Different battery sizes require different circuit board dimensions. Peel off the plastic film, and you'll find many components arranged on it.
Some might ask, what exactly is the purpose of this circuit board? Well, in terms of battery modules, no one would dare to use it without it!
We learned about lithium battery charging and discharging in high school chemistry; it mainly relies on the movement of ions between the positive and negative electrodes. During charging and discharging, ions repeatedly insert and extract between the two electrodes. During charging, lithium ions are generated at the positive electrode, and these ions are extracted from the positive electrode, pass through the electrolyte, and enter the negative electrode, leaving the negative electrode in a lithium-rich state. Discharging is the reverse. However, without the protection plate shown in the diagram above, overcharging and over-discharging are very likely to occur. Overcharging occurs when too many ions are transferred from the positive electrode, and over-discharging occurs when too many ions are transferred from the negative electrode.
Overcharging and over-discharging both severely damage batteries and pose potential hazards to users. Therefore, to prevent overcharging and over-discharging, battery manufacturers often add a protective plate to the top of the battery to avoid these harmful effects.
It is precisely because of the presence of the circuit protection board that lithium batteries are unlikely to overcharge or over-discharge under normal use. Normally, the charging cutoff voltage for lithium batteries is 4.2V, meaning the battery voltage when fully charged is 4.2V. However, the protection voltage set on the circuit board is often higher, sometimes 4.5V. Overcharging occurs when the battery voltage exceeds 4.5V due to an accident. The discharging cutoff voltage is 3.0V, but the protection voltage set on the circuit board is lower, sometimes 2.8V. Over-discharging occurs when the voltage drops below 2.8V due to an accident.
The diagram shows a simple circuit board schematic I drew, as simple as it gets. P+ and P- are VBAT+ and VBAT- on the battery connector, which are the main power supplies directly connected to the outside world (the red and black lines in Figure 1). U1 is the control IC chip. Based on changes in the external voltage, it makes a judgment and then turns on or off the Q1 and Q2 MOSFETs to ensure the normal operation and disconnection of the circuit. B+ and B- are the positive and negative terminals of the battery cell. ID and TH are optional and not every battery protection board has them. For ID, the MCU learns the battery type information by reading the resistance value of the ID pin resistor. The TH terminal is connected to an NTC resistor. Based on the NTC voltage, the corresponding temperature value can be obtained using ADC conversion to ensure that charging can be stopped and the circuit can be shut down when the battery is high-temperature.
The following section discusses how the circuit board functions under different battery conditions:
1. Normal working status
Under normal operating conditions, both "CO" and "DO" of U1 in the circuit are high-level outputs, and both MOSFETs are conducting normally. This forms a complete circuit, allowing the battery to charge and discharge normally. Furthermore, the on-resistance of the MOSFETs is very small, around 10 milliohms. Their on-resistance has minimal impact on circuit performance.
2. Overcharge protection
The battery charging process consists of three stages: trickle charging, constant current charging, and constant voltage charging. Trickle charging involves charging the battery until the cell voltage reaches the specified voltage, typically 3V, during which the current is very small. Once the battery voltage reaches 3.0V, it enters constant current charging, where the current remains constant and the voltage continuously increases. When the voltage reaches 4.2V, it switches to constant voltage charging, where the current gradually decreases until fully charged. (The voltage values may vary slightly depending on the specific requirements).
If the charging circuit loses control due to an accident during the charging process, causing the cell voltage to exceed 4.2V and meet the overcharge voltage delay time, it enters an overcharge state, which is quite dangerous.
However, with a protection board, there's no need to worry. The control IC on it detects the voltage difference between VDD and VSS. When the voltage difference exceeds the protection value, the IC controls the "CO" pin to switch from a high level to a low level. At this time, the MOSFET is turned off, forming an open circuit, thus providing charging protection.
There is a certain delay in the process of the control IC turning off the MOSFET when it detects that the battery voltage exceeds the specified value. This delay is to avoid misjudgment caused by interference. In the case of flat battery, the delay is generally 1 second.
Measures to recover from overcharging
a. When the load is connected between P+ and P-, the battery will discharge to the outside through the diode, forming a circuit, because the MOSFET has a built-in body diode, until it discharges to the normal voltage. Then the MOSFET will turn on and can be used again.
b. Connect the load to B+ and B-, causing the voltage across the battery cell to drop to the overcharge recovery voltage of the protection IC.
3. Over-discharge protection
When a battery discharges to an external load, its voltage will continuously decrease. When the battery reaches a set value (usually 3.0V), its capacity will be completely depleted.
During the discharge process, if over-discharge occurs, that is, after discharging to 3.0V and the battery is still discharging and reaches the voltage value set by the protection board (usually 2.8V), the control IC detects the cell voltage and changes the "DO" pin from a high level to a zero level, causing the Q2 MOSFET to turn off, thereby cutting off the discharge circuit. At this time, the battery cannot discharge to the load, thus playing a protective role.
Methods to recover from past experiences.
Due to the presence of the Q2 body diode, the battery can be charged through the body diode by inserting the charger. After the voltage of the battery cell is restored to 2.8V, the protection board is deactivated, the MOSFET is turned on, and a complete circuit is formed. The device is turned on after the battery reaches 3.0V.
4. Overcurrent protection
Due to the chemical characteristics of lithium-ion batteries, battery manufacturers set upper limits for both charging and discharging current. Exceeding these limits will result in permanent damage to the battery.
Taking discharge as an example, during normal battery discharge, the discharge current flows through the MOSFETs connected in series. Due to the inherent on-resistance of the MOSFETs, a voltage is generated across them, with a value of U = 2I/Rds, where Rds is the on-resistance of a single MOSFET. The control IC continuously monitors the voltage value at the VM pin. If the load causes excessive loop current for some reason, resulting in the on-state voltage drop of the MOSFETs exceeding the specified value, its "DO" pin will switch from high to zero, turning off the Q2 MOSFET and thus cutting off the loop, reducing the loop current to zero, thereby providing overcurrent protection.
5. Short circuit protection
When the battery discharges to a load, if the circuit current becomes so large that the voltage detected at the VM pin exceeds a specified value, the control IC determines that the load is short-circuited. Its "DO" pin will quickly switch from high to zero, causing the Q2 MOSFET to turn off, thus cutting off the discharge circuit and providing short-circuit protection. The delay time for short-circuit protection is very short, typically on the order of microseconds. Furthermore, the current value for short-circuit protection is much greater than that for overcurrent protection.
