1. BMS (Battery Management System)
Definition: It is the "brain" of the battery system, responsible for real-time battery status monitoring, energy management, communication and diagnostics, safety protection, and balancing control, ensuring safe, efficient, and long-life operation of the battery system.
Note:
- A BMS consists of both hardware and software.
- The performance of the BMS directly determines the safety, reliability, and economic efficiency of the entire system.
2. SOC (State of Charge) - Simply put: Remaining Capacity
Definition: The percentage of the battery's current remaining capacity to its rated capacity, i.e., SOC = (Remaining Capacity / Rated Capacity) x 100%.
Note:
- SOC is a key basis for the BMS's protection mechanisms, charge and discharge strategies, balancing control, and status feedback.
- The SOC value is estimated by the BMS through an algorithm, not directly measured. Therefore, an accurate SOC estimation strategy is paramount for the BMS.
- The SOC calculation formula is: SOC = Remaining Capacity / Total Capacity. As a battery degrades, its maximum capacity decreases. To more accurately reflect the battery's current remaining charge, the total capacity should be the actual total capacity over its current lifespan, also known as the real-time capacity. This calculated SOC more accurately reflects the battery's remaining charge, helping to more accurately assess battery life and other aspects, providing users with more reliable power information.
3. State of Health (SOH)
Definition: The ratio of a battery's current actual capacity to its initial rated capacity, i.e., SOH = (Current Actual Capacity / Initial Rated Capacity) × 100%.
Notes:
- This is a key indicator that measures the extent of battery performance degradation relative to its initial state. It primarily reflects the degradation of core performance indicators such as battery capacity and internal resistance. Users can intuitively assess the battery's aging status, providing a basis for maintenance and replacement decisions.
- Like SOC, SOH is also estimated through an algorithm.
- Currently, the industry generally assumes that a SOH of 70% marks the end of life for an energy storage system.
4. DOD (Depth of Discharge)
Definition: The percentage of a battery's discharged capacity as a percentage of its rated capacity, i.e., DOD = (Discharged Capacity/Rated Capacity) x 100%.
Note:
- This is a key indicator for measuring the degree of discharge in a battery system, providing a visual indication of the energy storage system's discharge capacity.
- Different DODs can also affect the performance of lithium batteries (compared to lead-acid batteries, the impact of DOD on lithium batteries is much smaller, but it cannot be completely ignored).
5. Charge/Discharge Rate (C-Rate)
Definition: The ratio of the charge/discharge current to the rated capacity. For example, 0.5C means charging/discharging at a current of half the battery's capacity.
- The maximum charge/discharge rate represents the upper limit of the energy storage system's allowable charge/discharge capacity. However, in practice, this value is not always maintained; actual demand determines the maximum charge/discharge rate.
- The charge/discharge rate visually represents the maximum charge/discharge capacity of the energy storage system. The operating capacity of the entire energy storage system serves as an important basis for matching device power.
- Energy storage systems are mostly 0.5C, while 1C is more commonly used for frequency modulation and amplitude modulation services.
- The maximum charge and discharge rate of a battery cell indicates its capacity. The BMS can redefine this value based on actual needs to determine the capacity of the energy storage system.
6. Cycle Count
Cycle count is a core metric for measuring the service life of an energy storage system. However, most products on the market currently have ambiguities in the definition, estimation, and experimental data of cycle count. Only by understanding the basics of cycle count can we judge the quality of an energy storage system or, more accurately, determine whether a product's marketing is fraudulent.
This article analyzes the relevant provisions of "GB/T 36276-2023 Lithium-ion Batteries for Power Storage":
"Rated power charge and discharge cycles" is defined as the guaranteed number of cycles at which the battery's energy decays to the rated value under specified conditions when cyclically charged and discharged at rated power.
This national standard specifies the criteria for cycle performance testing, but does not clearly define the number of cycles required for service life. This leaves ample room for many energy storage system manufacturers to misrepresent cycle counts.
The definition of cycle counts has three key elements:
<1> Under specified conditions:
- Ambient temperature: This generally assumes a cell temperature of 25±2°C. In practice, consistent temperature control is difficult to achieve, and this may differ from test conditions.
- Charge and discharge cut-off voltage: According to relevant data, the charge and discharge cut-off voltage for energy storage cells is 2.5-3.65V. This varies among battery modules: 2.7-3.65V is the most common, but 2.8-3.55V and 2.8V-3.6V are also common.
- Definition of a cycle: A complete charge and discharge (discharge capacity = rated capacity) constitutes one cycle. Some common ambiguous descriptions include:
- DOD 90%, cycle count 10,000: Is the discharge capacity at 90% DOD defined as the operating capacity of the energy storage system? If it is based on the operating capacity at 90% DOD, is the cycle count defined?
- 10,000 cycles: This is a completely ambiguous test condition, not specifying the depth of discharge or whether the rated capacity is used as a cumulative cycle.
- Is the cycle count tested for the cell, module, battery cluster, or energy storage system?
<2> Rated power cyclic charge and discharge:
Different charge and discharge powers can also affect the battery's cycle count. For example, for the same energy storage system, the cycle counts obtained with 0.5C charge and discharge versus 0.2C charge and discharge will definitely differ.
<3> Guaranteed Attenuation Value:
Common values such as 70% SOH and 80% SOH, which define the state of health at the end of a battery's life, can significantly impact the cycle count.
Mate Solar believes that the cycle count of energy storage systems is rarely based on empirical data; conclusions are often drawn from theoretical experiments. Some manufacturers exaggerate or confuse claims. End users should be vigilant, fully understand the situation upfront, and clearly define warranty agreements to protect their own interests.
Three-Level BMS Architecture
7. BMU (Battery Management Unit): A common name, lacking a strict, standardized name.
A BMU is typically installed inside the battery pack. Its primary function is to collect cell voltage and temperature data within the pack and implement battery balancing strategies.
8. BCMU (Battery Cluster Management Unit): A common name, lacking a strict, standardized name. Also known as BCU/ESBCM.
A BCMU is often installed inside a high-voltage protection box. Its primary function is to collect information from the first-level BMU, collecting battery cluster voltage, current, and insulation data, and controlling the battery pack protection contactors.
9. BSMU (Battery Stack Management Unit): Battery System Management Unit (BSU), commonly known as a battery system management unit (BSU), lacks a strict, unified standard. It may be referred to as BSU, ESMU, BAMS, or BAU.
It is often installed in the battery cluster combiner cabinet. Its primary function is to collect, store, and display information transmitted by the second-level BCMU. It also provides real-time alarms, control of the main circuit breaker, contact feedback, and real-time communication with the PCS, EMS, and on-site monitoring.