I. Problems during lithium battery charging
During lithium battery charging, excessively high or low voltage can damage the battery and may even cause an explosion. Therefore, a protection circuit is needed to control voltage and current to ensure the safe charging of lithium batteries.
II. Principle of Lithium Battery Charge and Discharge Protection Circuit
The principle of a lithium battery charge/discharge protection circuit is to protect the battery by controlling the voltage and current during charging and discharging. During charging, the protection circuit monitors the battery voltage; when the voltage reaches a set value, it cuts off the charging circuit to prevent overcharging. During discharging, the protection circuit monitors the battery voltage; when the voltage drops to a set value, it cuts off the discharging circuit to prevent over-discharging.
III. Composition of Lithium Battery Charge and Discharge Protection Circuit
A lithium battery charge/discharge protection circuit consists of three main parts: a protection chip, a MOSFET, and an inductor. The protection chip is the core component of the entire circuit, responsible for detecting voltage and current and controlling the switching of the MOSFET. The MOSFET controls the battery's charging and discharging; when the protection chip detects abnormal voltage or current, it protects the battery by controlling the MOSFET's switching state. The inductor is used for filtering, reducing noise and interference in the circuit and ensuring stable operation of the protection circuit.
IV. Summary
A lithium battery charge/discharge protection circuit is an important circuit protection device that protects the lithium battery from damage during the charging and discharging process. Its principle is to protect the battery by controlling the charging and discharging voltage and current. The protection circuit consists of a protection chip, a MOSFET, and an inductor. The protection chip is the core component of the entire charge/discharge protection circuit, responsible for detecting voltage and current and controlling the switching of the MOSFET.
I. Charge and discharge requirements of lithium batteries
1. Lithium battery charging
The maximum charging termination voltage for a single lithium battery is 4.2V. Overcharging is not allowed, otherwise the battery will be ruined due to excessive loss of lithium ions at the positive electrode. When charging lithium batteries, a dedicated constant current and constant voltage charger should be used. First, charge at a constant current until the voltage across the lithium battery reaches 4.2V, then switch to constant voltage charging mode. When the constant voltage charging current drops to 100mA, charging should be stopped.
The charging current (mA) can be 0.1 to 1.5 times the battery capacity. For example, for a 1350mAh lithium battery, the charging current can be controlled between 135mA and 2025mA. A typical charging current can be selected at around 0.5 times the battery capacity, and the charging time is approximately 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; a portion must remain at the negative electrode to ensure they can readily intercalate during the next charge. Otherwise, battery life will be shortened. To ensure some lithium ions remain in the graphite layer after discharge, the minimum discharge termination voltage must be strictly limited; in other words, lithium batteries cannot be over-discharged. The discharge termination voltage of a single lithium battery is typically 3.0V, and should not be lower than 2.5V. The battery discharge time is related to the battery capacity and the discharge current. Battery discharge time (hours) = battery capacity / discharge current, and the lithium battery discharge current (mA) should not exceed three times the battery capacity. For example, for a 1000mAh lithium battery, the discharge current should be strictly controlled below 3A; otherwise, the battery will be damaged.
II. Composition of the protection circuit
The protection circuit typically consists of a control IC, a MOSFET switch, a fuse, resistors, capacitors, and other components, as shown in Figure 2. Under normal circumstances, the control IC outputs a signal to turn on the MOSFET, connecting the battery cell to the external circuit. When the battery cell voltage or circuit current exceeds a specified value, it immediately turns off the MOSFET to protect the battery cell.
The circuit has overcharge protection, over-discharge protection, overcurrent protection, and short-circuit protection functions. Its working principle is analyzed as follows:
1. Normal state
Under normal conditions, both the "CO" and "DO" pins of N1 output high voltage, and both MOSFETs are in the ON state. The battery can be freely charged and discharged. Since the on-resistance of the MOSFETs is very small, typically less than 30 milliohms, their on-resistance has little impact on the circuit performance. In this state, the current consumption of the protection circuit is in the μA range, typically less than 7μA.
2. Overcharge protection
Lithium-ion batteries, as a type of rechargeable battery, require a constant current/constant voltage charging method. Initially, charging is done at a constant current. As charging progresses, the voltage rises to 4.2V (some batteries require a constant voltage of 4.1V depending on the positive electrode material), then switching to constant voltage charging until the current gradually decreases. If the charger circuit malfunctions during charging, the battery voltage may exceed 4.2V and continue constant current charging. In this case, the battery voltage will continue to rise. When the battery voltage exceeds 4.3V, the chemical side reactions in the battery will intensify, potentially leading to battery damage or safety issues.
In a battery with a protection circuit, when the control IC detects that the battery voltage reaches 4.28V (this value is determined by the control IC, and different ICs have different values), its "CO" pin will change from high voltage to zero voltage, causing V2 to switch from conducting to turning off, thereby cutting off the charging circuit and preventing the charger from charging the battery, thus providing overcharge protection. At this time, due to the presence of the body diode VD2 built into V2, the battery can discharge to an external load through this diode.
