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I. Lithium-ion Batteries vs. 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 generally lithium dioxide. During charging, lithium ions move from the positive electrode to the negative electrode and are inserted into the graphite layers. During discharging, lithium ions detach from the outer surface of the negative electrode within the graphite crystal and move towards the positive electrode. Therefore, during the charging and discharging processes of this battery, lithium always appears in the form of lithium ions rather than metallic lithium. Hence, this type of battery is called a lithium-ion battery, abbreviated as Li-ion battery.

Lithium-ion batteries have the following advantages: small size, large capacity, light weight, no pollution, high single-cell voltage, low self-discharge rate, and a large number of battery cycling times. However, they are relatively expensive. Nickel-cadmium batteries are gradually being phased out due to their low capacity, serious self-discharge, and environmental pollution. Nickel-metal hydride batteries have a relatively high performance-to-price ratio and do not pollute the environment, but their single-cell voltage is only 1.2V, which limits their application range.

II. Characteristics of Lithium-ion Batteries

1. Higher weight-to-energy and volume-to-energy ratios.

2. High voltage: The voltage of a single lithium-ion battery is 3.6V, which is equal to the series voltage of three nickel-cadmium or nickel-metal hydride rechargeable batteries.

3. Low self-discharge, allowing for long-term storage. This is the most outstanding advantage of this battery.

4. No memory effect. Lithium-ion batteries do not have the so-called memory effect like nickel-cadmium batteries, so there is no need to discharge them before charging.

5. Long lifespan. Under normal operating conditions, the charging/discharging cycle times of a lithium-ion battery are far greater than 500 times.

6. Capable of fast charging. Generally, lithium-ion batteries can be charged with a current of 0.5 to 1 times the battery capacity, shortening the charging time to 1 to 2 hours.

7. Can be connected in parallel at will.

8. Since the battery does not contain heavy metal elements such as cadmium, lead, and mercury, it is environmentally friendly and is the most advanced green battery nowadays.

9. High cost. Compared with other rechargeable batteries, lithium-ion batteries are relatively expensive.

III. Internal Structure of Lithium-ion Batteries

Lithium-ion batteries generally have two shapes: cylindrical and rectangular.

The inside of the battery adopts a spiral winding structure, and a very fine and highly permeable polyethylene film isolation material is used to separate the positive and negative electrodes. The positive electrode includes a lithium-ion collecting electrode composed of lithium and cobalt dioxide and a current collecting electrode composed of an aluminum film. The negative electrode consists of a lithium-ion collecting electrode composed of flaky carbon materials and a current collecting electrode composed of a copper film. The battery is filled with an organic electrolyte solution. In addition, safety valves and PTC components are installed to protect the battery from damage during abnormal conditions and output short circuits.

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

IV. Charging and Discharging Requirements of Lithium-ion Batteries

1. Charging of Lithium-ion Batteries:
   According to the structural characteristics of lithium-ion batteries, the maximum charging cut-off voltage should be 4.2V. Overcharging should be avoided, otherwise, too many lithium ions will be removed from the positive electrode, causing the battery to be scrapped. The charging and discharging requirements are relatively high, and a dedicated constant-current and constant-voltage charger can be used for charging. Generally, constant-current charging is carried out until 4.2V per cell, and then it switches to constant-voltage charging. When the constant-voltage charging current drops below 100mA, the charging should be stopped.

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

2. Discharging of Lithium-ion Batteries:
   Due to the internal structure of lithium-ion batteries, during discharging, lithium ions cannot all move towards the positive electrode. A certain amount of lithium ions must be retained on the negative electrode to ensure that lithium ions can smoothly enter the channels during the next charging. Otherwise, the battery life will be shortened accordingly. To ensure that some lithium ions remain in the graphite layer after discharging, the minimum cut-off voltage for discharging must be strictly limited. That is to say, lithium-ion batteries should not be over-discharged. The general cut-off voltage for discharging is 3.0V per cell, and the minimum should not be lower than 2.5V per cell. The length of the battery discharging time is related to the battery capacity and the magnitude of the discharging current. Battery discharging time (hours) = battery capacity / discharging current. The discharging current of lithium-ion batteries (mA) should not exceed 3 times the battery capacity (for example, for a 1000mAh battery, the discharging current should be strictly controlled within 3A). Otherwise, the battery will be damaged.

