1. Capacity (Unit: Ah)
This is a parameter that everyone is more concerned about. Battery capacity is one of the important performance indicators to measure battery performance. It indicates the amount of electricity released by the battery under certain conditions (discharge rate, temperature, termination voltage, etc.) (JS-150D can be used for discharge testing), that is, the capacity of the battery. It is usually measured in Ampere hours (abbreviated as A·H, 1A·h=3600C). For example, if a battery is 48V 200ah, it means that the battery can store 48V*200ah=9.6KWh. That is 9.6 kWh.
Battery capacity is divided into actual capacity, theoretical capacity and rated capacity according to different conditions.
a. Actual capacity
It refers to the amount of electricity that the battery can provide under a certain discharge regime (certain sinking depth, certain current density and termination voltage). The actual capacity is generally not equal to the rated capacity. It is directly related to temperature, humidity, charge and discharge rate, etc. In general, the actual capacity is smaller than the rated capacity. Sometimes it is even much smaller than the rated capacity.
b. Theoretical capacity
It refers to the amount of electricity given by all active materials participating in the battery reaction. That is the capacity under the most ideal condition.
c. Rated capacity
It refers to the capacity of the motor or electrical appliance indicated on the nameplate that can continue to work for a long time under rated working conditions. Usually it refers to the apparent power of the transformer, the active power of the motor, and the apparent power or reactive power of the phase modulation equipment. The units are VA, kVA, MVA.
In battery applications, the plate geometry, termination voltage, temperature, discharge rate, etc. will all have an impact on battery capacity. For example, in the winter in the north, if you use your mobile phone outdoors, the battery capacity will decrease rapidly.
2. Energy density (unit: Wh/kg or Wh/L)
Battery energy density is the ratio of the energy that can be charged to a given electrochemical energy storage device to the mass or volume of the energy storage medium. The former is called “mass energy density” and the latter is called “volume energy density”. The units are watt·hour/kg Wh/kg and watt·hour/liter Wh/L respectively.
The power here is the integral of the capacity (Ah) and operating voltage (V) mentioned above. In application, the indicator of energy density is more instructive than capacity.
Based on current lithium-ion battery technology, the energy density level that can be achieved is approximately 100~200Wh/kg. This value is still relatively low and has become a bottleneck for lithium-ion battery applications in many situations.
This problem also occurs in the field of electric vehicles. With both volume and weight strictly limited, the energy density of the battery determines the maximum single driving range of an electric vehicle. So the unique term “mileage anxiety” emerged. If the single driving range of electric vehicles is to reach 500 kilometers (similar to traditional fuel vehicles), the energy density of the battery cells must reach more than 300Wh/kg.
The improvement of energy density of lithium-ion batteries is a slow process, far lower than Moore’s Law in the integrated circuit industry. This creates a scissor gap between the performance improvement of electronic products and the energy density improvement of batteries, which continues to expand over time.
3. Charge and discharge rate (unit: C)
The charge-discharge rate is a measure of charging speed. This indicator will affect the continuous current and peak current of the lithium-ion battery when it is working. Its unit is generally C (abbreviation for C-rate), such as 1/10C, 1/5C, 1C, 5C, 10C, etc. For example, the rated capacity of a battery is 20Ah, and if its rated charge and discharge rate is 0.5C, it means that this battery can be repeatedly charged and discharged with a current of 20Ah*0.5C=10A, up to the cut-off voltage of charging or discharging. .
If its maximum discharge rate is 10C@10s and its maximum charge rate is 5C@10s, then the battery can be discharged at a current of 200A for 10 seconds and charged at a current of 100A for 10 seconds.
The more detailed the charge and discharge rate index is defined, the greater the guidance significance for use. In particular, lithium-ion batteries serve as the power source for electric vehicles. Continuous and pulse rate indicators under different temperature conditions need to be specified to ensure that lithium-ion batteries are used within a reasonable range.
4. Voltage (unit: V)
Here we introduce the voltage of lithium-ion batteries, including open circuit voltage, operating voltage, charging cut-off voltage, discharge cut-off voltage and other parameters.
a. Open circuit voltage
The open circuit voltage is the potential difference between the positive and negative electrodes of the battery without any external load or power supply connected to the battery. This is the open circuit voltage of the battery.
b. Working voltage
The working voltage is the measured potential difference between the positive and negative electrodes when the battery is connected to an external load or power supply. When the battery is in working condition and current flows through it. The working voltage is related to the circuit composition and the working status of the equipment, and is a changing value.
Generally speaking, due to the existence of the internal resistance of the battery, the operating voltage in the discharge state is lower than the open circuit voltage, and the operating voltage in the charging state is higher than the open circuit voltage.
c. Charge/discharge cut-off voltage
The charge/discharge cut-off voltage refers to the highest and lowest operating voltage the battery is allowed to reach. Exceeding this limit will cause some irreversible damage to the battery, leading to a reduction in battery performance, and in severe cases, even causing fires, explosions and other safety accidents.
