What's done for each cell test?

Published by Mooch in the blog Mooch's blog. Views: 3276

Updated 10/24/15 to include pulsed discharge tests.

For every cell I test, these are the steps I follow...

Continuous Current Tests
  • I use two of each cell for testing.
  • Photograph the wrap from one cell and top if a button has been spot-welded on.
  • Remove the wrap and photograph the case, top, and bottom.
  • Attach the thermocouple (temperature sensor) halfway down the cell with Kapton tape, making sure to cover the tip of the thermocouple to prevent any flowing air from cooling it.
  • Clean the test rig contacts with a Scotch-Brite pad and then a 90% alcohol wipe.
  • Mount the cell in the low-resistance test rig.
  • Charge 18650's and 26650's to 4.20V at 2.5A until the current drops to 100mA. 18350's are charged at 0.5A unless the manufacturer specifies a higher rate.
  • Run three constant-current (CC) discharges down to 2.80V to check basic cell functionality, including capacity and temperature. 18650's and 26650's are discharged at 10A. 18350's-18500's are discharged at 5A.
  • If all three discharges are essentially identical then I continue. If something keeps changing for each discharge I keep running them until the cell's performance has stabilized. So far, every cell has stabilized within three discharges.
  • For each discharge I measure the actual current level using a 0.25% tolerance current shunt and a Fluke 8846A meter. This not only confirms the starting current level but by using the min/max/avg functions of the meter I can confirm that the current level has not drifted.
  • Run CC discharges, down to 2.80V, at every 5A increment above that until the cell reaches 100°C or the voltage just quickly collapses.
  • Note the maximum cell temperature reached for each discharge.
  • After each discharge let the cell cool to below 40°C before recharging.
  • Recharge each cell to 4.20V, stopping when the charge current has dropped to 100mA.
  • Determine the cell's continuous discharge rating (CDR) by noting the current level that brings the temperature closest to the 78°C average (74°C-82°C range) I measured for the Samsung, Sony, LG cells I tested at their CDR.
  • Run an additional two CC discharges at the cell's CDR to check for voltage sag, loss of capacity, or increasing temperature. These are all signs of cell damage and indicate that the cell's rating is too high.
  • Run an additional two CC discharges at 5A above the cell's CDR to check for voltage sag, loss of capacity, or increasing temperature. These are all signs of cell damage and indicate that it's being discharged at beyond its rating. It also gives us an idea of hard it can be abused.
  • Take the second cell, run the three initial discharges, and then discharge at 10A and at the CDR of the cell. If the results are within 2% of the first cell then the first cell's discharge graph is used. If the discharges of the second cell are different from the first I do not post any test results until I can source another set of cells to test and compare.

Pulsed Current Tests
  • Discharge the second 18650 or 26650 cell at 30A, each pulse is 5 seconds on/30 seconds off, down to 2.50V. 18350-18500 cells are started at 10A.
  • A lower cutoff voltage is used for the pulse testing to give those cells that have a significant increase in voltage when hot (due to lowered internal resistance) a chance to warm up.
  • Run pulsed current discharges, down to 2.80V, at every 5A (for 18350-18500) or 10A (for 18650-26650) increment above that until the cell reaches 100°C or the voltage drops to 2.50V for the first pulse.
  • After each discharge let the cell cool to below 40°C before recharging.
  • Recharge each cell to 4.20V, stopping when the charge current has dropped to 100mA.
  • Note the maximum cell temperature reached for each discharge.

I don't have a standard yet for determining the pulse rating for a cell. When I have enough pulsed current discharge data I will give each cell I test a pulse rating. In the mean time you can view the discharge graphs to see what the voltage drop is for the cells I have been testing recently. All of the Samsung, Sony, and LG cells are being retested to add this pulsed current data.


Important Notice!
Testing batteries at their limits is dangerous and should never, ever be attempted by anyone who has not thoroughly studied the dangers involved and how to minimize them. My safety precautions are the ones I have selected to take and you should not assume they will protect you if you attempt to do any testing. Do the research and create your own testing methods and safety precautions.
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