Battery Science Popularization | Detailed Explanation of Energy Storage Battery Parameters

Batteries are one of the most important components in electrochemical energy storage systems. With the reduction of battery costs, improvements in energy density, safety, and lifespan, energy storage has also ushered in large-scale applications. This article will help you understand several important parameters of energy storage batteries.

Battery Capacity

Battery capacity is one of the key performance indicators of battery performance. There is a distinction between rated capacity and actual capacity. The amount of electricity a battery releases under certain conditions (discharge rate, temperature, cut-off voltage, etc.) is called the rated capacity (or nominal capacity). Common units of capacity are mAh and Ah = 1000 mAh. For example, a battery with 48V and 50Ah capacity indicates that the battery capacity is 48V × 50Ah = 2400Wh, which is also 2.4 kilowatt-hours (kWh).

Battery Discharge C-Rate

C-rate is used to represent the battery’s charge and discharge capability. The charge and discharge rate = charge and discharge current / rated capacity. For example, a battery with a rated capacity of 100Ah discharging at 50A has a discharge rate of 0.5C. 1C, 2C, 0.5C are battery discharge rates, indicating the speed of discharge. The capacity is fully discharged in one hour, known as 1C discharge; if it takes two hours to discharge fully, it is known as 1/2 = 0.5C discharge. Generally, different discharge currents can be used to test the battery capacity. For a 24Ah battery, a 1C discharge current is 24A, and a 0.5C discharge current is 12A. The higher the discharge current, the shorter the discharge time.

When referring to the scale of an energy storage system, it is usually represented by the system’s maximum power/system capacity (kW/kWh). For example, an energy storage power station scale is 500kW/1MWh, where 500kW refers to the maximum charge and discharge power of this energy storage system, and 1MWh refers to the power station’s system capacity. If discharged at a rated power of 500kW, the power station’s capacity would be fully discharged in 2 hours, and the discharge rate would be 0.5C.

SOC (State of Charge)

The battery’s State of Charge (SOC) refers to the ratio of the remaining capacity after a period of use or long-term storage to the capacity when fully charged, usually expressed as a percentage, which is simply the remaining battery power.

SOH (State of Health)

SOH (State of Health) represents the current battery’s ability to store electrical energy relative to a new battery, which is the ratio of the current battery’s full charge energy to the new battery’s full charge energy. The definition of SOH is mainly reflected in several aspects, such as capacity, charge, internal resistance, cycle count, and peak power, with energy and capacity being the most widely applied.

Generally, when the battery’s capacity (SOH) drops to about 70% to 80%, it can be considered to have reached the End of Life (EOL). SOH is an indicator that describes the current health status of the battery, while EOL indicates that the battery has reached the end of its life and needs to be replaced. By monitoring the SOH value, one can predict the time when the battery will reach EOL and carry out corresponding maintenance and management.

DOD (Depth of Discharge)

Depth of Discharge (DOD) is used to measure the percentage of the battery’s discharge capacity relative to the battery’s rated capacity. For the same battery, the set DOD depth is inversely proportional to the battery cycle life; the deeper the discharge, the shorter the battery cycle life. Therefore, it is important to balance the required operation time of the battery and the need to extend the battery cycle life.

If the change in SOC from completely discharging to fully charging the battery is recorded as 0% to 100%, in actual applications, it is best to keep each battery operating within the 10% to 90% range; going below 10% may lead to over-discharge, causing irreversible chemical reactions that affect battery life.

SOE (State of Energy)

SOE, full name State of Energy, is a parameter that describes the remaining energy of the battery system or energy storage system. Unlike SOC (State of Charge), which mainly focuses on the proportion of the remaining battery capacity to its total capacity, SOE focuses more on the actual available energy of the system, considering factors such as battery efficiency, temperature, and aging that affect the actual available energy.

In applications such as electric vehicles and energy storage power stations, SOE is an important parameter. It can help users or systems more accurately understand the current energy status of the battery system or energy storage system, thereby making more reasonable decisions on charging, discharging, or usage. For example, in electric vehicles, by monitoring SOE, one can estimate the driving range of the vehicle and avoid running out of power during driving; in energy storage power stations, by monitoring SOE, one can reasonably arrange the charging and discharging plans of the energy storage system to improve the utilization rate and economy of the energy storage system.

It should be noted that the estimation of SOE is more complex than that of SOC because it requires consideration of more factors, such as battery efficiency, temperature, aging, etc. Therefore, in actual applications, more complex algorithms and models are needed to estimate SOE. At the same time, due to the different characteristics and usage environments of different battery systems or energy storage systems, the estimation methods and accuracy of SOE will also vary.

In summary, SOE is an important parameter that describes the remaining energy of the battery system or energy storage system, which is of great significance for improving the utilization rate and economy of the system. With the continuous development of electric vehicles and energy storage technology, the estimation methods and applications of SOE will also continue to improve and expand.

