Technical specifications of lithium-ion batteries

Lithium-ion batteries are widely used in various fields as an ideal carrier of electrical energy. When we use lithium-ion batteries, we will pay attention to some technical indicators, as the main factor to measure its performance “good or bad”. These main technical indicators are as follows:

1. Capacity

Battery capacity is an important performance indicator to measure the performance of the battery, which indicates the amount of power the battery can release under certain conditions. Capacity is usually measured in Ampere-hours (Ah), for example, if a battery is 48V 200ah, it means that the battery can store 48V*200ah=9.6KWh, i.e. 9.6 kWh of electricity.

Battery capacity is divided into actual capacity, theoretical capacity and rated capacity according to different conditions: theoretical capacity refers to the battery active material; participate in the battery reaction given by the amount of electricity, that is, the most ideal state capacity; rated capacity refers to the nameplate indicated that the motor or electrical appliances in the rated operating conditions can be a long time to continue to work the capacity; the actual capacity of the battery in a specific discharge conditions can provide the amount of electricity, which with the temperature, It is directly related to temperature, humidity, charging and discharging multiplication rate, etc., and is usually lower than the rated capacity.

2. Energy Density

Energy density refers to the energy that can be stored per unit mass or volume of the battery. Energy density has two forms: mass energy density (Wh/kg) and volume energy density (Wh/L).

The current lithium-ion battery technology has an energy density of about 100 to 200 Wh/kg, which restricts the wide application of lithium-ion batteries, especially in the field of electric vehicles, where a high energy density is the key to improving range.

3. Charge/discharge ratio

Charge/discharge ratio is a measure of the speed of charging and discharging. This indicator affects the continuous and peak currents of lithium-ion batteries, and its unit is usually C.

For example, if the rated capacity of a battery is 20Ah, and its rated charge/discharge multiplier is 0.5C, it means that the battery can be charged and discharged repeatedly with a current of 20Ah*0.5C=10A, until the cut-off voltage of charging or discharging. If its maximum discharge multiplier is 10C@10s and its maximum charge multiplier is 5C@10s, then this battery can be discharged with a current of 200A for a duration of 10 seconds and charged with a current of 100A for a duration of 10 seconds.

The more detailed the definition of the charging and discharging multiplier index, the greater the significance of guidance for use. Especially for lithium-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 lithium-ion batteries are used within a reasonable range.

4.Voltage

Lithium-ion battery voltage, open circuit voltage, operating voltage, charging cut-off voltage, discharge cut-off voltage and other parameters, the unit is V. Open circuit voltage is the battery without any external load or power supply, measuring the potential difference between the positive and negative poles of the battery, which 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 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, calendar life and depth of discharge

Depth of discharge is the percentage of the battery discharge and the rated capacity of the battery, the battery standard 80% DOD means that 80% of the power discharge.

Lithium-ion battery life will gradually decline with the use and storage, and there will be more obvious performance. Still take smart phones as an example, after using the phone for a period of time, you can obviously feel the cell phone battery “not durable”, at the beginning may only charge once a day, and later may need to charge twice a day, which is the embodiment of the battery life continues to decline.

The life of a lithium-ion battery is divided into two parameters: cycle life and calendar life. Cycle life is generally measured in Times, which characterizes the number of times a battery can be charged and discharged cyclically. Generally under ideal temperature and humidity, deep charge and discharge (80% DOD) with rated charge and discharge current, and calculate the number of cycles experienced when the battery capacity decreases to 20% of the rated capacity.

Calendar life is the time span for the battery to reach the end-of-life condition (e.g., 20% capacity decay) after specific operating conditions under the use of environmental conditions. Calendar life is closely related to specific usage requirements, which usually require specific usage conditions, environmental conditions, storage intervals, and so on. Calendar life has more practical significance than cycle life, but because the calculation of calendar life is very complicated and time-consuming, battery manufacturers usually only give cycle life data.

6. Internal Resistance

The internal resistance of lithium-ion battery refers to the resistance of the current flowing through the battery when the battery is in operation, the unit is Ω, which includes ohmic internal resistance and polarization internal resistance. Ohmic internal resistance is composed 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.

Batteries with high internal resistance have high internal power consumption and serious heat generation during charging and discharging, which will cause accelerated aging and life decay of lithium-ion batteries, and also 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, the 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 ° C ~ 60 ° C), if beyond the reasonable range of use, the performance of lithium-ion batteries will have a greater impact.

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, efforts should be made to create a suitable operating temperature range to maximize the performance of lithium-ion batteries.

In addition to operating temperature restrictions, the storage temperature of lithium-ion batteries is also subject to strict constraints, long-term storage at high or low temperatures will cause irreversible effects on battery performance.