2008 Hi-Power 100ah cell. Cell resting at 2.5 volts for a few months was chosen for the test. It is a known good cell being what it is and being it is an early LiFePO4 cell of lesser quality than the more current cells. But still decent.
I took the cell and placed it out in the open during the evening to bring the temp of the cell down to the current temp of the evening air which was 28 degrees. I was hoping for lower and it had been lower a few nights before. I figured this would get me to a cold point well below the freezing point. Success but it was short lived. Once we started the charge procedure the temps rose but not by much but above the freezing point. I decided to place the cell on a block of ice and place another on top to keep the cell as cool as possible.
My technique for charging was to charge at 15 amps but really needed to start it at 19 amps and let it taper to 15 amps. The technique is consistent and works quite well. Once the cell reached 3.65 volts I turned down the amperage to 10 amps then to 5 amps and each time allowed to reach 3.65 again. Once it reached 3.65 @ 5amps we terminated the charge.
I did this on two consecutive days. The second day what changed was the medium I used to keep the cell cold. I placed the cell in a bucket of water and placed that bucket in our chest freezer and let it freeze overnight. In the morning I pulled the bucket out with the frozen water and cell imbedded within. The temp of the freezer was 5 degrees F when I pulled it out. I began charging again using the same technique.
We logged sort of every 10 to 20 minutes or so until the charge cycle was complete. Exactly the same both days.
The problem today was I found that the bucket of water was not fully frozen and therefore was not as cold as expected. I did check the terminal temps and never saw over 50 degrees.
As for the discharge I used my old Starter/Generator for our discharge medium. This allowed a discharge current of 25 amps at the start and on the first day we reached 30 amps at the end of the charge when we terminated at 2.5 volts. Todays discharge resulted in an amperage of 40 amps at the end of the discharge cycle and the same 2.5 volts.
I logged that as well and have two graphs overlapping. I have the numbers on both tests so you can see that the results were nearly the same each time.
Both the Charge and discharge times were quite close.
I currently have the cell once again in the freezer and we will let it remain for a few days if not a week. That will be a good long time to be sure it's cold. Our freezer will reach 0 degrees F. It will in fact go below 0 degrees F by a few degrees. It's nice and cold.
Attached is my PDF with graphs and log numbers.
Pete
