Lithium battery charger circuit diagram

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1. Lithium Batteries and Nickel-Cadmium/Nickel-Metal Hydride Rechargeable Batteries:

The negative electrode of a lithium-ion battery is typically made of graphite crystals, while the positive electrode is often composed of lithium dioxide. During the charging process, lithium ions move from the positive electrode to the negative electrode and are embedded in the graphite layers. Conversely, during discharge, lithium ions detach from the negative electrode's graphite surface and move towards the positive electrode. Thus, throughout the charging and discharging cycle, lithium remains in the form of lithium ions, not metallic lithium. This is why these batteries are referred to as lithium-ion batteries or simply lithium batteries.

Lithium batteries boast several advantages, including compact size, high capacity, lightweight design, no environmental pollution, high single-cell voltage, low self-discharge rates, and a long lifespan with numerous charge cycles. However, they tend to be more expensive. Nickel-cadmium batteries are gradually being phased out due to their low capacity, significant self-discharge issues, and environmental harm. Nickel-metal hydride (NiMH) batteries offer a favorable cost-performance ratio and are environmentally friendly, but their cell voltage is only 1.2V, limiting their applications.

Second, the Characteristics of Lithium Batteries:

1. They possess a higher energy density relative to weight and volume. 2. Their voltage is high; a single lithium cell typically has a voltage of 3.6V, equivalent to the combined voltage of three nickel-cadmium or nickel-metal hydride cells. 3. They can be stored for long periods with minimal self-discharge, which is one of their most remarkable features. 4. They lack a memory effect. Unlike nickel-cadmium batteries, lithium batteries do not suffer from memory effects, meaning they don’t require prior discharge before charging. 5. They have a long lifespan; under normal operating conditions, lithium batteries can endure over 500 charge/discharge cycles. 6. They can be fast-charged. Typically, a charging current of 0.5 to 1 times the battery capacity can reduce charging time to just 1 to 2 hours. 7. They can be used in parallel configurations. 8. Being free from toxic heavy metals like cadmium, lead, and mercury, they are environmentally friendly and considered the most advanced green battery technology. 9. They are costly compared to other rechargeable batteries.

Third, the Internal Structure of Lithium Batteries:

Lithium batteries commonly come in two shapes: cylindrical and rectangular. Internally, they adopt a spiral-wound structure, where a very fine, highly permeable polyethylene film serves as an insulating spacer between the positive and negative electrodes. The positive electrode comprises a lithium cobalt oxide collector and an aluminum current collector, while the negative electrode consists of a sheet-like carbon material collector and a copper current collector. The battery is filled with an organic electrolyte solution. Additionally, safety valves and PTC components are included to protect the battery from damage during abnormal conditions or output short circuits.

A single lithium cell has a voltage of 3.6V, and its capacity is not infinite. Therefore, single lithium cells are often connected in series and parallel to meet various application requirements. For instance, connecting five cells in series increases the overall voltage.

Fourth, the Charging and Discharging Requirements of Lithium Batteries:

1. Lithium Battery Charging: Given the structural characteristics of lithium batteries, the maximum charging termination voltage should be 4.2V, and overcharging should be avoided. Excessive charging could result in the depletion of lithium ions from the positive electrode, leading to battery failure. Charging and discharging requirements are stringent, and a dedicated constant current and constant voltage charger should be used. Typically, constant current charging reaches 4.2V per cell, followed by switching to constant voltage charging. Charging should cease when the constant voltage charging current drops below 100mA.

Charging current (mA) = 0.1 to 1.5 times the battery capacity (for example, a 1350mAh battery can have a charging current controlled between 135 to 2025mA). The standard charging current is approximately 0.5 times the battery capacity, resulting in a charging time of about 2 to 3 hours.

2. Lithium Battery Discharge: Due to the internal structure of lithium batteries, not all lithium ions can move to the positive electrode during discharge. Some ions must remain at the negative electrode to facilitate smooth insertion during subsequent charging. If lithium ions are completely depleted, the battery’s lifespan will be significantly reduced. To ensure some lithium ions remain in the graphite layer after discharge, the minimum discharge termination voltage must be strictly controlled. Typically, the discharge termination voltage is 3.0V per cell, with a minimum threshold of 2.5V per cell. The duration of battery discharge depends on the battery capacity and discharge current. Battery discharge time (hours) = battery capacity / discharge current. The lithium battery discharge current (mA) should not exceed three times the battery capacity (for instance, a 1000mAh battery should have its discharge current strictly controlled within 3A); otherwise, the battery may be damaged.

Currently, commercially available lithium battery packs come equipped with matching charging and discharging protection boards, simplifying external charge and discharge current control.

