How to correctly charge a lithium-ion battery?

The rate of charge or discharge is usually expressed in terms of battery capacity. This speed is called the C rate. The C rate is equal to the charge or discharge current under certain conditions and is defined as follows:

I=M&TImes;Cn

Among them:

I = charge or discharge current, A

M=C multiple or fraction

C = the value of the rated capacity, Ah

N = number of hours (corresponding to C).

A battery that discharges at a 1x C rate will release the nominal rated capacity within one hour. For example, if the nominal capacity is 1000 mAhr, the discharge rate of 1 C corresponds to a discharge current of 1000 mA, and the rate of C/10 corresponds to a discharge current of 100 mA.

Generally, the battery capacity determined by the production mark refers to the capacity of 5 hours when n=5. For example, the above battery can provide 5 hours of operation time at a constant current discharge of 200 mA. In theory, the battery can provide 1 hour of operation time at 1000 mA constant current discharge. However, in practice, the efficiency of the large battery is reduced, and the working time at this time will be less than 1 hour.

So how can I properly charge a lithium-ion battery? The most suitable charging process for lithium-ion batteries can be divided into four phases: trickle charging, constant current charging, constant voltage charging and charging termination.

Phase 1: Trickle Charge - Trickle charge is used to precharge (recovery charge) the fully discharged battery unit. When the battery voltage is lower than about 3V, the battery is first charged with a constant current of up to 0.1C.

Phase 2: Constant Current Charging - When the battery voltage rises above the trickle charge threshold, the charging current is increased for constant current charging. The current for constant current charging is between 0.2C and 1.0C. The current during constant current charging is not required to be very accurate, and a quasi-constant current is also acceptable. In a linear charger design, the current often rises as the battery voltage rises to minimize heat dissipation problems on the pass transistor.

Constant current charging greater than 1C does not shorten the entire charging cycle time, so this is not desirable. When charging at a higher current, the battery voltage rises more rapidly due to the overvoltage of the electrode reaction and the voltage rise on the internal impedance of the battery. The constant current charging phase will be shorter, but since the time of the constant voltage charging phase will increase accordingly, the total charging cycle time will not be shortened.

Phase 3: Constant Voltage Charging - When the battery voltage rises to 4.2V, the constant current charging ends and the constant voltage charging phase begins. For best performance, the regulation tolerance should be better than +1%.

Stage 4: Charging Termination - Unlike nickel batteries, continuous trickle charging of Li-ion batteries is not recommended. Continuous trickle charging can cause plate plating effects on metallic lithium. This can make the battery unstable and can cause sudden automatic disintegration.

There are two typical methods of terminating the charge: using the minimum charge current or using a timer (or a combination of both). The minimum current method monitors the charging current during the constant voltage charging phase and terminates charging when the charging current is reduced to the range of 0.02C to 0.07C. The second method is timed from the beginning of the constant voltage charging phase, and the charging process is terminated after two hours of continuous charging.

The above four-stage charging method requires about 2.5 to 3 hours to complete charging of the fully discharged battery. Advanced chargers also use more security measures. For example, if the battery temperature exceeds the specified window (usually 0 ° C to 45 ° C), charging will be suspended.

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