The principle of equalization charging in lithium-ion battery pack protection boards; what are the different types of equalization charging circuits for lithium-ion battery packs? Common equalization charging technologies include constant shunt resistor equalization charging, on-off shunt resistor equalization charging, uniform battery voltage equalization charging, switched capacitor equalization charging, buck converter equalization charging, and inductor equalization charging. When charging lithium-ion batteries in series, it is essential to ensure that each cell is charged evenly; otherwise, the performance and lifespan of the entire lithium-ion battery pack will be affected during use.
Lithium-ion battery pack protection board equalization charging principle
When the lithium-ion battery pack is charging, the positive and negative terminals of the external power supply of the lithium-ion battery pack protection board are connected to the positive and negative terminals bAT+ and bAT- of the battery pack, respectively. The charging current flows through the positive terminal bAT+ of the battery pack, the individual lithium-ion batteries 1 to N in the battery pack, the discharge control switch, the charging control switch, and the negative terminal bAT- of the lithium-ion battery pack protection board.
In the lithium-ion battery pack protection board system, the charging overvoltage protection control signal of the single-cell lithium-ion battery protection chip in the control circuit section is output in parallel after being isolated by an optocoupler, and supplies the gate voltage for the charging switching device in the main circuit.
If one or more lithium-ion batteries enter overvoltage protection mode during charging, the overvoltage protection signal controls the shunt discharge branch connected in parallel across the positive and negative terminals of the single lithium-ion battery to discharge, while simultaneously disconnecting the corresponding single lithium-ion battery connected in series in the charging circuit from the charging circuit.
The lithium-ion battery pack protection board system achieves balanced charging by controlling the switching devices of the shunt discharge branch through the protection chip while providing charging protection. This method is different from the traditional approach of achieving balanced charging at the charger end, which reduces the design and use cost of lithium-ion battery pack chargers.
Lithium-ion battery protection board power balancing function
1. When voltage differences arise among individual batteries in the battery pack due to battery matching or external environmental influences, the balancing circuit is permitted to operate if the voltage difference between the series-connected battery groups exceeds a set value. Balancing is initiated during charging, and the balancing resistor discharges the battery pack with the highest relative capacity. The balancing current is the balancing absorption current value, thereby reducing the rate of voltage rise in the battery pack.
2. When the voltage difference between the series-connected battery groups is less than the set value, the balancing circuit is prohibited from working, and there is no balancing power.
What are the different types of equalization charging circuits for lithium-ion battery packs?
Researchers both domestically and internationally have proposed numerous different circuit topologies for battery pack equalization charging circuit design. Based on the energy consumption of the circuit during the equalization process, battery pack equalization charging circuits can be divided into two main categories: energy dissipation type and energy non-dissipation type.
1. Energy dissipation-type equalization achieves equalization by connecting shunt resistors in parallel across each individual cell in the battery pack to discharge them. The shunt resistor discharge equalization circuit is the most straightforward equalization technique. This technique discharges the higher-capacity cells through the shunt resistors until all individual cells have the same capacity.
2. Compared with energy dissipative balancing, energy non-dissipative balancing circuits consume less energy but have a more complex circuit structure.
① The Flying Capacitor Generator connects a capacitor in parallel with each battery cell. By switching on and off, this capacitor can be connected in parallel to the same battery cell or to an adjacent battery cell. When the voltage of a battery cell is too high, the capacitor is first connected in parallel with the battery cell, so that the capacitor voltage matches that of the battery cell. Then, the capacitor is switched to an adjacent battery cell, and the capacitor discharges into the battery cell, thus achieving energy transfer.
② Switched Capacitor Balancing: For a power lithium battery pack composed of n individual cells connected in series, the switched capacitor balancing circuit requires n-1 capacitor elements and 2n switching devices. Taking the inconsistent terminal voltages of individual cells b1 and b2 as an example, there are two states during the control process. The disadvantage of this circuit is that it can only be used for balancing the terminal voltages between individual cells, and it can only realize the energy flow between adjacent individual cells. Therefore, when the number of cells connected in series is large, the balancing time is relatively long.
③ The double-layer capacitor equalization circuit is also a derivation and transformation of the switched capacitor circuit. The difference lies in the fact that this circuit uses two layers of switched capacitors to achieve energy transfer between batteries. Compared with the switched capacitor equalization circuit, the advantage of this circuit is that by using the newly added outer switched capacitor, a single battery can not only perform voltage equalization with adjacent single batteries, but also with non-adjacent single batteries, thus improving the equalization speed.
The above explains the principle of equalization charging in lithium-ion battery pack protection boards and the classification of equalization charging circuits for lithium-ion battery packs. Existing single-cell lithium-ion battery protection chips do not include equalization charging control functions. For multi-cell lithium-ion battery protection chips, the equalization charging control function requires an external CPU; this is achieved through serial communication with the protection chip, increasing the complexity and design difficulty of the protection circuit, reducing system efficiency and reliability, and increasing power consumption.