Fifth, Lithium Battery Protection Circuit:

The charge and discharge protection circuit for two lithium batteries is illustrated in Figure 1. It consists of two FETs and a dedicated protection integrated block S–8232. The overcharge control transistor FET2 and the overdischarge control transistor FET1 are connected in series in the circuit. The protection IC monitors the battery voltage and switches off the overcharge protection transistor FET1 when the battery voltage reaches 4.2V to stop charging. To prevent malfunctions, a delay capacitor is usually added externally. When the battery is discharged and the voltage drops to 2.55V, the overdischarge control transistor FET1 turns off, halting power supply to the load. Overcurrent protection involves turning off FET1 to stop discharging to the load when a large current flows through the load, protecting both the battery and the FET. Overcurrent detection uses the on-resistance of the FET as a sensing resistor, monitoring the voltage drop and stopping discharge if the voltage exceeds the set value. A delay circuit is also typically added to distinguish between inrush currents and short circuits. The circuit is fully functional and reliable, though it is somewhat complex and the dedicated integrated blocks are not easily obtainable, making it challenging for amateurs to replicate.

Sixth, a Simple Charging Circuit:

Many merchants now sell single-cell lithium batteries without chargers. These batteries offer excellent performance at affordable prices, making them ideal for DIY projects and replacements for lithium battery packs, appealing to electronics enthusiasts. Interested readers can refer to Figure 2 to build a charging board. The principle is to charge the battery with a constant voltage to avoid overcharging. The input DC voltage must be higher than the charged battery voltage by 3 volts. R1, Q1, W1, and TL431 form a precise adjustable voltage regulator circuit, Q2, W2, and R2 form an adjustable constant current circuit, and Q3, R3, R4, R5, and LED serve as charging indication circuits. As the rechargeable battery voltage rises, the charging current gradually decreases. Once the battery is fully charged, the voltage drop across R4 decreases, turning off Q3 and extinguishing the LED. To ensure the battery is sufficiently charged, continue charging for another 1-2 hours after the indicator turns off. Heat sinks should be used for Q2 and Q3 when operating. This circuit's advantages include simplicity in construction, ease of purchasing components, safe charging, intuitive displays, and no risk of damaging the battery. Adjusting W1 allows charging multiple series lithium batteries, and altering W2 adjusts the charging current over a wide range. The disadvantage is the lack of an overdischarge control circuit. Figure 3 is the printed circuit diagram of the charging board (view from the component side).

Seventh, Examples of Single-Cell Lithium Battery Applications:

1. For Battery Repair Replacement:

Many battery packs, such as those used in laptops, often sustain damage when individual cells fail. In such cases, suitable single-cell lithium batteries can be used as replacements.

2. Making a Highlight Flashlight:

I constructed a mini flashlight using a single 3.6V, 1.6Ah lithium battery paired with a white ultra-bright LED tube. It’s convenient to use, compact, and attractive. The large battery capacity allows me to use it for about half an hour each night, and it hasn't needed recharging for over two months. The circuit is depicted in Figure 4.

3. Replacing 3V Power Supply:

Since a single lithium battery has a voltage of 3.6V, it can replace two regular batteries to power small household appliances like radios, walkmans, and cameras. This not only reduces weight but also ensures long-term continuous use.

Eighth, Preservation of Lithium Batteries:

Lithium batteries should be fully charged before storage and can last for more than six months at 20°C. This suggests that lithium batteries are well-suited for storage in low-temperature environments. It’s been recommended to store rechargeable batteries in the freezer of a refrigerator.

Ninth, Precautions for Use:

Lithium batteries must never be dissected, drilled, punctured, cut, compressed, or overheated, as serious consequences may ensue. Lithium batteries without charging protection plates should not be short-circuited and are unsuitable for children to handle. Keep them away from flammable materials or chemicals. Proper disposal of disposable lithium batteries is essential. Fourth, the Charge and Discharge Requirements of Lithium Batteries:

1. Lithium Battery Charging: Based on the structural characteristics of lithium batteries, the maximum charging termination voltage should be 4.2V, and overcharging must be avoided. Otherwise, excessive lithium ions from the positive electrode could deplete, rendering the battery useless. Charging and discharging requirements are strict, and a dedicated constant current and constant voltage charger should be used. Typically, constant current charging reaches 4.2V per cell, followed by switching to constant voltage charging. Charging should cease when the constant voltage charging current drops below 100mA.

Charging current (mA) = 0.1 to 1.5 times the battery capacity (for example, a 1350mAh battery can have a charging current controlled between 135 to 2025mA). The standard charging current is approximately 0.5 times the battery capacity, resulting in a charging time of about 2 to 3 hours.

2. Lithium Battery Discharge: Due to the internal structure of lithium batteries, not all lithium ions can move to the positive electrode during discharge. Some ions must remain at the negative electrode to ensure smooth insertion during subsequent charging. If lithium ions are completely depleted, the battery’s lifespan will be drastically reduced. To ensure some lithium ions remain in the graphite layer after discharge, the minimum discharge termination voltage must be strictly controlled. Typically, the discharge termination voltage is 3.0V per cell, with a minimum threshold of 2.5V per cell. The duration of battery discharge depends on the battery capacity and discharge current. Battery discharge time (hours) = battery capacity / discharge current. The lithium battery discharge current (mA) should not exceed three times the battery capacity (for instance, a 1000mAh battery should have its discharge current strictly controlled within 3A); otherwise, the battery may be damaged.

Currently, commercially available lithium battery packs come equipped with matching charging and discharging protection boards, simplifying external charge and discharge current control.