Same with all rechargeable batteries, 18650 battery charge time equals the ratio of rated capacity to charge current. However, because of internal resistance of Lithium Battery during charge, charge time of 18650 battery should consider resistance overcome factor.
As per the resistance extent, a factor has been designed as comparison value of rechargeable battery charge time. Through long-term experiment, this factor has briefly turned into a ratio of charge current to rated capacity. Thus the above 18650 battery has become: 18650 battery charge time=rated capacity/charge current*factor.
According to national standard, battery should be charged under a current not higher than 0.2C. Based on this standard, the above factor is set 1.2. In fact, due to high capacity of 18650 Lithium Battery and different features of lithium battery charger from different manufacturers, usually this factor is set 1.5 during battery charge time calculation. Basically battery chargers are designed under this theory. For example, usually 18650 battery capacity is 1800-2600mAh. On the basis of the above formula, charger choice and 18650 battery charge time (under 0.2C discharge) should be so follow:
For 1800mAh battery, approximate 7.5-hour charge time with 350mA charger (1800÷350×1.5).
For 2200mAh battery, approximate 8.25-hour charge time with 400mA charger (8.5 hrs if 350mA charger used).
For 2600mAh battery, around 7.8-hour charge time with 500mA charger (2600÷500×1.5).
Considering different charger design, there are three alternative factors: 1.2, 1.3 and 1.5. The small the factor, the shorter the charge time will be.
Practically, 18650 battery charge time is set with PCM and protection IC during design. All that customers have to do is to use under the user instruction. Besides, it would be the best to stop charging after the color change of charger indicator because overcharge arising from too long charge time will damage charger and 18650 battery.
