Lithium battery circuit design

Lithium-ion batteries are widely used in portable electronic devices such as mobile phones, camcorders, laptops, cordless phones, power tools, remote control or electric toys, cameras, etc. due to their excellent properties.

1. Lithium batteries and nickel-cadmium and nickel-metal hydride rechargeable batteries:

The negative electrode of a lithium-ion battery is a graphite crystal, and the positive electrode is usually lithium dioxide. When charging, lithium ions move from the positive electrode to the negative electrode and embed into the graphite layer. When discharging, lithium ions detach from the negative electrode surface in the graphite crystal and move to the positive electrode. Therefore, during the charging and discharging process of the battery, lithium always appears in the form of lithium ions, rather than in the form of metallic lithium. Therefore, this type of battery is called a lithium-ion battery, or lithium battery for short.

Lithium batteries have the advantages of small size, large capacity, light weight, no pollution, high single-cell voltage, low self-discharge rate, and many battery cycles, but they are expensive. Nickel-cadmium batteries are gradually being eliminated due to their low capacity, serious self-discharge, and environmental pollution. Nickel-metal hydride batteries have a high performance-price ratio and do not pollute the environment, but the single cell voltage is only 1.2V, so they are limited in scope of use.

2. Characteristics of lithium batteries:

1. It has higher weight energy ratio and volume energy ratio;

2. High voltage. The voltage of a single lithium battery is 3.6V, which is equal to the voltage of three Ni-Cd or Ni-MH rechargeable batteries in series.

3. Low self-discharge and long-term storage are the most prominent advantages of this battery;

4. No memory effect. Lithium batteries do not have the so-called memory effect of nickel-cadmium batteries, so lithium batteries do not need to be discharged before charging;

5. Long life. Under normal working conditions, the number of charge/discharge cycles of lithium batteries is far greater than 500 times;

6. Fast charging. Lithium batteries can usually be charged with a current of 0.5 to 1 times the capacity, shortening the charging time to 1 to 2 hours;

7. Can be used in parallel at will;

8. Since the battery does not contain heavy metal elements such as cadmium, lead, mercury, etc., it does not pollute the environment and is the most advanced green battery in contemporary times;

9. High cost. Compared with other rechargeable batteries, lithium batteries are more expensive.

3. The internal structure of lithium battery  :

Lithium batteries generally come in two shapes: cylindrical and rectangular.

The battery is spirally wound inside, with a very fine and highly permeable polyethylene film isolation material between the positive and negative electrodes. The positive electrode includes a lithium ion collector composed of lithium and cobalt dioxide and a current collector composed of aluminum film. The negative electrode is composed of a lithium ion collector composed of flaky carbon material and a current collector composed of copper film. The battery is filled with an organic electrolyte solution. In addition, a safety valve and a PTC element are installed to protect the battery from damage in abnormal conditions and output short circuits.

The voltage of a single lithium battery is 3.6V, and its capacity cannot be infinite. Therefore, single lithium batteries are often connected in series or parallel to meet the requirements of different occasions.

4. Charging and discharging requirements of lithium batteries;

1. Charging of lithium batteries: According to the structural characteristics of lithium batteries, the maximum charging termination voltage should be 4.2V. Do not overcharge, otherwise the battery will be scrapped due to too many lithium ions taken away from the positive electrode. Its charging and discharging requirements are relatively high, and a dedicated constant current and constant voltage charger can be used for charging. Usually, constant current charging is switched to constant voltage charging after the constant voltage charging current drops to less than 100mA. Charging should be stopped.

Charging current (mA) = 0.1~1.5 times the battery capacity (e.g., for a 1350mAh battery, the charging current can be controlled between 135~2025mA). The normal charging current can be selected to be around 0.5 times the battery capacity, and the charging time is about 2~3 hours.

2. Discharge of lithium battery: Due to the internal structure of lithium battery, lithium ions cannot all move to the positive electrode during discharge. Some lithium ions must be retained in the negative electrode to ensure that the lithium ions can be smoothly embedded in the channel during the next charge. Otherwise, the battery life will be shortened accordingly. In order to ensure that some lithium ions remain in the graphite layer after discharge, the minimum discharge termination voltage must be strictly limited, that is, the lithium battery cannot be over-discharged. The discharge termination voltage is usually 3.0V/cell, and the minimum cannot be lower than 2.5V/cell. The length of battery discharge time is related to the battery capacity and discharge current. Battery discharge time (hours) = battery capacity/discharge current. The discharge current (mA) of a lithium battery should not exceed 3 times the battery capacity. (For example, for a 1000mAH battery, the discharge current should be strictly controlled within 3A) Otherwise, the battery will be damaged.

