RCinFLA
Solar Wizard
- Joined
- Jun 21, 2020
- Messages
- 3,561
I took the data posted by "Andy's Off-Grid Garage" and imported .csv files into Excel to produce attached graphs with additional important parameters noted.
The graphs in the .pdf file are for two 304AH and four 280AH cells Andy provided charge and discharge .csv files. The graphs are on individual PDF layers in the document that can be individually selected by Adobe Reader layers control panel on the left-hand vertical menu bar in Adobe Reader.
He did not record ambient temperature during the time the data is taken which is important as temperatures below 20 degs C reduces yielded cell capacity and increases overpotential of given cell current.
Rested open circuit voltage for a cell is approximately halfway between equal charge and discharge current curves on LFP cells, of which he used 40 amps for both charging and discharging. Overpotential voltage is nearly constant for given cell current from about 90% to 20% state of charge.
It is better to load test between 0.2 and 0.4 C(A). At low cell current there is more variation in the overpotential voltage, and it is in very low mV range voltage delta. 5 to 10 mVdc of overpotential voltage will occur for only a few mA's of cell current, which is why just passively paralleling cells will not fully balance cells. Overpotential is logarithmic to cell current.
If you insert several 5 minute zero cell current rest periods during test cycle you will allow cell to return to open circuit voltage equilibrium and within about 3 minutes after load current is reapplied you will get a more accurate overpotential voltage indication. 5 minutes rest periods at approximately 75%, 50%, and 25% state of charge are good locations.
Two items, besides total AH of capacity, that are important to examine is overpotential voltage for given cell current from rested open circuit voltage, and wH round trip charge-discharge efficiency.
Greater overpotential for given cell temperature/cell current, and poorer charge-discharge wH efficiency is generally a poorer quality cell. As a cell ages, these parameters will degrade.
Also attached is where these cells fall on the overpotential graph vs. cell current. Zoom into lower left-hand corner of graph to see cell placement dots. Interestingly, the worse performing cell is listed as 'certified'. The Hithium cell is best performing cell.
A truly new cell may need several charge-discharge cycles before Solid Electrolyte Interface layer is fully formed and alignment of anode and cathode capacity curves settle down.
The graphs in the .pdf file are for two 304AH and four 280AH cells Andy provided charge and discharge .csv files. The graphs are on individual PDF layers in the document that can be individually selected by Adobe Reader layers control panel on the left-hand vertical menu bar in Adobe Reader.
He did not record ambient temperature during the time the data is taken which is important as temperatures below 20 degs C reduces yielded cell capacity and increases overpotential of given cell current.
Rested open circuit voltage for a cell is approximately halfway between equal charge and discharge current curves on LFP cells, of which he used 40 amps for both charging and discharging. Overpotential voltage is nearly constant for given cell current from about 90% to 20% state of charge.
It is better to load test between 0.2 and 0.4 C(A). At low cell current there is more variation in the overpotential voltage, and it is in very low mV range voltage delta. 5 to 10 mVdc of overpotential voltage will occur for only a few mA's of cell current, which is why just passively paralleling cells will not fully balance cells. Overpotential is logarithmic to cell current.
If you insert several 5 minute zero cell current rest periods during test cycle you will allow cell to return to open circuit voltage equilibrium and within about 3 minutes after load current is reapplied you will get a more accurate overpotential voltage indication. 5 minutes rest periods at approximately 75%, 50%, and 25% state of charge are good locations.
Two items, besides total AH of capacity, that are important to examine is overpotential voltage for given cell current from rested open circuit voltage, and wH round trip charge-discharge efficiency.
Greater overpotential for given cell temperature/cell current, and poorer charge-discharge wH efficiency is generally a poorer quality cell. As a cell ages, these parameters will degrade.
Also attached is where these cells fall on the overpotential graph vs. cell current. Zoom into lower left-hand corner of graph to see cell placement dots. Interestingly, the worse performing cell is listed as 'certified'. The Hithium cell is best performing cell.
A truly new cell may need several charge-discharge cycles before Solid Electrolyte Interface layer is fully formed and alignment of anode and cathode capacity curves settle down.
