Linear and switch-mode charging solutions for Li-ion batteries

Lithium-ion charging

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

I=M×Cn

in:

I = charge or discharge current, A

M = multiple or fraction of C

C = rated capacity value, Ah

N = number of hours (corresponding to C).

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

Usually, the battery capacity specified by the manufacturer refers to the capacity when n=5, that is, the capacity of 5 hours of discharge. For example, the above battery can provide 5 hours of working time when discharged at a constant current of 200mA. Theoretically, the battery can provide 1 hour of working time when discharged at a constant current of 1000mA. However, in reality, due to the decrease in efficiency of large batteries during discharge, the working time at this time will be less than 1 hour.

So how can we charge lithium-ion batteries correctly? The most suitable charging process for lithium-ion batteries can be divided into four stages: trickle charge, constant current charge, constant voltage charge and charge termination. Refer to Figure 1.

Stage 1: Trickle charge - Trickle charge is used to pre-charge (recovery charge) a fully discharged battery cell. When the battery voltage is below about 3V, a maximum constant current of 0.1C is used to charge the battery.

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 does not require very precise current, and quasi-constant current is also acceptable. In linear charger designs, the current often increases with the increase in battery voltage to minimize heat dissipation issues on the pass transistor.

Constant current charging greater than 1C does not shorten the entire charging cycle time, so this practice is not advisable. When charging at a higher current, the battery voltage will rise more quickly due to the overvoltage of the electrode reaction and the voltage rise on the internal impedance of the battery. The constant current charging stage will become shorter, but because the time of the following constant voltage charging stage 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 optimal performance, the voltage regulation tolerance should be better than +1%.

Stage 4: Charge Termination - Unlike nickel batteries, continuous trickle charging is not recommended for lithium-ion batteries. Continuous trickle charging can cause the metallic lithium to undergo plate plating. This can make the battery unstable and can potentially lead to sudden, spontaneous rapid disintegration.

There are two typical charging termination methods: using the minimum charging current judgment or using a timer (or a combination of the two). The minimum current method monitors the charging current during the constant voltage charging phase and terminates the charging when the charging current decreases to the range of 0.02C to 0.07C. The second method starts from the beginning of the constant voltage charging phase and terminates the charging process after two hours of continuous charging.

The four-stage charging method described above takes about 2.5 to 3 hours to charge a fully discharged battery. Advanced chargers also use more safety measures. For example, if the battery temperature exceeds a specified window (usually 0°C to 45°C), charging will be suspended.
Lithium-ion Charging - System Considerations

A high-performance charging system is required to complete the charging process quickly and reliably. To achieve a reliable and cost-effective solution, the following system parameters should be considered during design:

Input Source

Many applications use very cheap wall adapters as input power. The output voltage depends mainly on the AC input voltage and the load current drawn from the wall adapter.

The AC bus input voltage on a standard US wall outlet typically varies from 90 VRMS to 132 VRMS. Assume a nominal input voltage of 120 VRMS with a tolerance of +10%, −25%. The charger must provide proper voltage regulation to the battery regardless of the input voltage. The charger input voltage is proportional to the AC bus voltage and the charge current:

VO=2VIN×a-1O(REQ+RPTC)-2×VFD

REQ is the sum of the resistance of the secondary winding and the reflected resistance of the primary winding (RP/a2). RPTC is the resistance of the PTC, and VFD is the forward voltage drop of the bridge rectifier. In addition, the transformer core loss will also slightly reduce the output voltage.

Applications that use a car adapter to charge face similar problems. The output voltage of a car adapter typically ranges from 9V to 18V.

Constant current charging rate and accuracy

The choice of topology for a particular application may be determined by the charging current. Many high constant current charging applications or multi-cell battery charging applications use switching charging solutions to achieve higher efficiency and avoid excessive heat generation. For size and cost considerations, low-end and mid-range fast charging applications tend to use linear solutions, which however lose more energy in the form of heat. For linear charging systems, the tolerance of constant current charging becomes extremely important. If the voltage regulation tolerance is too large, the pass transistor and other components will need to be larger, increasing size and cost. In addition, if the constant current charging current is too small, the entire charging cycle will be extended.

Output voltage stability accuracy

In order to make full use of the battery capacity as much as possible, the output voltage regulation accuracy is very critical. A small decrease in output voltage accuracy will also lead to a significant reduction in battery capacity. However, for safety and reliability reasons, the output voltage cannot be set too high. Figure 2 shows the importance of output voltage stability accuracy.

Charge Termination Method

Undoubtedly, overcharging is always a major concern for lithium-ion battery charging. Accurate charging termination method is critical for a safe and reliable charging system.

Battery Temperature Monitoring Generally, lithium-ion batteries should be charged in the temperature range of 0°C to 45°C. Charging the battery outside this temperature range can cause the battery to overheat. During the charging cycle, the pressure inside the battery increases and causes the battery to swell. Temperature is directly related to pressure. As the temperature rises, the pressure will also be too high, which may cause mechanical rupture or material leakage inside the battery, and in severe cases, it may cause an explosion. Charging the battery outside this temperature range can also impair the performance of the battery or shorten the expected life of the battery.

Usually, thermistors are used in lithium-ion battery packs to accurately measure the battery temperature. The charger detects the resistance of the thermistor. When the resistance exceeds the specified working range, that is, the temperature exceeds the specified range, charging is prohibited.

Battery discharge current or reverse leakage current

In many applications, the charging system remains connected to the battery even when input power is not present. The charging system must ensure that very little current is drawn from the battery when input power is not present. The maximum leakage current should be less than a few microamps, and typically less than one microamp.

Lithium-ion Charging - Application Examples

By fully considering the above system considerations in advance, a suitable charging management system can be developed.

Linear solution

Linear charging solutions are often used when there is a well-regulated input supply. In such applications, the advantages of linear solutions include ease of use, small size, and low cost. Since linear charging solutions are inefficient, the most important factor affecting the design is the thermal design. The thermal design is the thermal resistance between the input voltage, the charging current, and the pass transistor and the ambient cooling air. The worst case is when the device transitions from the trickle charge phase to the constant current charge phase. In this case, the pass transistor must dissipate the maximum heat, and a trade-off must be made between the charging current, system size, cost, and thermal requirements.

For example, an application requires charging a 1000mAh single-cell Li-Ion battery at a constant current of 0.5C or 1C using a 5V ±5% input power supply. Figure 3 shows how to use Microchip's MCP73843 to form a low-cost standalone solution that requires only a few external components to implement the required charging algorithm. The MCP73843 perfectly combines high-precision constant current charging, constant voltage regulation, and automatic charge termination.

To further reduce the size, cost and complexity of linear solutions, many external components can be integrated into the charge management controller. Advanced packaging can provide higher integration, of course, it will also sacrifice some flexibility. Such packaging requires advanced production equipment, and in many cases rework is avoided. Usually, charging current detection, transmission transistors and reverse discharge protection are integrated. In addition, such charge management controllers will also implement certain thermal regulation functions. The thermal regulation function can limit the charging current according to the device die temperature, so that the charging cycle time can be optimized while ensuring the reliability of the device. The thermal regulation function greatly reduces the workload of thermal design.

A fully integrated linear solution based on the Microchip MCP73861 is shown in Figure 4. The MCP73861 includes all the functions of the MCP73843, plus current sensing, a pass transistor, reverse discharge protection, and battery temperature monitoring.