Understand the core concepts of BMS

1, Battery material

What we usually call ternary lithium batteries or lithium iron phosphate batteries are named after the positive electrode materials of the batteries. The positive electrode materials of lithium-ion batteries mainly include lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium iron phosphate and ternary materials. Among them, ternary materials represent nickel cobalt manganese ternary materials, which are widely used in the field of electric vehicles. Lithium iron phosphate batteries refer to lithium-ion batteries with lithium iron phosphate as the positive electrode material, which are widely used in the field of electrochemical energy storage. The chemical formula of lithium iron phosphate is LiFePO4, and ternary lithium batteries are also called ternary polymer lithium batteries, and their chemical formula is Li(NiCoMn)O2. Compared with lithium iron phosphate batteries, ternary lithium batteries have higher energy density, but are more prone to thermal runaway.

2, Grouping method

Battery grouping refers to combining multiple batteries together through parallel and series connection to increase the battery capacity or voltage. Parallel connection connects the positive and negative electrodes of multiple batteries so that they share charge and load, thereby increasing the battery capacity. Series connection connects the positive and negative electrodes of multiple batteries in sequence so that their voltages are added together, thereby increasing the voltage of the battery pack. For example, if there are 4 batteries, each with a voltage of 1.5 volts and a capacity of 1000 mAh, then the voltage of the battery pack after parallel connection will be 1.5 volts and the capacity will reach 4000 mAh; the voltage of the battery pack after series connection will be 6 volts and the capacity will still be 1000 mAh. The grouping method is usually expressed in the form of xPyS, where x represents the number of single cells in parallel and y represents the number of single cells in series. P stands for Parallel and S stands for Series. For example, a grouping method of 3P10S means that 3 batteries are first connected in parallel to form a group, and then 10 groups are connected in series to form a module.

3, Capacity and charge/discharge rate

The battery capacity is expressed in ampere-hours (Ah), which means how long it takes to discharge using a certain current. For example, 100Ah means that it can discharge for one hour using a current of 100A.

4, Battery cell voltage

Voltage refers to the potential difference between the positive and negative electrodes of a battery cell, and the voltage of a battery cell will change with the increase or decrease of the amount of electricity. SoC is a parameter used to describe the use of battery capacity. When the battery is charged, the SoC gradually increases, and when it is discharged, the SoC gradually decreases. The voltage of a battery cell is related to the value of SoC. When the battery is discharged, the battery cell voltage varies with different amounts of electricity and SoC, so the voltage of the battery cell is a floating range. The voltage range of lithium iron phosphate batteries (LiFePO4 batteries) is generally 2.8V to 3.6V. The voltage of ternary lithium batteries is approximately 2.75V-4.2V.

5,SoX

SoX stands for State of X, which describes the state of the battery. X can be H, C, P or E, i.e. SoH, SoC, SoP or SoE. H stands for Health, C stands for Capacity, P stands for Power and E stands for Energy.

6, Battery barrel effect and balancing

Battery barrel effect means that the overall performance of the battery is subject to the worst performance of the battery cell. For example, during the charging process, when the battery cell with the smallest capacity is full, charging needs to be stopped to avoid damage caused by overcharging. The battery cell with larger capacity can only use part of its capacity, resulting in part of the battery pack being wasted. Therefore, it is necessary to control the consistency of the battery cells to ensure the overall performance of the battery. In order to minimize the inconsistency of the battery pack, the battery needs to be balanced. The sources of inconsistency include the internal differences of the battery itself and the differences in external usage status. At present, the existing balancing solutions are mainly divided into passive balancing and active balancing. Passive balancing is likely to have adverse effects on the battery management system and battery pack, while active balancing requires the configuration of corresponding circuits and energy storage devices, which are costly and bulky, and are not easy to promote and apply.

The main function of battery balancing is to consume or transfer the power of pre-charged cells during the charging and discharging process, so that other cells that are not fully charged can continue to charge, ensuring that the entire battery pack can be fully charged. Passive balancing is also called energy dissipation balancing. Its working principle is to connect a resistor in parallel to each battery cell. When a single cell has been fully charged in advance and the battery pack needs to continue to be charged, a resistor is connected to discharge it and consume excess energy. Active balancing is also called non-energy dissipation balancing. Its principle is to transfer the energy in the pre-charged cells to the cells that have not yet been fully charged, ensuring that each battery cell can be fully charged.