Matching cells from a bulk purchase

[Bo_Hunter]

I purchased 32 320Ah LiFEPo4 cells from the same supplier at the same time. They all arrived together. I tested their initial voltage and the cells were all between 3.205 and 3.209 volts from the factory. I have a small charger and topped every cell up to 3.209 volts and made four 24v packs. I finally had enough sunshine recently for the solar charge controller to charge all 4 packs up to the point the BMS cut off charging because of cell overvoltage. At this point, I read the voltage of each cell from all 4 packs. What I am thinking I need to do next is re-arrange the packs and combine the cells with the highest voltages together and "match" the cells this way. Seems like the internal resistance of some of the cells is significantly different than the others. Once the packs are rearranged, then allow the solar charge controller to get the entire packs up closer to 3.65v and then I can top balance with my small 10amp charger. Anything wrong with this plan? See attached images for gory details.

 

[Bo_Hunter]

I realize that after a few full cycles, the packs may shift again and this may need to be repeated a few times with full top balances done, but I think this will ensure I get the most available energy out of each pack over its lifespan. Or, it may be a bunch of useless work for extremely minimal gain, but I don't think so.

 

[rodrick]

You can take your 8 highest runners and make a pack out of them that will help but a lot of your cells haven’t even gotten to the point of balancing. voltage wise typically 3.45 volts balance starts anything earlier doesn’t really help much

 

[BigGun]

Just so much easier to Top Balance your cells in the beginning. You would of been closer if you'd of charged your cells to 3.50-3.6 before building the packs.

 

[S Davis]

If you only charged to 3.209 in parallel they are not top balanced, no surprise the state of charge is all over the place. They might arrive with close to the same voltage but that means nothing in the middle of the charge curve.

 

[Bo_Hunter]

You can take your 8 highest runners and make a pack out of them that will help but a lot of your cells haven’t even gotten to the point of balancing. voltage wise typically 3.45 volts balance starts anything earlier doesn’t really help much

Thanks. I agree that the other cells are not close enough to top balance yet. Unfortunately, I only have a small charger and it would take at least a week to get these size packs charged up (x4). So, I let the solar charge controller do the heavy lifting. I suspect once I get the cells re-arranged, I’ll find others that are more closely match and will play the swap game a few times until I get them as closely mated as possible.

 

[Bo_Hunter]

Just so much easier to Top Balance your cells in the beginning. You would of been closer if you'd of charged your cells to 3.50-3.6 before building the packs.

Agreed, but I don’t have a charger big enough for these packs and didn’t really want to invest in a 40amp (or larger) charger for this one-time event. I’ll match the cells as they are now and let the solar charge controller charge them up closer to top-balance ready and then do a full top balance across all cells with my little 10amp charger. It will still take at least overnight with these packs.