Lithium-ion battery is a widely used rechargeable battery. It has high operating voltage, small size, light weight, high energy density, long cycle life, can be fully charged quickly in a short time, and allows discharge temperature Wide advantages. In addition, lithium-ion batteries have the advantages of low self-discharge current, no memory effect, and no environmental pollution. Its global supply is continuously increasing.
Li-ion battery protection
The safety of lithium-ion battery-powered equipment is the most concerned issue at present, so its protection is very important. Lithium-ion battery protection mainly includes overcharge protection, overdischarge protection, overcurrent and short circuit protection.
1. overcharge protection
When the charger overcharges the lithium-ion battery, in order to prevent the internal pressure from rising due to temperature rise, the charging state needs to be terminated. To this end, the protection device needs to monitor the battery voltage, and when it reaches the battery overcharge voltage, it activates the overcharge protection function and stops charging.
2. Over-discharge protection
In order to prevent the over-discharge state of the lithium-ion battery, when the lithium-ion battery voltage is lower than its over-discharge voltage detection point, the over-discharge protection is activated, the discharge is suspended, and the battery is kept in a standby mode with low quiescent current.
3. over current and short circuit protection
When the discharge current of the lithium ion battery is too large or a short circuit occurs, the protection device will activate the overcurrent protection function.
Multi-cell Li-Ion Circuit Protection
The rated voltage of a single lithium-ion battery is 3.6V, which cannot meet the needs of high-voltage power supply applications. Therefore, multiple lithium-ion batteries are required to be used in series. For this reason, various manufacturers of power management control integrated circuits have introduced their own multi-cell lithium-ion battery (battery pack) protection integrated circuit chips, such as S-8204B (S-8204B of SII) of SEI -8204 series, another product of this series is S-8204A. The difference between the two is that S-8204A works with P-channel MOSFET, and S-8204B works with N-channel MOSFET). This type of product is characterized by monitoring the charge and discharge status of 3 or 4 lithium-ion batteries, which can achieve overcharge, overdischarge and overcurrent protection.
Take S-8204B as an example, it can perform high-precision detection of the voltage of each lithium-ion battery. It has a 3-stage overcurrent detection function. The external capacitor can set the overcharge detection delay time, overdischarge detection delay time, and discharge overcurrent. Detection delay time 1 and discharge overcurrent detection delay time 2 can also be switched in series using 3 / 4-cell lithium-ion batteries through the SEL terminal. However, its biggest feature is that it can be used in cascade. The next section will explain this function of S-8204B in detail.
Protection chip cascade
The battery protection chip mentioned above can protect up to 4 lithium-ion batteries. However, many applications require 5 to 12 lithium-ion batteries to work in series, such as power tools, electric bicycles and UPS. The answer is simple: use multiple lithium battery protection chips at the same time. As shown in Figure 1, two protection chips are connected in series, and two N-channel MOSFETs are used as control switches to protect 8 lithium-ion batteries. Three protection chips are connected in series to protect 12 lithium-ion batteries. This series connection of multiple protection chips is a cascade of protection chips. Taking S-8204B as an example, two S-8204Bs are used in combination, and two N-channel MOSFETs are used to control the low-voltage side. In this way, a single IC can select 3 and 4 functions to achieve 6 to 8 Lithium-ion battery protection in series. If 5 Li-ion batteries are connected in series, one S-8204B can be used in series with other lithium-ion battery protection chips to achieve protection functions. This flexible combination of multiple protection chips can complete the protection of any number of lithium-ion batteries.
Figure 1 Cascade of multi-cell lithium-ion batteries
In the following, the specific work of the protection chip cascade is introduced in detail. Taking S-8204B as an example, the CTLC terminal can control the output voltage of the COP terminal externally, and the CTLD terminal can control the output voltage of the DOP terminal externally. The CTLC terminal and CTLD terminal can control the output voltage of the COP terminal and DOP terminal separately. In addition, these control functions take precedence over the battery charge and discharge protection function inside the chip. If one of the eight batteries is overcharged, the output voltage of the COP terminal of the S-8204B connected to the battery will change, and this voltage change will be transmitted to the CTLC terminal of another S-8204B connected to it. , So that the output voltage of the COP pin of another S-8204B also changes, thereby controlling the charge control MOSFET to be turned off, and achieving overcharge protection of the lithium ion battery. If one of the eight batteries is over-discharged, the DOP terminal of the S-8204B connected to the battery sends an over-discharge signal to the CTLD terminal of the other S-8204B chip to change the state of the DOP terminal Finally, the discharge control MOSFET is turned off, and the discharge is ended. Figure 2 shows the working principle of the circuit using two S-8204Bs to achieve overcharge protection (in the case of N-channel MOSFET control), and Figure 3 is the working principle of overdischarge protection.
Figure 2 Working principle of the protection circuit when the lithium-ion battery is overcharged
Figure 3 Working principle of the protection circuit when the lithium-ion battery is over-discharged
Temperature control during charging and discharging
In addition, many designers need to consider the temperature control of the charging and discharging process. Charging and discharging the lithium-ion battery at high temperature will cause explosion danger; charging and discharging at low temperature will cause damage to the battery cell. In the above solution, a temperature control switch (such as S-5841) is connected to the CTLC terminal of S-8204B. When the temperature of the lithium-ion battery is too high, the control signal of the temperature control switch is sent to the COP through the CTLC terminal. Forcibly ends the charging process of the lithium-ion battery. Similarly, the temperature control switch can be connected to the CTLD terminal to protect the temperature during discharge.
There are also single-chip multi-cell lithium battery charging protection solutions on the market, such as Intersil's ISL9208, which can achieve 7-cell lithium-ion battery charging protection. Compared with the multi-chip series solution, the advantages of the single-chip solution are that the circuit is simple and it is easier to achieve better electrical performance, but the number of batteries that can be monitored is limited and the price is more expensive. Multi-chip cascading methods, such as the S-8204 series, do not have this number limitation, and the circuit configuration is flexible and the cost is not high.