Currently, both prismatic and cylindrical lithium-ion cells employ certain safety protection measures, such as explosion-proof vents, separators, and PTC (Potentially Transmitted Cell) devices. To further ensure the safety of finished lithium-ion batteries during use, protection against overcharge, over-discharge, overcurrent, and short circuits is crucial. Therefore, protection circuits are typically designed within the battery pack to effectively monitor the battery's charging and discharging status and, under certain conditions, shut down the charging and discharging circuits to prevent damage to the battery.
Typically, the protection circuit of a lithium-ion battery consists of a protection IC and two power MOSFETs. The protection IC monitors the battery voltage and switches to the external power MOSFETs to protect the battery when there is overcharging or over-discharging. The protection IC has functions such as overcharge protection, over-discharge protection, and overcurrent/short circuit protection.
1. Normal state
Under normal conditions, all MOSFETs in the circuit are in the ON state, allowing the battery to charge and discharge freely. Since the on-resistance of the MOSFETs is very small, typically less than 30 milliohms, their on-resistance has minimal impact on 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 require constant current/constant voltage charging. In the initial stage of charging, it is constant current charging. As the charging process progresses, the voltage will rise to 4.2V, and it will switch to constant voltage charging until the current becomes smaller and smaller.
During battery charging, if the charger circuit malfunctions, the battery voltage may exceed 4.2V and continue constant current charging. The battery voltage will continue to rise, and when it exceeds 4.3V, the chemical side reactions in the battery will intensify, potentially damaging the battery or causing safety issues, necessitating termination of charging. At this point, the protection IC detects the battery voltage. When it reaches 4.28V (assuming the battery overcharge point is 4.28V), it activates overcharge protection, switching the power MOSFET from on to off, thus stopping charging. Additionally, it's crucial to prevent false overcharge detection due to noise, avoiding misinterpretation as overcharge protection. Therefore, a delay time needs to be set, typically around 1 second. This delay time must not be shorter than the duration of the noise.
3. Over-discharge protection
During the discharge process of a battery to an external load, its voltage gradually decreases. In cases of over-discharge, the electrolyte decomposes, leading to a deterioration in battery characteristics. If the battery is allowed to continue discharging to a load under these conditions, it will cause permanent damage.
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), it will activate the over-discharge protection, causing the power MOSFET to switch from on to off to cut off the discharge, thus preventing over-discharge and keeping the battery in a low quiescent current standby mode, where the current is only 0.1μA. The over-discharge protection function will only be deactivated when the lithium-ion battery is connected to a charger and its voltage is higher than the over-discharge voltage. Furthermore, considering pulse discharge, the over-discharge detection circuit has a delay time to avoid malfunctions.
4. Overcurrent protection
If an overcurrent occurs for unknown reasons, it must be stopped immediately to ensure safety. When the discharge current is too high, the protection IC will activate overcurrent protection. Overcurrent detection uses the Rds(on) of the power MOSFET as an inductive impedance to monitor the voltage drop. If the voltage drops above the predetermined overcurrent detection voltage, discharge stops. The calculation formula is: V = I × Rds(on) × 2 (V is the overcurrent detection voltage, I is the discharge current). Similarly, overcurrent detection must have a delay time to prevent false triggering in case of sudden current inflow. This delay time is typically around 13 milliseconds. Usually, after an overcurrent occurs, if the overcurrent factor can be removed (e.g., immediately disconnected from the load), the system will return to normal and normal charging and discharging operations can resume.
5. Short circuit protection
A short circuit occurs for unknown reasons, and to ensure safety, the discharge must be stopped immediately. When a short circuit occurs, the protection IC will activate short-circuit protection. The delay time of 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.
Currently, the company is researching and developing various battery cells, and assembling finished batteries is also a long-term goal. Protection circuits will be one of the key components we are considering. With the wider application of lithium-ion batteries, the requirements for protection circuits are becoming increasingly stringent. Future protection ICs will further improve the precision of voltage detection, reduce current consumption, and enhance functions such as preventing malfunctions. As mobile phones become smaller, the size requirements for lithium-ion battery protection circuits are also decreasing. In the past two years, products integrating control ICs and MOSFETs into a single protection IC have emerged, such as DIALOG's DA7112 series. Some manufacturers have even packaged the entire protection circuit into a small IC, such as MITSUMI's products.
