recently after fully charged the large 18650 Lithium battery pack I assembled with all brand new cells, I left the pack in my garage to test its self-discharge without any load applied. the voltage of all the parallel group of cells are quite stable, except of one parallel group of cells dropped from 4.20V to 4.12V in one day. I disassembled the pack to remove the bad cell and replace it with one brand new good cell.
for the bad cell, its voltage continuously drops quickly, from 4.05V to 0.25V in 3 days. however its internal resistance is almost the same as that of the other good cells. for this bad cell, there is no liquid leakage, no physical damage or drop to the ground, no short circuit, no over heated and its appearance looks the same as that of the good cells. prior assembling, its voltage is the same as other brand new cells which is about 3.61V (I purchased a lot brand new 18650 cells at the end of 2018 and left them in my garage for about 5 months, and before started to assemble them more than one month ago, their voltage are all around 3.61V, very similar as that when I received them about 5 months ago), don't know why after assembling them together with a BMS with balance feature, fully charged the pack to about 4.20V, this cell became a bad cell.
with a 4.2V charger, can charge this bad cell from 0.25V to 2.0V in 20 seconds with current about 0.45A, then disconnect the charger, and its voltage drops down very quickly. this shows the cell can't really be charged, and can't hold the charge, correct?
is this cell the bad cell originally? or something happened to it in the assembling and charging process? it doesn't seem that this cell has built-in protection circuit. its internal resistance is the same at that of the good cells, and everything else looks normal to it, just its voltage keeps dropping very quickly. what electrical problem of it causes its voltage drops so quick? is it able to repair it?
for the bad cell, its voltage continuously drops quickly, from 4.05V to 0.25V in 3 days. however its internal resistance is almost the same as that of the other good cells. for this bad cell, there is no liquid leakage, no physical damage or drop to the ground, no short circuit, no over heated and its appearance looks the same as that of the good cells. prior assembling, its voltage is the same as other brand new cells which is about 3.61V (I purchased a lot brand new 18650 cells at the end of 2018 and left them in my garage for about 5 months, and before started to assemble them more than one month ago, their voltage are all around 3.61V, very similar as that when I received them about 5 months ago), don't know why after assembling them together with a BMS with balance feature, fully charged the pack to about 4.20V, this cell became a bad cell.
with a 4.2V charger, can charge this bad cell from 0.25V to 2.0V in 20 seconds with current about 0.45A, then disconnect the charger, and its voltage drops down very quickly. this shows the cell can't really be charged, and can't hold the charge, correct?
is this cell the bad cell originally? or something happened to it in the assembling and charging process? it doesn't seem that this cell has built-in protection circuit. its internal resistance is the same at that of the good cells, and everything else looks normal to it, just its voltage keeps dropping very quickly. what electrical problem of it causes its voltage drops so quick? is it able to repair it?
