Correct method for matching cells for multiple packs?

[riverbug]

Another question, hopefully not a dumb one. Would someone be able to clarify what the proper method is to match cells when creating packs? In this case 4 cells for 12V packs...

A year ago I purchased two separate sets of 8 280AH Eve cells from 18650batterystore about 6 months apart (so 16 cells total). Each order was supposed to be batched and matched (probably were, I don't know). Once I received each order of the 8 cells, I tested each cell to determine its AH capacity. I then "matched" the cells in groups of 4; based simply on AH capacity. In the end I had 4 12V packs, each with cells that most closely matched one another based on AH capacity from my testing.

I recently had a separate thread started asking about why these 4 packs discharge at different rates. And then today I had this epiphany that maybe when I matched the cells, I didn't do it correctly. Perhaps AH capacity isn't the factor that one should use when matching cells for a pack.

 

[sunshine_eggo]

Another question, hopefully not a dumb one. Would someone be able to clarify what the proper method is to match cells when creating packs? In this case 4 cells for 12V packs...

A year ago I purchased two separate sets of 8 280AH Eve cells from 18650batterystore about 6 months apart (so 16 cells total). Each order was supposed to be batched and matched (probably were, I don't know).

Probably weren't matched based on expectations. Where they of similar characteristics from a single production batch? Probably. Where they matched in any meaningful way besides voltage and internal resistance, probably not.

Once I received each order of the 8 cells, I tested each cell to determine its AH capacity. I then "matched" the cells in groups of 4; based simply on AH capacity. In the end I had 4 12V packs, each with cells that most closely matched one another based on AH capacity from my testing.

I recently had a separate thread started asking about why these 4 packs discharge at different rates.

I didn't see the thread. There are many contributing factors beyond cell characteristic matching that may influence performance.

And then today I had this epiphany that maybe when I matched the cells, I didn't do it correctly. Perhaps AH capacity isn't the factor that one should use when matching cells for a pack.

There are three components to match:

Capacity - pack will be limited by the lowest capacity cell - within 10% is typically workable.
Internal resistance - not hugely important, but excessive difference can reduce cell efficiency - again within 10% is typically workable.
Self-discharge - as close to the same as possible. IMHO, the most important component to retain a top balance AND the most annoying to get meaningful results.

 

[riverbug]

Probably weren't matched based on expectations. Where they of similar characteristics from a single production batch? Probably. Where they matched in any meaningful way besides voltage and internal resistance, probably not.



I didn't see the thread. There are many contributing factors beyond cell characteristic matching that may influence performance.



There are three components to match:

Capacity - pack will be limited by the lowest capacity cell - within 10% is typically workable.
Internal resistance - not hugely important, but excessive difference can reduce cell efficiency - again within 10% is typically workable.
Self-discharge - as close to the same as possible. IMHO, the most important component to retain a top balance AND the most annoying to get meaningful results.

Thanks very much for the response. I also just saw this thread on balancing (and refers to matching) so I'll dig through that more thoroughly as well. https://diysolarforum.com/threads/g...alance-and-charge-a-lfp-battery-part-1.74070/