1. BMS (Battery Management System)
Definition: The "brain" of the battery system. It is responsible for real-time monitoring of battery status, energy management, communication & diagnostics, safety protection, and cell balancing control, ensuring the battery system operates safely, efficiently, and with a long lifespan.
Key Notes:
The BMS consists of both hardware and software components.
The performance of the BMS directly determines the overall safety, reliability, and cost-effectiveness of the entire system.
2. SOC (State of Charge) - Simplified: Remaining Battery Capacity
Definition: The percentage of the battery's current remaining capacity relative to its rated capacity. Formula: SOC = (Remaining Capacity / Rated Capacity) * 100%.
Key Notes:
SOC is a critical input for BMS protection mechanisms, charge/discharge strategies, balancing control, and status feedback.
SOC is estimated by the BMS using algorithms, not directly measured. Therefore, an accurate SOC estimation strategy is paramount for the BMS.
The formula is SOC = Remaining Capacity / Total Capacity. As batteries degrade, their maximum releasable capacity decreases. To accurately reflect the current state of charge, the "Total Capacity" used in the formula should be the actual total capacity at the current state of health (i.e., real-time capacity). This provides a truer representation of remaining energy, enabling more precise assessment of battery range and delivering more reliable state-of-charge information to users.
3. SOH (State of Health)
Definition: The ratio of the battery's current actual capacity to its initial rated capacity. Formula: SOH = (Current Actual Capacity / Initial Rated Capacity) * 100%.
Key Notes:
SOH is a key indicator measuring the degradation of the battery's current performance relative to its initial state. It primarily reflects the deterioration of core parameters like capacity and internal resistance, allowing users to intuitively judge battery aging and make informed maintenance or replacement decisions.
Like SOC, SOH is also estimated via algorithms.
The industry commonly considers SOH 70% as the end-of-life (EOL) point for energy storage systems.
4. DOD (Depth of Discharge)
Definition: The percentage of the rated capacity that has been discharged from the battery. Formula: DOD = (Discharged Capacity / Rated Capacity) * 100%.
Key Notes:
DOD is a key metric indicating the depth to which the battery system has been discharged, directly reflecting the available discharge capability of the energy storage system.
Different DOD levels also impact lithium-ion battery performance (though the effect on Li-ion is significantly smaller than on lead-acid batteries, it is not negligible).
5. Charge/Discharge Rate (C-Rate)
Definition: The ratio of the charge or discharge current to the rated capacity. For example, 0.5C means charging or discharging at a current equal to half the battery's rated capacity value (e.g., 50A for a 100Ah battery).
Key Notes:
The maximum charge/discharge rate indicates the upper limit of the system's allowable charge/discharge capability. However, the system does not continuously operate at this maximum rate; actual operation is based on demand.
The C-rate directly indicates the working capability of the entire energy storage system and serves as a crucial basis for matching equipment power requirements.
Most energy storage systems operate at rates like 0.5C. Higher rates like 1C are more commonly used for frequency regulation and peak shaving services.
The maximum C-rate of a cell indicates its inherent capability. The BMS can redefine this value based on system design and requirements to determine the overall capability of the energy storage system.
6. Cycle Life (Number of Cycles)
Cycle life is a core metric for evaluating the lifespan of an energy storage system. However, definitions, estimation methods, and test data for cycle life are often ambiguous in the market. Understanding the fundamentals of cycle life is essential for evaluating system quality and identifying potentially misleading claims in product marketing.
Analysis based on relevant provisions in the Chinese National Standard GB/T 36276-2023 (Lithium-ion batteries for electrical energy storage):
"Rated Power Charge/Discharge Cycle Count" is defined as: The guaranteed number of cycles where the charge/discharge energy decays to the rated charge/discharge energy value when the battery is cycled at rated power under specified conditions.
While this standard specifies the test method for cycle performance, it does not rigidly define the cycle count representing "useful life." This ambiguity leaves significant flexibility for manufacturers in how they report cycle life figures.
Three Critical Elements Defining Cycle Life:
1. Specified Conditions:
Ambient Temperature: Tests are typically conducted with the cell temperature at 25±2°C. Real-world temperature control is challenging and often differs from test conditions.
Charge/Discharge Cut-off Voltages: Research indicates common cut-off voltages for energy storage cells are 2.5V-3.65V. Variations exist at the module level: 2.7V-3.65V is most common, but 2.8V-3.55V and 2.8V-3.6V are also used.
Definition of One Cycle: A full charge/discharge (where Discharged Capacity = Rated Capacity) constitutes one cycle. Ambiguous descriptions are common:
"10,000 cycles at DOD 90%": Is the capacity discharged at DOD 90% defined as the system's working capacity? If so, is the cycle count defined based on cycling this working capacity?
"10,000 cycles": Completely vague regarding test conditions – lacks specification of discharge depth per cycle and whether cycles are counted based on accumulating the rated capacity.
Level of Testing: Is the cycle count tested at the cell, module, battery rack (cluster), or full system level?
2. Rated Power Cycling:
Different charge/discharge power levels significantly impact cycle life results. For example, the same system cycled at 0.5C will yield a different cycle life than at 0.2C.
3. Degradation Threshold (SOH Endpoint):
Common EOL SOH values are 70% or 80%. The chosen SOH threshold defining "end of life" greatly influences the reported cycle count number.
7. BMU (Battery Management Unit)
Note: Terminology varies; no single strict standard exists.
Definition: Typically installed within a battery pack (PACK). Its primary functions include collecting individual cell voltages and temperatures within the PACK and executing cell balancing strategies.
8. BCMU (Battery Cluster Management Unit)
Note: Terminology varies; also commonly referred to as BCU, ESBCM, etc.
Definition: Often installed in a high-voltage protection box. Its primary functions include collecting data from the first-level BMUs, acquiring battery cluster voltage, current, and insulation resistance information, and controlling contactors for battery cluster protection.
9. BSMU (Battery System Management Unit)
Note: Terminology varies; also commonly referred to as BSU, ESMU, BAMS, BAU, etc.
Definition: Typically installed in the battery cluster combiner cabinet. Its primary functions include collecting and processing data transmitted from the second-level BCMUs, data storage and display, providing real-time alarms, controlling the main circuit breaker and monitoring its status (contact feedback), and enabling real-time communication with the PCS, EMS, and local monitoring systems.