Currently, all lithium battery packs sold on the market are sealed with matching charge and discharge protection boards. You only need to control the external charge and discharge current.

5. Lithium battery protection circuit:

The charge and discharge protection circuit of two lithium batteries is shown in Figure 1. It consists of two field effect tubes and a special protection integrated block S--8232. The overcharge control tube FET2 and the over-discharge control tube FET1 are connected in series in the circuit. The protection IC monitors the battery voltage and controls it. When the battery voltage rises to 4.2V, the overcharge protection tube FET1 is cut off and charging stops. To prevent malfunction, a delay capacitor is generally added to the external circuit. When the battery is in the discharge state and the battery voltage drops to 2.55V, the over-discharge control tube FET1 is cut off and stops supplying power to the load. Overcurrent protection is to control FET1 to cut off and stop discharging to the load when a large current flows through the load, in order to protect the battery and field effect tube. Overcurrent detection uses the on-resistance of the field effect tube as a detection resistor to monitor its voltage drop. When the voltage drop exceeds the set value, the discharge stops. A delay circuit is generally added to the circuit to distinguish between surge current and short-circuit current. The circuit has complete functions and reliable performance, but it is highly professional, and the special integrated block is not easy to buy, and it is not easy for amateurs to copy.

6. Simple charging circuit:

Now many merchants sell single-cell lithium batteries without charging boards. They have excellent performance and low price. They can be used for homemade products and the maintenance and replacement of lithium battery packs, so they are deeply loved by the majority of electronic enthusiasts. Interested readers can refer to Figure 2 to make a charging board. The principle is: use a constant voltage to charge the battery to ensure that it will not be overcharged. The input DC voltage is 3 volts higher than the voltage of the charged battery. R1, Q1, W1, TL431 form a precise adjustable voltage regulator circuit, Q2, W2, R2 form an adjustable constant current circuit, and Q3, R3, R4, R5, LED are charging indication circuits. As the voltage of the charged battery rises, the charging current will gradually decrease. When the battery is fully charged, the voltage drop on R4 will decrease, so that Q3 will be cut off and the LED will go out. To ensure that the battery is fully charged, please continue to charge for 1-2 hours after the indicator light goes out. Please install a suitable heat sink for Q2 and Q3 when using it. The advantages of this circuit are: simple production, easy to purchase components, safe charging, intuitive display, and no damage to the battery. By changing W1, multiple lithium batteries in series can be charged, and by changing W2, the charging current can be adjusted over a wide range. The disadvantage is: there is no over-discharge control circuit. Figure 3 is the printed circuit board of the charging board (a perspective view from the component side).

7. Application examples of single-cell lithium batteries

1. As a replacement for battery pack repair

There are many battery packs, such as those used in laptop computers. After repair, it was found that when this battery pack is damaged, only individual batteries have problems. You can choose a suitable single lithium battery for replacement.

2. Make a high-brightness miniature flashlight

The author has used a single 3.6V1.6AH lithium battery and a white ultra-high brightness light-emitting tube to make a miniature flashlight, which is easy to use, small and beautiful. And because of the large battery capacity, it is used for an average of half an hour every night, and it has been used for more than two months without charging. The circuit is shown in Figure 4.

3. Replace 3V power supply

Since the voltage of a single lithium battery is 3.6V, only one lithium battery can replace two ordinary batteries to power small household appliances such as radios, walkmans, cameras, etc. It is not only light in weight but also has a long continuous use time.

8. Storage of lithium batteries:

Lithium batteries need to be fully charged before storage. They can be stored at 20°C for more than half a year, which shows that lithium batteries are suitable for storage at low temperatures. Someone once suggested storing rechargeable batteries in the refrigerator, which is indeed a good idea.

IX. Precautions for use:

Lithium batteries must not be disassembled, drilled, punctured, sawed, pressurized, or heated, otherwise serious consequences may occur. Lithium batteries without charging protection boards must not be short-circuited and should not be played with by children. They should not be placed near flammable items or chemicals. Scrapped lithium batteries should be properly disposed of.