Suitable charging current for Li-ion batteries on a smart charger

XTAR Light

Apr 26, 2010
To charge the 18650/21700 Li-ion batteries, a smart charger adopts trickle (pre-charge)-constant current-constant voltage charging method. About the proper charging current for the Li-ion batteries, it should be discussed based on the three-stage charging stages on the charger.

*Trickle current charging with a small current makes the Li-ion batteries slowly reactive. In this pre-charge phase, the battery is charged at a low rate, typical of 10% the constant current mode, when the battery cell voltage is below 3V. If the battery voltage is higher than 3V, there is no trickle charging recovery stage.

*In constant current charging stage, when the Li-ion battery voltage reaches 3V, the charger will increase the constant current and gradually increase the voltage, which is the main stage of Li-ion
battery charging. Generally, a standard charge is typically defined as 0.5 C, where C stands for capacity. It means that the charge current should be equal to or less than half the battery capacity. For example, a 2500 mAh cell, the standard charging current would be 1250 mA.

*When the Li-ion battery voltage reaches 4.1 or 4.2 V, the charger switches to a constant voltage stage to avoid overcharging. At this point, the current going into the battery gradually decreases until it reaches around C/10 and C/30, at which point charging is terminated.

To reach the optimal charging current, it's also important to choose a proper adapter with the right power supply. The voltage supplied by the adapter should equal what your charger requires. And the current of the adapter should equal what the charger requires or be larger. For example, if your charger is rated at 5V 2A , you could charge it with a 5V 3A adapter. While, if you use a 5V 1A adapter to power up, the charger will charge the battery much slower or even refuse to work.