How can diodes prevent current backflow when battery modules are connected in parallel?

一, The triggering mechanism and hazards of current backflow
1. Trigger conditions
The essence of current backflow is the reverse flow of energy, and its core triggering condition is that the load terminal voltage (V_load) is higher than the power supply terminal voltage (V_supply). In a battery parallel system, typical scenarios include:

Voltage imbalance between battery packs: When a battery pack experiences a voltage increase due to differences in SOC (remaining charge) or inconsistent internal resistance, it may reverse charge to other low-voltage battery packs.
Load mutation: The reverse electromotive force generated when inductive loads such as motors and inductors are powered off may flow back to the battery pack through parallel paths.
Transient power switching: When switching between dual power sources (such as mains and backup battery), if the voltage of the backup power source rises faster than that of the main power source, it may cause a brief backflow.
2. Hazard analysis
The hazards of current backflow are directly related to the system power level:

Low voltage and low-power scenarios (such as consumer electronics): Backflowing current may penetrate the charging IC, causing the device to fail to charge or even burn out.
High voltage and high-power scenarios (such as industrial power supplies): Backflowing current can generate excessive circulation inside the battery, accelerate battery aging, and even cause thermal runaway.
Grid side equipment (such as photovoltaic inverters): Backflowing current may cause voltage fluctuations in the grid, affect the operation of other equipment, and even trigger grid protection tripping.
二, Technical principle and selection points of diode anti backflow
1. Unidirectional conductivity: Building basic protective barriers
The core characteristic of a diode lies in the unidirectional conductivity of its PN junction, which only allows current to flow from the anode (A) to the cathode (K), with reverse cutoff. In a battery parallel system, diodes prevent backflow through the following mechanism:

Forward conduction: When the battery pack voltage is higher than the load terminal voltage, the diode conducts to supply power to the load.
Reverse cutoff: When the voltage at the load end increases due to a fault or transient switching, the diode automatically cutoff, blocking the reverse current path.
2. Key parameter selection
According to the voltage, current, and efficiency requirements of the battery parallel system, the selection of diodes should focus on the following parameters:

Positive voltage drop (V_F): directly affects system efficiency. Ordinary diodes have a V_F of about 0.6-0.8V, while Schottky diodes can reduce it to 0.2-0.4V. For example, in a 48V energy storage system, using Schottky diodes (such as MBR1045CT) can reduce conduction losses by more than 60%.
Reverse recovery time (Trr): In high-frequency switching scenarios, Trr should be less than 10ns to avoid switching losses. The Trr of fast recovery diodes (such as FR107) is about 50ns, while Schottky diodes have near zero reverse recovery time.
Rated current (I2): It should be greater than 1.5 times the maximum operating current of the system. For example, in a 100A parallel system, diodes with I2 ≥ 150A (such as SS34) should be selected.
Surge current carrying capacity (I2FSM): It needs to cover transient high currents during system startup or failure. For example, in car BMS, it is necessary to select diodes with I2 FSM ≥ 300A to cope with sudden load changes.
三, Typical application scenarios and engineering practices
1. Parallel protection of electric vehicle battery packs
In the Tesla 4680 battery module, diodes and MOSFETs work together to achieve anti backflow and balanced control:

Anti backflow design: Schottky diodes (such as CBRD1045-40) are connected in series at the output end of each battery cell group, with a 40V withstand voltage covering the requirements of 12V/24V systems. When the voltage of a certain battery cell group rises abnormally, the corresponding diode will turn off to prevent reverse charging.
Balanced control: Passive balancing is achieved by connecting small signal diodes (such as BAS70-04) in parallel with balancing resistors. When the voltage of a certain battery cell is too high, the balancing circuit diode conducts, forming a bypass current to prevent overcharging.
2. Parallel connection of multiple battery packs in photovoltaic energy storage system
In the photovoltaic inverter of Sunac Power, the diode array realizes intelligent switching of multiple battery packs:

Priority control: Using back-to-back MOS tubes and diodes to achieve automatic switching between main battery packs (such as lithium batteries) and backup battery packs (such as lead-acid batteries). When the voltage of the main battery pack is below the threshold, the backup battery pack is automatically connected through a diode to avoid backflow.
EMI optimization: parallel RC absorption network (such as R=10 Ω, C=100nF), Suppress switch noise by 40dB to meet the IEC 61000-4-5 standard.
3. Data Center UPS System Anti Backflow
In Huawei data center UPS, the ideal diode controller (such as LM66100DCK) achieves zero voltage drop and backflow prevention:

Working principle: Simulating an "ideal diode" through an internal PMOS transistor, the voltage drop during forward conduction is only a few milliohms, and it quickly turns off during reverse conduction (response time<10 μ s).
Protection logic: When the mains power is cut off, the controller automatically detects a voltage drop and cuts off the reverse current path within 10 μ s to prevent battery energy from flowing back to the mains end.