Fabrication of multi-cell lithium battery charging circuit

At present, lithium-ion batteries and lithium polymer batteries (hereinafter referred to as lithium batteries) have been widely used due to their advantages such as high power density and long life. However, in order to extend the battery life, lithium batteries have higher requirements for charging, including the following points:

1. For lithium batteries in an over-discharged state, a small current trickle charge is required.

2. Lithium batteries within the normal voltage range need to be charged at a constant current.

3. When the battery is close to the charging limit voltage, constant voltage charging is required.

Single-cell lithium batteries are usually charged with a linear constant current, but for multi-cell lithium battery packs with a larger charging current, a switching power supply is required to improve efficiency. The following is a step-down charger suitable for three-cell lithium battery packs, which can charge three lithium battery packs in the above three modes with an input voltage between 15-25V.

The attached figure is a schematic diagram of the charging circuit, using CN3703 as the IC. When the battery voltage is detected to be lower than 8.4V, the circuit enters the trickle charging mode, and the charging current is automatically reduced to 15% of the normal setting current. The normal constant current is set by R1 and R2, and the calculation method is:

Ich=0.2/(R1//R2) is taken according to Figure 1, the constant current charging current is about 1A, and the maximum charging current supported by CN3703 can reach SA. In the constant voltage charging stage, CN3703 keeps the voltage constant at 12.6V with an accuracy of 1%, which meets the voltage accuracy requirements of lithium batteries.

In the figure, C4, C5, C1 and C3 are input and output filter capacitors respectively, D1 is used to prevent the input power supply from being reversely connected and causing circuit damage, and D2 is used to prevent the battery current from flowing back. D4 and D5 are green and red light-emitting diodes respectively, as full and charging indicators.

The value of R6 can set the charging current at the end of charging. If R6 is short-circuited as shown in the figure, the charging current at the end of charging is about 9% of the set constant current value. If the battery temperature needs to be detected, R7 can be replaced by a negative temperature coefficient thermistor and installed close to the battery. When the battery temperature is too high, the resistance of R7 decreases to a certain value, and the circuit stops charging.

Since CN3703 uses SSOP package, the whole circuit can be made of chip components. Except for C3, the other capacitors can be made of chip ceramic capacitors, and C3 can use aluminum electrolytic capacitors or tantalum capacitors. In addition, the rated current of the inductor is required to be greater than 1.5 times the constant current charging current, otherwise saturation may occur, causing severe heating. The area of ​​the completed circuit board is 22mmx40mm, and the highest device is the electrolytic capacitor, which is about 6mm.

After the production is completed, power on the test first (do not connect the battery first). Input a DC power supply between 15~25V at the input end. Under normal circumstances, the green indicator light is on. Use a multimeter to measure the battery terminal voltage, which should be around 12.6V. If there is an electronic load, you can connect the output to the electronic load. When the output voltage is set between 8.4A12.6V, the output current should be constant at around 1A, and the red indicator light is on. After the test is OK, you can connect three lithium battery packs for charging.