Between the control IC detecting that the battery voltage exceeds 4.28V and issuing the shutdown V2 signal, there is a delay period. The length of this delay period is determined by C3 and is usually set to about 1 second to avoid misjudgment due to interference.
3. Over-discharge protection
During the discharge process of a battery to an external load, its voltage will gradually decrease. When the battery voltage drops to 2.5V, its capacity has been completely discharged. If the battery is allowed to continue discharging to the load at this time, it will cause permanent damage to the battery.
During battery discharge, when the control IC detects that the battery voltage is below 2.3V (this value is determined by the control IC, and different ICs have different values), its "DO" pin will change from high voltage to zero voltage, causing V1 to switch from conducting to turning off, thereby cutting off the discharge circuit and preventing the battery from discharging to the load, thus providing over-discharge protection. At this time, due to the presence of the body diode VD1 built into V1, the charger can charge the battery through this diode.
Since the battery voltage cannot drop further under over-discharge protection, the current consumption of the protection circuit must be extremely low. At this time, the control IC will enter a low-power state, and the power consumption of the entire protection circuit will be less than 0.1μA.
There is also a delay between when the control IC detects that the battery voltage is below 2.3V and when it sends the shutdown V1 signal. The length of this delay is determined by C3 and is usually set to about 100 milliseconds to avoid misjudgment due to interference.
4. Overcurrent protection
Due to the chemical characteristics of lithium batteries, battery manufacturers stipulate that their maximum discharge current cannot exceed 2C (C = battery capacity/hour). When the battery discharges at a current exceeding 2C, it will cause permanent damage to the battery or safety problems.
During normal discharge of the battery to the load, the discharge current passes through the two MOSFETs connected in series. Due to the on-resistance of the MOSFETs, a voltage is generated across them. This voltage value is U = I * RDS * 2, where RDS is the on-resistance of a single MOSFET. The "V-" pin on the control IC detects this voltage value. If the load becomes abnormal for some reason, causing the loop current to increase, when the loop current becomes large enough that U > 0.1V (this value is determined by the control IC, and different ICs have different values), its "DO" pin will change from high voltage to zero voltage, causing V1 to change from on to off, thereby cutting off the discharge loop and making the current in the loop zero, thus playing an overcurrent protection role.
There is also a delay between the control IC detecting an overcurrent and issuing a shutdown signal for V1. The length of this delay is determined by C3 and is usually around 13 milliseconds to avoid misjudgment due to interference.
As can be seen from the above control process, the magnitude of the overcurrent detection value depends not only on the control value of the control IC, but also on the conduction impedance of the MOSFET. When the conduction impedance of the MOSFET is larger, the overcurrent protection value is smaller for the same control IC.
5. Short circuit protection
During battery discharge to a load, if the circuit current becomes so large that U > 0.9V (this value is determined by the control IC, and different ICs have different values), the control IC will determine that the load is short-circuited. Its "DO" pin will quickly switch from high voltage to zero voltage, causing V1 to switch from conducting to off, thereby cutting off the discharge circuit and providing short-circuit protection. The delay time for short-circuit protection is extremely short, typically less than 7 microseconds. Its working principle is similar to overcurrent protection, only the judgment method and protection delay time are different.
The above details the working principle of a single-cell lithium-ion battery protection circuit. The protection principle of multiple series-connected lithium-ion batteries is similar and will not be elaborated here. The control IC used in the circuit above is the R5421 series from Ricoh Corporation of Japan. In actual battery protection circuits, there are many other types of control ICs, such as the S-8241 series from Seiko, the MM3061 series from Mitsumi Corporation of Japan, the FS312 and FS313 series from Fuchuang Technology of Taiwan, and the AAT8632 series from Analog Technology of Taiwan, etc. Their working principles are largely the same, with only differences in specific parameters. Some control ICs integrate the filter capacitor and delay capacitor inside the chip to save on external circuitry, thus requiring very little external circuitry, such as the S-8241 series from Seiko. Besides the control IC, another important component in the circuit is the MOSFET, which acts as a switch. Since it is directly connected in series between the battery and the external load, its on-resistance affects the battery performance. When a better MOSFET is selected, its on-resistance is very small, the internal resistance of the battery pack is small, the load-carrying capacity is strong, and the power consumption during discharge is also less.
With the development of technology, portable devices are becoming smaller and smaller. As a result, the size requirements for the protection circuit of lithium-ion batteries are also getting smaller. In the past two years, products that integrate the control IC and MOSFET into a single protection IC have emerged, such as the DIALOG DA7112 series. Some manufacturers have even packaged the entire protection circuit into a small IC, such as the products of MITSUMI.