5. Cycle Life (Unit: times) and Depth of discharge (DoD)
a. Discharge depth
Depth of discharge refers to the percentage of battery discharge to the battery’s rated capacity. The depth of discharge of shallow cycle batteries should not exceed 25%. Deep cycle batteries can discharge 80% of the power. The battery starts discharging at the upper limit voltage and ends at the lower limit voltage.
Define all discharged power as 100%. The battery standard 80%DOD means that 80% of the power is discharged. For example, the initial SOC is 100%, and it stops when it reaches 20%. This is 80% DOD.
The life of lithium-ion batteries will gradually decrease with use and storage, and there will be more obvious performance. Still taking a smartphone as an example, after using a mobile phone for a period of time, you can clearly feel that the battery of the mobile phone is “not durable”. At the beginning, it may only be charged once a day, but later it may need to be charged twice a day. This means that the battery life is constantly declining. manifestation.
The life of lithium-ion batteries is divided into two parameters: cycle life and calendar life.
b. Cycle life
Cycle life is generally expressed in units of times, indicating the number of times the battery can be charged and discharged.
Of course, there are conditions here. Generally, under ideal temperature and humidity, deep charge and discharge (80% DOD) are performed at the rated charge and discharge current, and the cycle experienced when the battery capacity decays to 20% of the rated capacity is calculated. frequency.
c. Calendar life
The definition of calendar life is more complicated. The battery cannot be charged and discharged all the time, and must be stored and shelved. It is also impossible to be in ideal environmental conditions all the time and will experience various temperature and humidity conditions. The rate of charge and discharge is also changing all the time. So the actual service life needs to be simulated and tested. Simply put, the calendar life is the time span for the battery to reach end-of-life conditions (such as capacity decay to 20%) under specific usage conditions under usage environment conditions.
Calendar life is closely combined with specific usage requirements. Specific usage conditions, environmental conditions, storage intervals, etc. usually need to be stipulated.
Calendar life is more practical than cycle life, but the calculation of calendar life is very complicated and takes too long. Therefore, generally battery manufacturers only provide cycle life data. If you need calendar life data, you usually have to pay extra and have to wait a long time.
6. Internal resistance (unit: Ω)
The internal resistance of a lithium-ion battery refers to the resistance to current flowing through the interior of the battery when the battery is working. It includes ohmic internal resistance and polarization internal resistance. Polarization internal resistance also includes electrochemical polarization internal resistance and concentration difference electrode. Reduce internal resistance.
Ohmic internal resistance consists of electrode materials, electrolytes, diaphragm resistance and contact resistance of various parts.
Polarization internal resistance refers to the resistance caused by polarization during electrochemical reactions, including resistance caused by electrochemical pole polarization and concentration polarization.
The unit of internal resistance is generally milliohms (mΩ). Batteries with large internal resistance consume large internal power and generate serious heat during charging and discharging, which will cause accelerated aging and lifespan reduction of lithium-ion batteries. It will also limit high rate charging and discharging applications. Therefore, the smaller the internal resistance is, the better the life and rate performance of the lithium-ion battery will be.
7. Self-discharge
Self-discharge is a phenomenon in which a battery will lose power if it is left idle for a long time. When the battery is placed, its capacity is constantly decreasing. The rate of capacity decrease is called the self-discharge rate, which is usually expressed as a percentage: %/month.
Self-discharge is something we don’t want to see. A fully charged battery will have much less power after being stored for a few months. So we hope that the self-discharge rate of lithium-ion batteries is as low as possible.
Special attention needs to be paid here. Once the self-discharge of a lithium-ion battery causes the battery to over-discharge, the impact is usually irreversible. Even if it is recharged, the battery’s usable capacity will be greatly lost and its lifespan will decline rapidly.
Therefore, if a lithium-ion battery is left unused for a long time, the battery must be charged regularly to avoid over-discharge due to self-discharge, which will greatly affect its performance.
8. Operating temperature range
Due to the characteristics of the internal chemical materials of lithium-ion batteries, lithium-ion batteries have a reasonable operating temperature range (common data is between -20℃~60℃). If used beyond a reasonable range, it will have a greater impact on the performance of lithium-ion batteries.
Lithium-ion batteries made of different materials have different operating temperature ranges. Some have good high-temperature performance, while others can adapt to low-temperature conditions.
The working voltage, capacity, charge and discharge rate and other parameters of lithium-ion batteries will change very significantly with changes in temperature. Long-term use at high or low temperatures will also accelerate the lifespan of lithium-ion batteries. Therefore, efforts to create a suitable operating temperature range can maximize the performance of lithium-ion batteries.
In addition to the operating temperature limit, the storage temperature of lithium-ion batteries is also strictly restricted. Long-term high or low temperature storage will have an irreversible impact on battery performance.
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