PACK lithium battery basic parameters introduction!

??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 the performance of the battery, it indicates that under certain conditions (discharge rate, temperature, termination voltage, etc.) the amount of electricity discharged from the battery (can be used JS-150D discharge test), that is, the capacity of the battery, usually in amperage - hour unit (abbreviation, expressed in A-H, 1A-h = 3600C). For example, if a battery is 48V 200ah, then it means that the battery can store 48V*200ah=9.6KWh, i.e. 9.6 degrees of electricity. Battery capacity is divided into actual capacity, theoretical capacity and rated capacity according to different conditions.

Actual capacity refers to the amount of electricity a battery can give under a certain discharge regime (a certain sedimentation level, a certain current density and termination voltage). The actual capacity is generally not equal to the rated capacity, which is directly related to the temperature, humidity, charging and discharging multiplier. Generally, the actual capacity is a little smaller than the rated capacity, sometimes even much smaller than the rated capacity;

Theoretical capacity refers to the amount of electricity given by all active substances participating in the battery reaction. That is, the capacity under the most ideal state;

Rated capacity refers to the capacity of a motor or appliance indicated on the nameplate that can continue to work for a long time under rated operating conditions. It usually refers to apparent power for transformers, active power for motors, and apparent or reactive power for phase-regulating equipment, and is expressed in VA, kVA, and MVA;

In application, the geometry of the pole plate, termination voltage, temperature, and discharge rate all have an effect on the battery capacity, for example, in the winter in the north, if a cell phone is used outdoors, the battery capacity will drop rapidly.

??2. Energy Density (Energy Density, unit: Wh/kg or Wh/L)

Energy density, battery energy density, for a given electrochemical energy storage device, the energy that can be charged with the mass or volume of the energy storage medium ratio. The former is called "mass energy density", the latter is called "volumetric energy density", the unit is watt-hours / kg Wh / kg, watt-hours / liters Wh / L. Here, the power, is the above mentioned capacity (Ah) and the operating voltage (V) of the integral. When it comes to applications, the metric of energy density is more instructive than capacity.

Based on the current lithium-ion battery technology, the energy density level can be achieved at about 100~200Wh/kg, which is still relatively low and has become a bottleneck for lithium-ion battery applications in many occasions. This problem also occurs in the field of electric vehicles, in the volume and weight are subject to strict limitations, the energy density of the battery determines the maximum driving range of electric vehicles, so the "mileage anxiety" this unique term. If the single driving range of electric vehicles is to reach 500 kilometers (comparable to traditional fuel vehicles), the energy density of the battery monomer must reach more than 300Wh/kg.

The improvement of lithium-ion battery energy density is a slow process, much lower than the integrated circuit industry's Moore's Law, which results in the performance of electronic products and battery energy density improvement between the existence of a scissor gap, and with the time continue to expand.

??3. Charge/Discharge rate (unit: C)

Charge/Discharge rate is a measure of the speed of charging. This indicator affects the continuous current and peak current of the lithium-ion battery when working, and its unit is generally C (C-rate abbreviation), such as 1/10C, 1/5C, 1C, 5C, 10C, etc.. For example, if the rated capacity of a battery is 20Ah, and if its rated charge/discharge multiplier is 0.5C, it means that this battery, can be charged and discharged repeatedly with a current of 20Ah*0.5C=10A, up to the cut-off voltage of charging or discharging. If its maximum discharge multiplier is 10C@10s and maximum charge multiplier is 5C@10s, then the battery can be discharged with a current of 200A for 10 seconds, and charged with a current of 100A for 10 seconds. The more detailed the definition of the charging and discharging multiplier indexes is, the more significant it is for the guidance of the use. Especially for Li-ion batteries, which are used as the power source of electric transportation vehicles, it is necessary to define the continuous and pulse multiplication indexes under different temperature conditions to ensure that the Li-ion batteries are used within a reasonable range.

??4. Voltage (V)

The voltage of lithium-ion battery has some parameters such as open-circuit voltage, operating voltage, charging cut-off voltage, discharging cut-off voltage and so on.

Open-circuit voltage is not connected to any load or power supply outside the battery, measure the potential difference between the positive and negative terminals of the battery, this is the open-circuit voltage of the battery.

Working voltage is the battery external load or power supply, in the working state, there is a current flow, measured by the potential difference between the positive and negative electrodes. Working voltage is related to the composition of the circuit and the operating state of the equipment, is the value of change. Generally speaking, due to the existence of the internal resistance of the battery, the operating voltage is lower than the open-circuit voltage in the discharged state and higher than the open-circuit voltage in the charging state.