Currently, the lithium-ion battery packs sold on the market are all equipped with built-in charging and discharging protection boards. Only the external charging and discharging currents need to be controlled.

V. Protection Circuits of Lithium-ion Batteries

The charging and discharging protection circuit for two lithium-ion batteries is shown in Figure 1. It is composed of two field-effect transistors and a dedicated protection integrated circuit block S--8232. The overcharging control transistor FET2 and the over-discharging control transistor 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 overcharging protection transistor FET1 is cut off, stopping the charging. To prevent misoperation, a delay capacitor is usually added to the external circuit. When the battery is in the discharging state and the battery voltage drops to 2.55V, the over-discharging control transistor FET1 is cut off, stopping the power supply to the load. Over-current protection means that when a large current flows through the load, the control FET1 is cut off, stopping the discharging to the load. The purpose is to protect the battery and the field-effect transistors. Over-current testing uses the on-resistance of the field-effect transistor as a test resistor to monitor its voltage drop. When the voltage drop exceeds the set value, the discharging is stopped. A delay circuit is usually added to the circuit to distinguish between surge currents and short-circuit currents. This circuit has complete functions and reliable performance, but it is highly professional, and the dedicated integrated circuit block is not easy to purchase, making it difficult for amateur enthusiasts to replicate.

VI. Simple Charging Circuits

Currently, many merchants sell single lithium-ion batteries without charging boards. They have excellent performance and a low price, and can be used for DIY products and the replacement of lithium-ion battery packs during repairs. Therefore, they are very popular among electronics enthusiasts. Interested readers can refer to Figure 2 to make a charging board. The principle is: Use a stable voltage to charge the battery to ensure that there is no overcharging. The input DC voltage only needs to be 3 volts higher than the voltage of the battery being charged. R1, Q1, W1, and TL431 form a fine adjustable regulated voltage circuit. Q2, W2, and R2 form an adjustable constant-current circuit. Q3, R3, R4, R5, and LED form a charging indication circuit. As the voltage of the battery being charged rises, the charging current will gradually decrease. When the battery is fully charged, the voltage drop on R4 will decrease, causing Q3 to be cut off and the LED to go out. To ensure that the battery is fully charged, please continue charging for 12 hours after the indicator light goes out. When using, please install suitable heat sinks for Q2 and Q3. The advantages of this circuit are: simple construction, easy availability of components, safe charging, intuitive indication, and no damage to the battery. By changing W1, it can be used to charge multiple lithium-ion batteries connected in series. By changing W2, the charging current can be adjusted over a wide range. The disadvantage is: there is no over-discharging control circuit. Figure 3 is the printed circuit board diagram of this charging board (a perspective view from the component side).

VII. Examples of the Use of Single Lithium-ion Batteries

1. As a replacement for battery pack repairs
   There are many battery packs, such as those used in laptop computers. After repair, it is found that only individual batteries in the battery pack are faulty. Appropriate single lithium-ion batteries can be selected for replacement.

2. Making a high-brightness mini flashlight
   The author once used a single 3.6V 1.6AH lithium-ion battery in combination with a white ultra-high-brightness light-emitting diode to make a mini flashlight. It is convenient to use, compact and beautiful. And because of the large battery capacity, even if it is used for half an hour on average every night, it has not needed to be charged for more than two months so far. The circuit is shown in Figure 4.

3. Replacing a 3V power supply
   Since the voltage of a single lithium-ion battery is 3.6V, only one lithium-ion battery is needed to replace two ordinary batteries to supply power to small household appliances such as radios, Walkmans, and cameras. It is not only light in weight but also has a long continuous operation time.

VIII. Storage of Lithium-ion Batteries

Lithium-ion batteries should be stored after being fully charged. They can be stored for more than six months at 20°C. It can be seen that lithium-ion batteries are suitable for storage at low temperatures. Someone once suggested storing rechargeable batteries in the refrigerator's refrigeration compartment, which is indeed a good idea.

IX. Precautions for Use

Lithium-ion batteries must not be disassembled, drilled, punctured, sawed, pressurized, or heated. Otherwise, serious consequences may occur. Lithium-ion batteries without a charging protection board must not be short-circuited and should not be played with by children. They should not be close to flammable or chemical substances. Scrap lithium-ion batteries should be properly disposed of.

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