atatistcheff
Solar Enthusiast
- Joined
- Sep 20, 2019
- Messages
- 176
I need some help from someone with experience building LiFePO4 packs from used cells. Here is my situation:
I recently bought 8 Headway 8Ah LiFePO4 cells to build into a 4s2p battery for my UPS. I got these from batteryhookup.com. These are "engineering samples" and have a 2017 manufacture date. They advertise something like 90-102% capacity on these cells. When I first got them I measured the voltage on each cell. The high was 3.332 and the low was 3.240 so not crazy different - that's a 0.092V range across all cells. I hooked them up in parallel for a few hours and then built the pack and connected the 4s BMS and let them sit overnight. The next morning I measured the voltage across the four pairs of cells and they were as follows:
1 - 3.330
2 - 3.301
3 - 3.289
4 - 3.331
The range was then 0.042V which is well within the BMS spec of +-0.05V
I then did a capacity test at about 25A (3C) rate. The BMS cut off the power after 9.2Ah was used according to my hall coulomb meter attached to the battery positive wire. At 100% capacity this pack should have 16Ah @ 12V. That's only 57% of the rated capacity of the pack.
In testing voltages I found that pair 3 was at 2.5V which is why the BMS cut off the power. The other cell pairs were between 3.16 and 3.25 volts.
I tried bottom balancing the cells by bringing pair 3 up to 3.2V but again the BMS cut the power while the other cells were well above the 2.5V cutoff. So it seems pretty clear that I have one or two cells that are not cutting the mustard.
So my questions are:
Is my methodology solid?
If I do have a bad cell or two what is the best way to determine each individual cell capacity? I can power an inverter with 12V but trying to do a test with 3.2V is a bit problematic. I'd like to confirm each cell but I need a good way to do that.
Other than ask batteryhookup to replace the cell(s) which are under capacity is there anything else I can do to help this pack perform better?
Thanks in advance for reading this lengthy message!
I recently bought 8 Headway 8Ah LiFePO4 cells to build into a 4s2p battery for my UPS. I got these from batteryhookup.com. These are "engineering samples" and have a 2017 manufacture date. They advertise something like 90-102% capacity on these cells. When I first got them I measured the voltage on each cell. The high was 3.332 and the low was 3.240 so not crazy different - that's a 0.092V range across all cells. I hooked them up in parallel for a few hours and then built the pack and connected the 4s BMS and let them sit overnight. The next morning I measured the voltage across the four pairs of cells and they were as follows:
1 - 3.330
2 - 3.301
3 - 3.289
4 - 3.331
The range was then 0.042V which is well within the BMS spec of +-0.05V
I then did a capacity test at about 25A (3C) rate. The BMS cut off the power after 9.2Ah was used according to my hall coulomb meter attached to the battery positive wire. At 100% capacity this pack should have 16Ah @ 12V. That's only 57% of the rated capacity of the pack.
In testing voltages I found that pair 3 was at 2.5V which is why the BMS cut off the power. The other cell pairs were between 3.16 and 3.25 volts.
I tried bottom balancing the cells by bringing pair 3 up to 3.2V but again the BMS cut the power while the other cells were well above the 2.5V cutoff. So it seems pretty clear that I have one or two cells that are not cutting the mustard.
So my questions are:
Is my methodology solid?
If I do have a bad cell or two what is the best way to determine each individual cell capacity? I can power an inverter with 12V but trying to do a test with 3.2V is a bit problematic. I'd like to confirm each cell but I need a good way to do that.
Other than ask batteryhookup to replace the cell(s) which are under capacity is there anything else I can do to help this pack perform better?
Thanks in advance for reading this lengthy message!