EV Charging
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ower batteries are at the heart of today’s technological revolution, EV power battery performance is also important,especially as we march toward a future powered by electric vehicles (EVs) and renewable energy. But what makes one battery better than another? Why do some last longer, charge faster, or perform better under stress? The answer lies in understanding the key performance indicators (KPIs) that define a power battery’s quality and efficiency. In this article, we’re going to break down these KPIs into digestible bites, so you’ll walk away with a solid grasp of what really matters when it comes to power batteries.
1.Battery Voltage
A very important factor affecting EV power battery performance is battery voltage.The battery voltage mainly includes terminal voltage, nominal (rated) voltage, open circuit voltage, operating voltage, charge termination voltage and discharge termination voltage.
Terminal voltage: The terminal voltage of a battery refers to the potential difference between the positive and negative terminals of the battery.
Open circuit voltage: The terminal voltage of the battery under open circuit conditions is called open circuit voltage, that is, the terminal voltage of the battery when there is no load.
End-of-charge voltage: When the battery is fully charged, the active substances on the plates have reached a saturated state. If the battery continues to charge, the voltage of the battery will not rise. The voltage at this time is called the end-of-charge voltage. The end-of-charge voltage of aluminum-acid batteries is 2.7~2.8V, the end-of-charge voltage of metal hydride nickel-based batteries is 1.5V, and the end-of-charge voltage of lithium-ion batteries is 4.25V.
Nominal voltage: also known as rated voltage, refers to the voltage that the battery should reach when working under standard conditions. The nominal voltage is determined by the electrode potential of the plate material and the concentration of the internal electrolyte. The nominal voltage of lead-acid batteries is 2V, the nominal voltage of nickel metal hydride batteries is 1.2V, the nominal voltage of lithium iron phosphate batteries is 3.2V, and the nominal voltage of lithium manganese oxide batteries is 3.7V.
Working voltage: also called load voltage, refers to the terminal voltage of the battery in the discharge state after the load is connected. The working voltage at the beginning of battery discharge is called initial voltage.
Discharge termination voltage: When the battery is discharged under the discharge conditions specified by certain standards, the battery voltage will gradually decrease. When the battery is no longer suitable for further discharge, the lowest working voltage of the battery is called the discharge termination voltage. If the battery continues to discharge after the voltage is lower than the discharge termination voltage, the voltage across the battery will drop rapidly, forming a deep discharge. The products formed on the plates will not be easy to recover during normal charging, thus affecting the battery life. The discharge termination voltage is related to the discharge rate. The discharge current directly affects the discharge termination voltage. Under the specified discharge termination voltage, the greater the discharge current, the smaller the battery capacity. The discharge termination voltage of the metal hydride nickel battery is 1V, and the discharge termination voltage of the lithium-ion battery is 3.0V.
2. Battery Capacity
When we talk about a battery’s capacity, think of it as the fuel tank of your car. It’s a measure of how much energy the battery can store. Just like a larger fuel tank lets you drive farther, a higher battery capacity means you can use your device or vehicle longer before needing to recharge.
Capacity definition: Capacity refers to the total amount of electricity released by a fully charged battery under specified conditions, in units of Ah or kAh, which is equal to the product of discharge current and discharge time. The amount of active material in a unit cell determines the amount of charge contained in the unit cell, and the content of active material is determined by the materials and volume used in the battery. Generally, the larger the battery volume, the higher the capacity. The capacity of a battery can be divided into rated capacity, n-hour rate capacity, theoretical capacity, actual capacity, state of charge, etc.
1. Rated capacity: Rated capacity refers to the capacity released when a fully charged battery is discharged at room temperature with a current of 1 (A) and reaches the termination voltage.
2. N-hour rate capacity: n-hour rate capacity refers to the amount of electricity released when a fully charged battery is discharged at an n-hour rate discharge current and reaches the specified termination voltage.
3. Theoretical capacity: Theoretical capacity is the highest theoretical value obtained by calculating the mass of active materials according to Faraday’s law. In order to compare different series of batteries, the concept of specific capacity is often used, that is, the theoretical amount of electricity that a battery per unit volume or unit mass can provide, with the unit of Ah/L or Ah/kg.
4. Actual capacity: Actual capacity is also called available capacity, which refers to the amount of electricity that a battery can output under certain conditions. It is equal to the product of discharge current and discharge time, and its value is less than theoretical capacity. Actual capacity reflects the amount of electricity that the battery actually stores. The larger the battery capacity, the longer the driving range of the electric vehicle. During use, the actual capacity of the battery will gradually decay. National standards stipulate that a newly manufactured battery with an actual capacity greater than the rated capacity is a qualified battery.
