I was recently shipped a $9,000.00 24V LiFePO4 marine
battery that had been severely over discharged. The eight 160Ah prismatic cells inside ranged from 0.760V to 0.813V when I received the battery.
Due to these voltages, and an overall pack voltage of just 6.117V, for a 24V nominal pack, I did not want to attempt to charge this as a bank
I disemboweled the battery & removed the cells and began charging them independently at 2A to 3.0V. Once at 3.0V they were allowed to sit for 24-30+ hours and resting voltage monitored. If the voltages remained stable I continued to charge them to 100%.
Not only had these eight cells been over-discharged
(though never hitting 0V) they had also been over-charged
This bank had been charged at 29.2V or the equivalent of 14.6V for a 12V pack or 3.65VPC. The owner thinks
the system held 29.2V for about 4 hours. The case design did a poor job of containing the lower third of the cells and as a result the end cells were bulged a bit down low, not bad
but definitely bulged
from over charging.
I then decided to begin capacity testing one of the cells. I charged cell #2 to 3.55V and allowed current to taper to 5A. I then applied a 25A constant load at 75F and discharged the cell to 2.5V and counted the Ah's & time.
The first capacity test came in at about 110Ah but I did not record
because I was just considering scrapping the cells
. Remembering that I had seen recovery of capacity in LFP before, after multiple 100% discharge/recharge events
, I decided to push on with Cell #2.
Discharge - 25A @ 75F to 2.5V - Recharge - 40A to 3.55V & 5A
Capacity Test #1 = 110.? Ah (did not write it down forgot tenths)
Capacity Test #2 = 119.8Ah
Capacity Test #3 = 131.4Ah
Capacity Test #4 = 139.6Ah
Capacity Test #5 = 145.2Ah
Capacity Test #6 = 145.9Ah
Once capacity stopped climbing I ended the capacity testing.
It should be noted that even after a complete 100% discharge to 2.5V (note I do not recommend this
) the resting voltage of this cell bounced back to 3.019V. This is a clear reason why voltage is a horrible indicator of SOC with LFP.
A fair number of folks, here and elsewhere, have noted diminished capacity under fractional "C" use and I suspect much of this capacity is still inside
the cells waiting to be reawakened.
My own cells on my own bank have barely budged in capacity in nearly 800 cycles and are 2009 cells. I do however cycle them deeply before a capacity test. I may repeat this same test strategy on my own bank on the next capacity test.
In this very abused cell
I was able to take it from 68.75% of its 160Ah rating all the way back to 91.2% of its as new
factory rating. I suspect when new these prismatic cells delivered 110% +/- (many prismatic cells do) but I can't say for sure, so am using 160Ah as a baseline. These cells have been in hard use since 2010 and have March 23, 2010 or an almost 6 year old date code.
I am astounded at how well these abused
cells responded to the six 100% discharges followed by compete 100% recharges.
If your cells are not meeting your assumed
capacity cycle the heck out of them a few times and repeat the test.
I found this quite interesting hopefully others can glean something from it too...