The charge/discharge cut-off voltage is the maximum and minimum operating voltage that the battery is allowed to reach. Exceeding this limit value will cause some irreversible damage to the battery, leading to the degradation of battery performance, and in serious cases, even causing fire, explosion and other safety accidents.

??5. Cycle Life (unit: times) and Depth of discharge (DoD)

Depth of discharge (DoD) is the percentage of the battery's discharge to the battery's rated capacity. The depth of discharge of a shallow cycle battery should not exceed 25%, while a deep cycle battery can discharge 80% of its capacity. The battery starts discharging at the upper limit voltage and terminates discharging at the lower limit voltage. Define all discharged charge as 100%. Battery standard 80% DOD means to discharge 80% of the charge. For example, if the initial SOC is 100% and I put it at 20% and stop, that's 80% DOD.

The life of a lithium-ion battery will gradually decay with use and storage, and it will be more obvious. Still take smart phones as an example, after using the phone for a period of time, you can obviously feel the phone battery "not durable", the beginning may only charge once a day, the back may need to charge twice a day, which is the embodiment of the continuous decline in battery life.

Lithium-ion battery life is divided into two parameters: cycle life and calendar life. Cycle life is generally measured in cycles, which characterizes the number of times a battery can be charged and discharged. Of course, there are conditions here, generally in the ideal temperature and humidity, with the rated charge and discharge current for the depth of charge and discharge (80% DOD), calculate the number of cycles experienced when the battery capacity declines to 20% of the rated capacity.

The definition of calendar life is a bit more complicated, the battery can't always be charging and discharging, there are storage and shelving, and can't always be in the ideal environmental conditions, it will go through all kinds of temperature and humidity conditions, and the charging and discharging multiplier 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 the end-of-life condition (e.g., the capacity decreases to 20%) after a specific use condition under the use environment. Calendar life is closely related to specific usage requirements, which usually require specific usage conditions, environmental conditions, storage intervals, and so on.

Calendar life is more meaningful than cycle life, but because calendar life is so complex and time consuming to measure, battery manufacturers generally only provide cycle life data. If you need to get the calendar life data, you usually have to pay extra and wait for a long time.

??6. Internal resistance (unit: Ω)

The internal resistance of lithium-ion battery refers to the resistance of the current flowing through the battery when the battery is working, which includes ohmic internal resistance and polarization internal resistance, and polarization internal resistance includes electrochemical polarization internal resistance and concentration polarization internal resistance.

Ohmic internal resistance consists of electrode material, electrolyte, diaphragm resistance and contact resistance of each part. Polarization internal resistance refers to the resistance caused by polarization during electrochemical reaction, including the resistance caused by electrochemical polarization and concentration polarization.

The unit of internal resistance is generally milliohm (mΩ). Batteries with large internal resistance have high internal power consumption and serious heat generation during charging and discharging, which will cause accelerated aging and life span degradation of lithium-ion batteries, and at the same time limit the application of charging and discharging with large multiplication rate. Therefore, the smaller the internal resistance is made, the better the life and multiplication performance of the lithium-ion battery will be.

??7. Self-discharge

Self-discharge is the phenomenon that a battery will lose power if it is left unused. When the battery is placed, its capacity is decreasing, the rate of capacity decline is called the self-discharge rate, usually expressed as a percentage: % / month.

Self-discharge is what we do not want to see, a fully charged battery, put a few months, the power will be much less, so we hope that the lithium-ion battery self-discharge rate the lower the better.

Here we need to pay special attention to, once the self-discharge of lithium-ion batteries lead to battery over-discharge, the impact is usually irreversible, even if re-charging, the battery's usable capacity will have a great loss, life will be a rapid decline. So long-term placement of unused lithium-ion batteries, the battery must remember to charge regularly to avoid over-discharge due to self-discharge, performance is greatly affected.

??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 between -20 ℃ ~ 60 ℃), if used beyond the reasonable range, it will have a greater impact on the performance of lithium-ion batteries.

Lithium-ion batteries of different materials, the operating temperature range is also different, some have good high temperature performance, and some can adapt to low temperature conditions. The operating voltage, capacity, charge/discharge ratio and other parameters of lithium-ion batteries will change significantly with temperature. Prolonged use at high or low temperatures will also accelerate the decay of the life of lithium-ion batteries. Therefore, it is important to maximize the performance of lithium-ion batteries by striving to create a suitable operating temperature range.

In addition to operating temperature restrictions, lithium-ion batteries are also subject to strict storage temperatures. Long-term storage at high or low temperatures can have an irreversible effect on battery performance.