When charging your batteries, temperature is important. Never charge a battery that has been stored below 0℃ as you risk an explosion at these temperatures.
Charge Temperature: 0℃ ~ 45℃
Discharge Temperature: -20℃ ~ 60℃
Storing Temperature : below 60℃
When charging, check the temperature of the battery by hand.
If charging at a 1C charge rate, the battery shouldn’t get warm. Excessive heat means there is something wrong, STOP CHARGING!
After used batteries, you may find your batteries are warm to touch. It may be a good idea to give the batteries a chance to cool a little before recharging. ( the way in summer weather)
*Damaged cells and batteries
Never try charging a battery that has been crash damaged, you run the risk of fire!
Cells that are obviously swollen or have physical damage should never be used and careful disposal is required.
*Never leave a charger unattended!
The most important point above all, knowing how potentially dangerous a Li-ion battery can be if something goes wrong, NEVER leave your batteries on charge unattended.
No matter how safe you think your charger is, things can and really do go wrong from time to time.
As already stated, heat is a big factor. If a battery is pushed beyond 60℃ during discharge or charging, problems occur due to metallic lithium generation, which damages the cell and again causes battery damage.
The maximum voltage for a cell is 4.235V per cell, but this only possible when cell temperature is at around 60℃. The maximum voltage at room temperature is around 4.2V per cell. Going beyond these figures, the over-abundance of electrons will again break chemical bonds and free lithium to bond with oxygen creating lithium oxide and again, more damaged!