The easiest way to tell that a Li-Ion cell is nearing it’s EOL, is that it will not hold a charge well, will not perform as well as it used to, and will get noticeably warmer when charging.
With a means to measure cell voltage, you can tell if a cell is holding charge well, or not by measuring the cell’s voltage after charging. Newer LiCo/ICR, or LiMn/IMR cells will typically charge up to 4.18-4.20 Volts when charged with a proper CC/CV charger. Newer cells will also stay at this voltage for quite some time. Older cells will loose some voltage soon after charging. If cell voltage drops to ~4.10 Volts after charging to 4.18-4.20 Volts, after a day or so, the cell is well on it’s way towards EOL. In general, cells that drop to 4.00 Volts, are considered at EOL and should be replaced. Also, it is generally accepted that Li-Ion cells should be disposed of /recycled when the charged capacity has dropped to 80% of the original capacity of the cell.
Li-ion battery, unless abused e.g. Extreme temperature exposures, overdischarged, overcharged, etc. It’s not likely to blow up.Usually, when a Li-Ion cell is used up, it just doesn’t work as well. It doesn’t hold voltage under load as well as it did when new, exhibits a higher rate of self discharge, and warms up when charging (this is an indication of high internal resistance, which is associated with cell degradation). So, in most cases the cell simply lacks the performance it once had.
The problem with old worn out Li-Ion cells is that the chemical components within the cell tend to become unstable. Add to this the fact that these cells get much warmer when charging, and you may have a problem.
All Li-Ion cells use a volatile electrolyte. They also have the ability to create oxygen within the cell. Add heat created during charging (or discharging, as well) of a cell which has a high internal resistance, and you increase the chances of a “vent with flame” incident occurring.