Have you mixed date codes and what can you report?

[Roswell Bob]

I am curious as to the variability in same design batteries over time. If you smoked a battery and had to replace one in a string then what have you observed? Has new cell performed ok? Did the cell balancing mechanism do it's job? I expect as processes matures and recipes are tweaked the variability between batches would tend to lessen. I haven't been on the LiFePO4 bandwagon long so I have no history to speak of. If you have some history I would like to hear. Have you observed a tightening of specifications over the years? Would you expect performance of different date codes to converge as time goes by? Will the Grade B cells become extinct on their own? If you have any comments regarding the effectiveness of balancers with unmatched cells then please share. Thank you.

 

[joanna_lannie]

Seems like a lot of questions. But I think your ultimate goal is to confirm one thing: what is the result of mixing new and old batteries. In fact, the problem is not big, but there will still be a situation where the new battery is slightly restricted by the old battery because the battery pack is a whole. Of course, the degree of restriction will not be very large, but batteries that differ by more than a few years are another matter.

 

[Roswell Bob]

Seems like a lot of questions. But I think your ultimate goal is to confirm one thing: what is the result of mixing new and old batteries. In fact, the problem is not big, but there will still be a situation where the new battery is slightly restricted by the old battery because the battery pack is a whole. Of course, the degree of restriction will not be very large, but batteries that differ by more than a few years are another matter.

Yes. I see another post where battery was dropped and case caved in. I was wondering if one cell could be replaced without a problem. I have found with many electrical components that as time goes by the variability decreases. I see your point about the ageing of cells could make matching new to old more of a problem. Thank you.

 

[RCinFLA]

Matching of used cells dependents on how the cells were used. The more charge-discharge cycles, greater charge and discharge currents used, and ambient temps exposed to affect their aging rate. There is always the possibility they were exposed to abuse like over-charge, or being dropped. As cells age their internal impedance rises due to the reduction in ease of lithium-ion migration through cell to support demanded cell current.

You can check matching by measuring difference of unloaded rested open circuit cell voltage to moderate load current terminal voltage slump after 3 minutes of moderate constant cell discharge loading of between 0.2 to 0.4 C(A) current rate. Needs to be done with cells at same temperature, preferably close to 20-25 degs C and near mid-state of charge.

Matched cells will have the same delta terminal voltage slump after reaching equilibrium cell terminal voltage at the given cell current rate. The closer the terminal delta voltage slump for given load current the better. Greater than 20% difference is not well matched. An old cell can increase in voltage slump 3x to 5x compared to its new condition. This makes heavily used cells unusable for anything more than light load current.

When load is applied, the cell terminal voltage will slump with an exponential decay. It will level out in one to three minutes. Depending on load current level, the voltage will immediate drop about 20-40% of its final equilibrium voltage slump due to internal cell conductor resistance then exponentially decay for one to three minutes before it levels out at equilibrium cell voltage. The terminal voltage exponential decay is caused by the lithium-ion migration reaching equilibrium.

Matching of cells is not just having same AH of capacity between cells. The greater the voltage slump with moderate load current the less extractable capacity is achieved from cell.

The more unmatched the cells the harder it is to keep them in state of charge balance with use.

 

[Roswell Bob]

Matching of used cells dependents on how the cells were used. The more charge-discharge cycles, greater charge and discharge currents used, and ambient temps exposed to affect their aging rate. There is always the possibility they were exposed to abuse like over-charge, or being dropped. As cells age their internal impedance rises due to the reduction in ease of lithium-ion migration through cell to support demanded cell current....

This explains very well. Sounds like the newer cell would work fine, but pack Amp-hours will still be limited to the performance of the used cells. BMS is going to trip when once cell voltage is at UV threshold. Thank you.