What is a factory defect?
It’s a battery that is either manufactured incorrectly, leading to failure in use, or a cell that has a manufacturing defect that becomes apparent in or after use. A manufacturing error could be a weak connection, poor solder joint, missed cell connection, and/or improperly installed component.
Weak connections, like a poor solder joint, don’t allow for the proper flow of power out into the system it is powering. Heat is generated which can cause a cell to deteriorate in use. The heat buildup should be noticeable early on but it may be a few cycles before there is obvious damage to a cell.
Missed cell connections are most common in multi-P (or paralleled cell) batteries. Connecting in parallel requires connecting multiple cells together on both positive and negative connections. While uncommon, it is possible for one of those cells to not be properly connected with the others. This will cause a cell imbalance in charge and discharge. This type of error should be recognizable very early in the use process as your battery will get further out of balance with each charge/discharge. While a balance process can bring the lines back into line with each other, that separation will become apparent again on its next use.
A cell defect can be harder to detect as it can initially voltage test well but, once load is applied, that cell will likely begin to show it’s defect by not properly maintaining voltage under load with the other cells or continuing to discharge after load is removed.
Self-discharge is a symptom of a micro short in most cases and will cause a cell to over discharge over a period of time.
Defective cells are rare and most of them are identified and removed before battery pack assembly. So, in the case of a cell defect, you will see the deterioration of a single cell, not an entire battery.
Improperly installed components would be reverse polarity connections (plug or main lead), weak solder connections, incorrect/reverse balance lead install, or bad connector crimps on main connectors or balance leads.