot charge/discharge one another.

Thank you!

[Bikemad]

If the battery did not have a BMS to protect the weaker cells, there is a real possibility of the new cells damaging the older cells as they are more likely to become exhausted far quicker the new ones. In extreme cases it can even result in the weaker old cells being charged with a reverse polarity by the available power still being supplied from the good cells.

Without a BMS, the weaker cells could also be overcharged during the charging process as their voltages will probably rise far quicker than the new cells.

As long as your cells are protected with a BMS it should ensure that the older cells are not overcharged during charging, or excessively discharged during use.

Even with several new cells, your battery capacity will still only be as good as the individual capacity of the worst of the old cells.
A chain (battery) is a made up of many individual links (cells) and it can only ever be as strong as it's weakest link.



Alan
 

[Morgen 3Eman]

Nice training aid, Alan.  Concise, clear, and obvious to the most casual observer.

Well done.

TTFN,
Dennis

[Just]

Thank you Alan! You posted really fine pictures representing the problem!

Actually I bought very expensive (about $80) BMS, which should do all the possible jobs including over- charge and discharge protection. In practice, it only balances (hopefully) the cells during charging, but doesn't protect them during a discharge! Due to its bad operation, some of my cells were discharged to 0.22V, 0.58V, 1.38V, and 1.73V! Even some of them were recovered by charging them with a small current, their capacity decreased dramatically! Some of the cells remained dead...