5. State of charge: The state of charge (SOC) refers to the ratio of the remaining power of the battery under a certain discharge rate to the rated capacity under the same conditions, reflecting the characteristics of the battery capacity change. SOC = 1 means that the battery is fully charged. As the battery discharges, the charge of the battery gradually decreases. At this time, the battery charge state can be expressed by the relative amount of the percentage of the SOC value to represent the change state of the charge in the battery. Generally, the high efficiency area of battery discharge is 50%~80% SOC.
3.Battery Internal Resistance
Definition: The internal resistance of a battery refers to the resistance encountered when the current flows through the battery. It is generally the sum of the resistances of the electrolyte, positive and negative electrode groups, and separators in the battery.
1. The greater the internal resistance of the battery, the more energy the battery consumes itself and the lower the battery efficiency.
2. Batteries with large internal resistance generate severe heat when charging, causing the battery temperature to rise sharply, which has a great impact on both the battery and the charger.
3. As the battery is used for a longer time, the internal resistance of the battery will increase to varying degrees due to the consumption of electrolytes and the decrease in the activity of the chemical substances inside the battery, thus affecting the performance of the EV Power Battery. The internal resistance of the battery is measured using a special instrument.
4.Insulation resistance is the resistance between the battery terminals and the battery box or vehicle body.
4. Battery Energy
Definition: Battery energy refers to the electrical energy that a battery can output under a certain discharge regime, in units of Wh or kWh. It affects the driving range of electric vehicles. Battery energy is divided into total energy, theoretical energy, actual energy, specific energy, energy density, charging energy, and discharging energy.
Total energy: Total energy refers to the total electrical energy output of the battery during its life cycle.
Theoretical energy: Theoretical energy is the product of the theoretical capacity of the battery and the rated voltage. It refers to the energy output by the battery under the discharge conditions specified by certain standards.
Actual energy: Actual energy is the product of the actual capacity of the battery and the average operating voltage, which indicates the energy that the battery can output under certain conditions.
Specific energy: Specific energy is also called mass specific energy, which refers to the electrical energy that can be output per unit mass of the battery. The unit is Wh/kg. Specific energy can be divided into theoretical specific energy and actual specific energy. Theoretical specific energy refers to the energy that can be theoretically output when 1kg of battery reaction material is fully discharged; actual specific energy refers to the actual energy that can be output by 1kg of battery reaction material. The actual specific energy of the battery is much smaller than the theoretical specific energy. The specific energy of the battery is a comprehensive indicator that reflects the quality level of the battery. The specific energy of the battery affects the vehicle mass and driving range of the electric vehicle, and is an important indicator for evaluating whether the power battery of the electric vehicle meets the predetermined driving range.
Energy density: Energy density, also known as volumetric energy, refers to the electrical energy that a battery can output per unit volume, with the unit being Wh/L.
Charging energy: Charging energy refers to the electrical energy input into the battery through the charger.
Discharge energy: Discharge energy refers to the electrical energy output when the battery is discharged.
5.Battery Power
Definition: Battery power refers to the amount of energy output per unit time under a certain discharge system, measured in W or kW. Battery power determines the acceleration performance and climbing ability of electric vehicles,also affects EV power battery performance.
Specific power: The power that a battery can output per unit mass is called specific power, also known as mass specific power, and its unit is W/kg or kW/kg.
Power density: The output power obtained from the unit mass or unit volume of the battery is called power density, and the unit is W/kg or W/L. The output power obtained from the unit mass of the battery is called mass power density. The output power obtained from the unit volume of the battery is called volume power density.
6.Output Efficiency
As an energy storage device, power batteries convert electrical energy into chemical energy when charging and release electrical energy when discharging. In this reversible electrochemical conversion process, there is a certain amount of energy loss, which is usually expressed by the battery’s capacity efficiency and energy efficiency.
Capacity efficiency: Capacity efficiency refers to the ratio of the capacity output during battery discharge to the capacity input during charging (ηc is the capacity efficiency of the battery; Co is the capacity output during battery discharge, Ah; Ci is the capacity input during battery charging, Ah). The main factor affecting battery capacity efficiency is side reactions. When the battery is charged, part of the power is consumed in the decomposition of water. In addition, self-discharge and the shedding, agglomeration, and porosity shrinkage of electrode active materials also reduce capacity output.
