What Is The Best Charging Current And How To Calculate The Charging Time Of Lithium Battery

Mar 08, 2022 | XTAR

People often ask, what’s the best charging current for my batteries? How long does it take to fully charge the battery? Now let us discuss this issue in detail.

First, how to determine the current when charging the lithium battery?

It should be based on battery capacity. According to the structural characteristics of the lithium battery, the charging current and charging voltage are dynamically changed. The maximum charging termination voltage should be 4.2V. Do not overcharge, otherwise the battery will be damaged and serious danger may occur. So it is more perfect to use a professional charger which use the trickle (pre-charge)-constant current-constant voltage charging method. Usually the charging current we manually select is actually the charging current in the constant current charging stage.

The optimal charging current for lithium batteries is actually divided into three phases:

1. Trickle Recover: when the initial (no-load) voltage of the lithium battery is lower than the pre-charge voltage, the battery activity weakens. Setting a precharge stage is helpful to extend the battery life, just like the first gear of a car. For a single lithium-ion battery, this voltage is generally 3.0V, and the charging current can be set to about 100mA or 10% of the constant current charging current. (If the battery voltage is higher than 3.0V, there is no trickle recover stage.)

2. Constant Current Charging: The so-called constant current means that the current is constant and the voltage is gradually increased. Now it comes into fast charging phase. Most constant-current charging currents are set between 0.2C and 1C. Here, 0.5C is generally selected. The reason for choosing 0.5C is that this current is a good balance between charging time and charging safety. For lithium batteries It is also generally suitable.

Here C refers to the battery capacity, such as 3000mAh battery, 0.2C is 600mA, 1C is 3A. The conventional charging current can be selected to be about 0.5 times the battery capacity, and can be charged to 75-85% in 2 hours. Adding the constant-voltage charging time, the entire charging time is about 2.5 to 3 hours.

Such as:

The common Panasonic capacity type battery 18650 3500mAh and can be charged at 0.2C-0.5C, that is, the charging current is 700mA-1.75A.

The power type Samsung 30Q, 18650 3000mAh can be charged at 0.5C-1C, that is, the charging current is 1.5A-3A.

The new battery can select the top value, and the old battery should select the lower.

3. Constant Voltage Charging: For a single-cell lithium-ion battery, when the battery reaches a certain voltage value, it enters the constant voltage charging stage. This voltage value is generally 4.0-4.2V. At this stage, the battery voltage is gradually approaching 4.2V, the voltage difference between the charger and the battery becomes smaller, and the current gradually decreases. When the cut-off current set by the charger is reached, the charger is internally disconnected and the charging is completed, and the current is 0A.

At present, some smart chargers can intelligently detect the battery condition and automatically select the most appropriate charging current, and no longer need to manually adjust the current.

Second, how to calculate the charging time of lithium battery?

The charging current of the lithium battery is usually marked on the charger. If you want to calculate the charging time, divide the battery capacity by the charging current, and add 0.5-1 hours at the end. Why add 0.5-1 hours? Because when the charger reaches the limited voltage of the charger, constant voltage charging will be performed, the charging current becomes smaller, and the charging becomes slower, so the charging time will add about 1 hour.

The charging time of 18650 battery = nominal capacity / charging current + 1h. However, this is a theoretical value, the actual value is different due to the following three factors:

A. the settings of different manufacturers’ chargers are different;

B. whether the lithium battery parameters are true;

C. the battery is old or new.


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