Key technologies of BMS battery management system

BMS

A battery management system (BMS) is a system that manages batteries. It usually has the function of measuring battery voltage to prevent or avoid abnormal conditions such as over-discharge, over-charge, and over-temperature. With the development of technology, many functions have been gradually added.

The battery management system is closely integrated with the power battery of the electric vehicle. It uses sensors to detect the voltage, current, and temperature of the battery in real time. It also performs leakage detection, thermal management, battery balancing management, alarm reminders, calculates the remaining capacity (SOC), discharge power, reports the battery degradation degree (SOH) and the remaining capacity (SOC) status, and uses algorithms to control the maximum output power according to the voltage, current, and temperature of the battery to obtain the maximum mileage, and uses algorithms to control the charger to charge with the optimal current. It communicates in real time with the on-board master controller, motor controller, energy control system, on-board display system, etc. through the CAN bus interface. The three main key technologies of BMS are as follows:

1. SOC estimation

That is, accurately estimate the remaining battery power, ensure that the SOC is maintained within a reasonable range, prevent damage to the battery due to overcharging or over-discharging, and predict at any time how much energy is left in the hybrid vehicle energy storage battery or the state of charge of the energy storage battery. The SOC estimation accuracy is high, and for the same amount of battery, a longer driving range can be achieved. Therefore, high-precision SOC estimation can effectively reduce the required battery cost.

SOC is the transmission information calculated based on the monitored external characteristic information. While SOC informs the owner of the current power level, it also lets the car know its own power level, prevents overcharging and over-discharging, improves balance consistency, increases output power and reduces extra redundancy. The bottom layer of the system is calculated by complex algorithms to ensure the safe, continuous and stable operation of the car and improve safety. Therefore, it is very important to accurately estimate the SOC value, and its algorithm is one of the core competitiveness of related companies.

2. Balance control

To ensure the parameter consistency of battery cells, it is to charge the single cells evenly so that each battery in the battery pack reaches a balanced and consistent state. Balancing control is divided into active balancing and passive balancing. Active balancing is to balance the capacity or voltage differences between battery cells during the charging, discharging or placement of the battery pack to eliminate various inconsistencies generated inside the battery. In this process, energy transfer is involved. There are generally two methods of energy transfer. One is to balance the energy of single cells with high energy to batteries with low energy, and the other is to transfer the energy of single cells with high voltage (capacity) to a backup battery, and then transfer it from the backup battery to other batteries with lower voltage (capacity).

In traditional energy-consuming BMS systems, the balancing method is mainly passive balancing, which uses single-cell batteries in parallel to shunt energy-consuming resistors, and balancing can only be done during the charging process. Its working principle is to detect the difference between the series-connected single-cell batteries by collecting the voltage, and take the "upper threshold voltage" of the set charging voltage as the benchmark. As long as any single-cell battery reaches the "upper threshold voltage" first during charging and detects the difference with the batteries in the adjacent group, that is, the battery with the highest single-cell voltage in the battery group, discharges the current through the energy-consuming resistor connected in parallel to the single-cell battery, and so on, until the single-cell battery with the lowest voltage reaches the "upper threshold voltage" for a balancing cycle.

3. Thermal Management

Make the battery work within the appropriate temperature range and reduce the temperature difference between each battery module. Thermal management mainly includes determining the optimal operating temperature range of the battery, battery thermal field calculation and temperature prediction, heat transfer medium selection, thermal management system heat dissipation structure design and fan prediction stable point selection.