Energy efficiency: Energy efficiency is also called electrical energy efficiency, which refers to the ratio of the energy output when the battery is discharged to the energy input when it is charged (ηE is the energy efficiency of the battery; Eo is the energy output when the battery is discharged, Wh; Ei is the energy input when the battery is charged, Wh.). The reason that affects energy efficiency is the internal resistance of the battery, which increases the battery charging voltage and decreases the discharge voltage. The energy loss of the internal resistance is lost in the form of battery heat.
Output efficiency: As an energy storage device, power batteries convert electrical energy into chemical energy when charging and store it, and release the electrical energy when discharging. In this reversible electrochemical conversion process, there is a certain amount of energy loss. It is usually expressed in terms of battery capacity efficiency and energy efficiency.
Self-discharge rate: The self-discharge rate refers to the rate of capacity decline of the battery during storage, that is, the speed at which the battery loses capacity due to self-discharge when there is no load. It indicates the characteristics of capacity change after the battery is shelved. The self-discharge rate is expressed as the percentage of capacity reduction per unit time. (ηΔc is the battery self-discharge rate; Ca is the capacity of the battery before storage, Ah; Cb is the capacity of the battery after storage, Ah; Tt is the storage time of the battery, usually in days or months.)
Discharge rate: The size of the battery discharge current is often expressed by “discharge rate”, that is, the battery discharge rate is expressed by the discharge time or the number of hours required to discharge the rated capacity with a certain discharge current. It can be seen that the shorter the discharge time, that is, the higher the discharge rate, the greater the discharge current. The discharge rate is equal to the ratio of the rated capacity to the discharge current. According to the size of the discharge rate, it can be divided into low rate (<0.5C), medium rate (0.5~3.5C), high rate (3.5~7.0C), and ultra-high rate (>7.0C). For example, if the rated capacity of a battery is 20Ah, if it is discharged with a current of 4A, it will take 5h to discharge the rated capacity of 20Ah, that is, it is discharged at a rate of 5, represented by the symbol C/5 or 0.2C, which is a low rate.
7. Battery Life
The service life refers to the effective life span of the battery under specified conditions. The battery’s service life ends when the battery has an internal short circuit or is damaged and cannot be used, or when the capacity does not meet the specification requirements. The battery’s service life includes the service life and the use cycle.
The service life is the time that the battery can be used, including the storage time of the battery.
The usage cycle refers to the number of times a battery can be reused, also known as the cycle life.
Conclusion
So, what have we learned? When evaluating or designing power batteries, it’s essential to consider a wide range of performance indicators. Capacity, energy density, power density, cycle life, charging speed, safety, cost-effectiveness, and environmental impact all play critical roles in determining a battery’s overall quality and suitability for specific applications.
As technology continues to advance, these KPIs will evolve, pushing the boundaries of what batteries can do. Whether you’re a consumer looking for the best electric vehicle or a manufacturer designing the next generation of energy storage systems, understanding these KPIs is key to making informed decisions.
The future of power batteries is bright—charged with innovation, sustainability, and endless possibilities.
Recommended reading: Main types of EV power batteries
popular questions
FAQ`s.
Q1: What is the most important performance indicator for power batteries?
A1:The importance of a performance indicator depends on the specific application. For example, energy density is critical for electric vehicles, while cycle life is essential for renewable energy storage systems.
Q2: How does fast charging impact battery life?
A2: Fast charging generates more heat, which can degrade the battery over time, potentially reducing its cycle life. However, many modern batteries include thermal management systems to mitigate this impact.
Q3: Are higher capacity batteries always better?
A3: Not necessarily. While higher capacity means more stored energy, it can also result in larger and heavier batteries, which might not be ideal for all applications.
Q4: What are solid-state batteries, and how do they differ from traditional batteries?
A4: Solid-state batteries use a solid electrolyte instead of a liquid one, which can lead to higher energy density and improved safety. They are still in the development stage but hold promise for the future of battery technology.
Q5: How can I ensure the batteries I use are environmentally friendly?
A5: Look for batteries from manufacturers that prioritize sustainable practices, such as using recycled materials and offering recycling programs for used batteries. Additionally, choosing batteries with longer cycle lives can reduce environmental impact over time.
Derek Ke
Hi, I’m Derek Ke, founder of Moreday.com, an expert in solar-protected electrical products and electric vehicle charging.
Over the past 15 years, we have helped nearly 500 customers (such as farms, residential, industrial, and commercial) in 60 countries solve new energy and green power problems. We aim to share more knowledge about solar power generation and new energy with everyone so that green electricity can enter thousands of households.
