Common terminology of Lithium-ion battery

Capacity

The capacity of the battery is under certain discharge conditions, which is represented by the symbol C. The common unit is ampere hour, abbreviated as Ah or mAh. The capacity of battery can be divided into theoretical capacity, rated capacity and actual capacity. The theoretical capacity is the highest theoretical value obtained by calculating the mass of active substance according to Faraday’s law. In order to compare different series of batteries, the concept of specific capacity is commonly used, that is, the theoretical capacity per unit volume or unit mass of battery, like Ah/kg (mAh/g) or Ah/L (mAh/cm3).

The actual capacity refers to the power output of the battery under certain conditions. It is equal to the discharge current multiplying by discharge time, and its value is less than the theoretical capacity.

The rated capacity is also called guaranteed capacity, which is the minimum capacity that the battery should discharge under certain discharge conditions according to the standards issued by the state or relevant departments.

Internal resistance

The resistance is called the internal resistance of the battery. The internal resistance of the battery is not a constant, but changes with time during the discharge process, because the composition of active substances, electrolyte concentration and temperature are constantly changing. The internal resistance of battery includes OHMIC resistance and polarization resistance. The polarization internal resistance includes electrochemical polarization and concentration polarization. The existence of internal resistance makes the terminal voltage of the battery is lower than the EMF and open circuit voltage when discharging, while the terminal voltage is higher than the EMF and open circuit voltage when charging. The OHMIC resistance obeys Ohm’s law; the polarization resistance increases linearly with the increase of the logarithm of the current density, but not linearly.

Load capacity

When the positive and negative terminals of the battery are connected to the load, the output power to drive the load is the load capacity of the battery.

Internal pressure

The internal pressure refers to the internal air pressure of the battery, which is caused by the gas generated during the charging and discharging process of the sealed battery. It is mainly affected by the battery material, manufacturing process, battery structure and other factors. The main reason is the gas generated and accumulate by the decomposition of the water in the battery and organic solution in the battery.

C-rate

C is the first letter of capacity, which is used to indicate the current value when the battery is charged and discharged. For example, when the rated capacity of a rechargeable battery is 1000mah, it means that the discharge time lasts for 1 hour at 1000mA (1C); the discharge time lasts for 5 hours at 200mA (0.2C). The charging can also be calculated according to this.

Cut-off discharge voltage

The cut-off discharge voltage refers to the lowest working voltage that the battery is not suitable for further discharge. Basing on different battery types and different discharge conditions, the requirements of battery capacity and life are also different, so the specified battery cut-off discharge voltage is not the same.

Open circuit voltage (OCV)

When the battery does not discharge, the potential difference between the two electrodes of the battery is called the open circuit voltage. The open circuit voltage of the battery will vary according to the materials of the positive and negative electrodes and the electrolyte of the battery. If the materials of the positive and negative electrodes are identical, the open circuit voltage of the battery will be the same no matter how large the volume of the battery is and how the geometric structure changes.

Working voltage

Working voltage refers to the voltage displayed in the discharge process after the battery is connected to the load, which is also known as discharge voltage. The battery working voltage at the beginning of discharge is called initial voltage. After switching on the load, the working voltage of the battery is lower than the open circuit voltage due to the existence of OHMIC resistance and polarization over potential.

Depth of discharge (DoD)

In the process of using the battery, the percentage of the capacity released by the battery to its rated capacity is called the discharge depth. The discharge depth has a deep relationship with the charging life of the secondary battery. The deeper the discharge depth is, the shorter the charging life is. Therefore, deep discharge should be avoided as far as possible in use.

Over discharge In the process of discharge, if the battery exceeds the cut-off discharge voltage and continues to discharge, the internal pressure of the battery will increase, the reversibility of positive and negative active substances will be damaged, and the capacity of the battery will be significantly reduced.

Over charge

When the battery is charged, if the battery is still charged after fully charged, it may lead to the increase of the internal pressure, the deformation and leakage of the battery, and the performance of the battery will be significantly reduced and damaged.

Energy density

The energy released by the average unit volume or mass of a battery. Generally, under the same volume, the energy density of lithium-ion battery is 2.5 times of that of nickel cadmium battery and 1.8 times of that of nickel hydrogen battery. Therefore, under the condition of equal battery capacity, lithium-ion battery will have smaller volume and lighter weight than nickel cadmium and nickel hydrogen battery.

Self-discharge

Whether in use or not, the battery power loss occurs due to various reasons. The monthly self-discharge rate of industry standard of lithium-ion battery is less than 12%. The self-discharge rate is related to the placement performance of the battery, and is also related to the internal resistance structure and material performance of the battery

Cycle life

The battery is fully discharged after it is fully charged, and the cycle is carried out until the capacity decreases to 75% of the initial capacity. At this time, the number of cycles is the cycle life of the battery. The cycle life is related to the charging and discharging conditions of the battery.

Discharge platform

After the lithium-ion battery is fully charged, discharge to 3.6V and the capacity is recorded as C1. And then discharge to 3.0V and the capacity is marked as C0. C1/C0 is called the discharge platform of the battery. Industry standard of 1C discharge platform is more than 70%.

Discharge rate

The discharge current of the battery is usually expressed by “discharge rate”, that is, the discharge rate of the battery is expressed by the discharge time or the number of hours required for fully discharge at a certain discharge current. Therefore, the shorter the discharge time, the higher the discharge rate, and the greater the discharge current. (Discharge rate = rated capacity / discharge current) According to the discharge rate, it can be divided into low rate (< 0.5C), medium rate (0.5-3.5C), high rate (3.5-7.0C), ultra-high rate (> 7.0C) For example, if the rated capacity of a battery is 20Ah, and 4A current is used to discharge, it will take 5h to fully discharge, that is to say, discharge at the rate of 5 times, which is indicated by the symbol C/5 or 0.2C, which is low rate.

Memory effect Memory effect is related to Ni Cd battery. Because the negative electrode in traditional process is sintered and the cadmium grain is big, if the nickel cadmium battery is recharged before they are completely discharged, the cadmium grains are easy to gather into blocks and form a secondary discharge platform when the battery is discharged.

The battery will store this discharge platform and use it as the end point of discharge in the next cycle, although the capacity of the battery itself can make the battery discharge to a lower platform. The battery will only remember this low capacity in the subsequent discharge process. Similarly, in each use, any incomplete discharge will deepen this effect and make the battery capacity lower.         There are two ways to eliminate this effect, one is to use low current deep discharge (such as 0.1C to 0V) and the other is to charge and discharge with high current (such as 1C) several times.

Ni MH battery and Li ion battery have no memory effect.