Design of a repair device for lithium-ion batteries

Background of the birth of mobile phone battery repair device:

Due to the development of contemporary society, almost everyone is using mobile phones. Mobile phones have become an indispensable daily necessity for us. However, people have gradually discovered that mobile phone batteries cannot meet our usage requirements during frequent use. They often cannot be charged after a period of use, and the discharge time becomes shorter. A newly charged battery runs out of power after a short period of use, so a repair device for repairing mobile phone batteries was born.

Design Principle

(I) Introduction of mobile phone batteries

Lithium-ion batteries are commonly used in mobile phone batteries. This is mainly because lithium-ion batteries are lithium-ions (Li-Ion) and lithium polymers (Li-Pol). They are light in weight, large in capacity, and low in internal resistance. However, we need to charge them according to their characteristics, otherwise they will be severely damaged and their rechargeable times will be greatly reduced.

During the charging process, we need to be careful not to overcharge or over-discharge lithium ions. It has its own minimum voltage and maximum voltage. Generally, the minimum voltage is between 2.2V and 2.3V, and the maximum voltage is between 4.2V and 4.3V. Therefore, we need to pay attention to setting the discharge termination voltage and the charge termination voltage when designing the circuit.

(II) Design ideas

The designed circuit is mainly divided into two stages: discharge and charge. The general charger only charges the battery, but the repair instrument has an additional discharge process.

The discharge stage is mainly to completely discharge the residual power in the battery and make its voltage reach the discharge termination value. In the discharge stage, pay attention to the setting of the discharge voltage termination value. If the battery is over-discharged, it will cause great damage to the battery.

During the charging phase, our chargers are different from ordinary chargers. We use pulse signals with periodic frequency changes to charge the battery. This is mainly because the pulse signal can gradually activate the chemical substances in the battery, so that the battery can reach the effect when it leaves the factory. Similarly, the charging voltage termination value cannot be exceeded during charging. Overcharging may also cause permanent damage to the battery.

Production process

(I) Design of discharge circuit

1. Design requirements

①The discharge current can generally be 0.2 times the rated capacity of the battery.

② The discharge termination voltage must be accurately grasped to avoid damaging the battery due to over-discharge.

③Manually start and stop the discharge.

2. Design points

① When the battery is discharged to the termination voltage, the discharge circuit should be automatically disconnected or automatically switched to charging state.

② It is convenient to monitor the discharge working status.

The following is a diagram of the discharge circuit design:

Discharge circuit diagram

The driving circuit part can be composed of two relays to form a self-locking circuit. When the discharge is completed, the switch is automatically disconnected and transferred to the charging part.

(II) Design of charging circuit

1. Design indicators

①Adjustable constant current charging, maximum charging current 500mA.

②When the discharge is finished, it automatically switches to constant current charging state.

③When charging reaches the charging termination voltage, the constant current circuit stops working and locks the "charging end" state.

2. Design points

① Use a voltage comparator to monitor the battery voltage to ensure that when the charging termination voltage is reached, the charging current can be turned off in time and the "charging end" state can be locked.

②It has both automatic function and manual start and stop function.

The charging circuit is combined with the pulse signal to charge the circuit. The pulse signal here can be realized by using a 555 timer, and three 555s are used to form a pulse signal with a periodic frequency change. The circuit diagram is shown in the figure.

The connection of the entire circuit needs to have a logical relationship, which can be achieved using a NAND gate. In the design, we used a 4012 four-phase input NAND gate.

Pulse signal circuit diagram

(III) Design logic

1. Discharge circuit logic relationship

The potential at point A is low in the discharge state and high in the charge state. We will record the potential at this point for now.

2. Logical relationship of charging circuit

Charging circuit logic diagram

It can be seen that the potential at point B is low in the discharge state and high in the charge state, which is the same as the potential at point A. Therefore, we connect the potentials of these two points to the input of the NAND gate.

3. Overall logical relationship

Connect the A and B points and the pulse signal to the input of the NAND gate, and then output the signal through a NAND gate. In this way, a logical relationship can be formed. See Table 1. Connect to the two ends of the battery, and a switch TWH8778 can be used here to connect.

Table 1: Output signal logic relationship

The other circuit parts mainly include the power control part, which can use rectification, filtering and voltage stabilization to convert the AC signal into the DC power we need. The charging current can be charged by a constant current source.

The circuits and devices used in this design are all common circuits, which are very helpful for current students majoring in electrical engineering. Through this kind of electronic production, they can consolidate their professional knowledge and strengthen their understanding of each part of the circuit and the performance and usage of each device.

Repair equipment for lithium-ion batteries is indispensable in future development. Because of the environmental protection and energy-saving characteristics of lithium-ion batteries, it will be the focus of future development. Many household electrical appliances will gradually adopt lithium-ion batteries.

In addition, the charger can also be modified to charge nickel-cadmium batteries or nickel-metal hydride batteries, which can further expand its functions and leave a lot of room for thought.