OCV (Open Circuit Voltage)

OCV (Open Circuit Voltage) refers to the terminal voltage of the battery when it is in an open-circuit state (i.e., when the battery is neither discharging nor charging). In battery technology, OCV is an important parameter that reflects the electromotive force or voltage level of the battery in a specific state.

For rechargeable batteries, OCV will change with the battery’s charge and discharge state (SOC, State of Charge) and the health status of the battery (such as battery aging, increased internal resistance, etc.). During the charging process, as the battery’s charge increases, OCV will gradually rise; during the discharging process, as the battery’s charge decreases, OCV will gradually fall.

Measurement of OCV is very important for the Battery Management System (BMS) because it can help the system understand the current state of the battery, thereby enabling accurate capacity estimation, charging control, discharging control, and fault diagnosis. For example, in electric vehicles, the BMS will monitor the battery’s OCV in real-time and adjust the charging strategy according to changes in OCV to ensure that the battery can be charged safely and efficiently.

In addition, OCV can also be used to assess the health status of the battery. As the battery is used and ages, its internal resistance will gradually increase, leading to a reduced range of OCV changes during charging and discharging. By monitoring the trend of OCV changes, one can determine the remaining capacity and degree of aging of the battery, providing a basis for battery maintenance and replacement.

It should be noted that the measurement of OCV requires ensuring that the battery is in an open-circuit state, that is, there is no current flowing between the positive and negative poles of the battery. Therefore, in actual applications, it is usually necessary to measure OCV after the battery has stopped charging and discharging for some time to ensure the accuracy of the measurement.

ACR/DCR

ACR (Alternate Current Resistance) and DCR (Direct Current Resistance) are two important parameters in battery performance evaluation, which respectively reflect the internal resistance characteristics of the battery in AC and DC circuits.

ACR: ACR refers to the internal resistance of the battery in an AC circuit, reflecting the degree to which the battery impedes AC. It is usually measured using a sinusoidal current signal of a specific frequency (such as 1kHz), at which time the battery’s internal resistance can be approximated as ohmic resistance, that is, the sum of the internal resistances of all parts of the battery.

The measurement results of ACR are affected by various factors such as the internal structure of the battery, electrolyte, electrode materials, etc.

DCR: DCR refers to the internal resistance of the battery in a DC circuit, reflecting the relationship between voltage and current under constant current. DCR measurement usually involves applying a constant DC to the battery terminals and measuring the resulting voltage drop.

DCR includes not only ohmic resistance but also electrochemical resistance and diffusion resistance, etc., thus more comprehensively reflecting the internal resistance characteristics of the battery.

OVP (Over Voltage Protection)

OVP (Over Voltage Protection), also known as overvoltage protection, is a mechanism designed to protect batteries and subsequent circuits from damage when the battery voltage exceeds a certain safety threshold. It achieves this by cutting off or limiting the power supply through specific circuit designs and protective measures. The principle is like overvoltage protection in electrical power systems, but it focuses more on the application scenario of batteries.

With the popularization of electronic products and the continuous development of battery technology, batteries, as key components for energy storage and supply, are increasingly being emphasized for their safety. Overvoltage in batteries can not only lead to battery damage but also potentially trigger serious consequences such as fires and explosions. Therefore, overvoltage protection (OVP) has become an essential method for ensuring battery safety and extending battery service life.

OCP (Over Current Protection)

(Over Current Protection), also known as overcurrent protection, is a circuit protection mechanism used to prevent currents in a circuit from exceeding a predetermined value, thereby avoiding equipment damage or dangerous situations like fires. Overcurrent protection is widely applied in various fields, including electrical power systems, electronic devices, and motor drives.

The working principle of overcurrent protection (OCP) is based on current detection and comparison. When the current in the circuit exceeds the preset threshold, the overcurrent protection device quickly responds by cutting off the power supply, reducing voltage, or adjusting circuit parameters to limit the current, thus safeguarding the safety of the circuit and equipment.

OTP (Over-Temperature Protection)

OTP (Over-Temperature Protection) is an essential safety protection mechanism in charging equipment, designed to prevent damage to devices or safety incidents due to excessive temperatures during the charging process. The over-temperature protection mechanism monitors the temperature of the charging device and takes appropriate measures, such as reducing charging power, stopping the charge, or cutting off the power supply, when the temperature exceeds a preset safety threshold. This mechanism is typically integrated into the control chips or power management modules of chargers, using temperature sensors to monitor the device’s temperature in real-time and compare it with the preset threshold.

During the charging process, equipment temperatures gradually rise due to the heat generated by the current passing through resistances and the exothermic reactions within the battery. If the temperature becomes too high and is not promptly controlled, it can lead to battery damage, circuit aging, or even cause severe consequences like fires. Therefore, over-temperature protection (OTP) during charging is of great significance for ensuring charging safety and extending the service life of devices.