Originally Posted by Maine Sail
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...
Those are interesting observations, but I wouldn't mix the topic of abused cells with reduced capacity due to fractional C use. Predictably, we already see some who, in the wake of this, start concluding that being sloppy/reckless is in fact ok.
When cells get discharged below 2.0V resting voltage, like the ones you got hold of, the copper substrate of the anode starts dissolving into the electrolyte. It happens gradually over time. When you recharge, no matter how "careful" you are, that copper doesn't return where it originated from. Instead it plates various parts
of the cell inside. Specifically, it can form crystals and later cause the cell to short out.
Capacity, on the other hand, has little to do with the copper. It is a matter of lithium moving between the active materials, carbon and iron phosphate. So even if those cells exhibit "some capacity", it doesn't mean they are not copper plated and electrochemically damaged with all the consequences this can have.
Those cells are scrap, because all bets are off now. Working or not, they are a liability and I really can't see the sense in trying to gauge "how much" of a liability in order to keep using them. It is ridiculous considering what happens when things do go wrong.
Overcharging, if severe enough, causes electrolyte breakdown and gas pressure build-up. This causes the casings to swell, but evenly. Bulging low down might have a lot more to do with lack of clamping, vibrations, thermal cycling etc, especially after 6 years in service
. The materials inside the cells have sagged.
When it comes to cell voltage vs SOC, 2.5V or 3.0V is a fully discharged cell, there is virtually no capacity to speak of between the two. OCV is the only true indication of the state of charge of a cell
and it happens to be easy to read near the top and near the bottom.
Since I personally couldn't care less if my bank is at 45% or 65% (where voltage is too problematic to correlate to SOC), the voltmeter does a great job at telling me if SOC is up or down and I found that it is all I ever need
You must look at a true OCV curve
, not the manufacturer's charge or discharge curve, this one was not
recorded in open circuit conditions and it includes the effect of internal resistance.
OCV reads different whether the bank was last charging or discharging.
The curves above were recorded by a university in a lab with fully stabilised voltages, so they waited for ages before taking each reading etc.
At very small C-rates, like often found on marine
banks, the current doesn't skew those values much. I find that I use the blue curve to assess charging in the upper range and the orange one to get an idea of how much I have left when the voltage starts dropping (but not while running alternators or heavy loads obviously, because of the effect of internal resistance again).
For example, when I see 13.1V under light discharge, I know I am about 60-65% down and it is highly reliable
, much more than any Coulomb counter gizmo that needs constant resetting.
Similarly, when the solar panels
are charging gently and the voltage has climbed to 13.4V, I know I am in the 80% SOC.
Last but not least, this is true regardless of capacity: it naturally tracks any capacity fade.