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Capacity (Capacity)

The amount of electricity that a battery can deliver under certain discharge conditions is called the battery's capacity, denoted by the symbol C. The commonly used units are ampere-hours, abbreviated as Ah (ampere-hour) or mAh (milliampere-hour).

The capacity of a battery can be divided into theoretical capacity, rated capacity, and actual capacity.

Theoretical capacity is the highest theoretical value calculated according to Faraday's law based on the mass of the active material. To compare different types of batteries, the concept of specific capacity is often used, that is, the theoretical amount of electricity that a battery of unit volume or unit mass can deliver, with units of Ah/kg (mAh/g) or Ah/L (mAh/cm³).

Actual capacity refers to the amount of electricity that a battery can output under certain conditions. It is equal to the product of the discharge current and the discharge time, with the unit of Ah, and its value is less than the theoretical capacity.

Rated capacity, also known as guaranteed capacity, is the minimum capacity that a battery should deliver under certain discharge conditions according to the standards promulgated by the state or relevant departments.

Internal Resistance (Impedance)

Definition: When current passes through the interior of a battery, it encounters resistance, which causes the battery's voltage to drop. This resistance is called the battery's internal resistance.

The internal resistance of a battery is not a constant; it changes continuously with time during the discharge process because the composition of the active material, the concentration of the electrolyte, and the temperature are all constantly changing.

The internal resistance of a battery includes ohmic internal resistance and polarization internal resistance, and the polarization internal resistance further includes electrochemical polarization and concentration polarization. Due to the existence of internal resistance, the terminal voltage of a battery during discharge is lower than the electromotive force and the open-circuit voltage of the battery, and during charging, the terminal voltage is higher than the electromotive force and the open-circuit voltage.

Ohmic resistance obeys Ohm's law; polarization resistance increases with the increase of current density, but not in a linear relationship. It usually increases linearly with the logarithm of the current density.

Open Circuit Voltage (Open circuit voltage, OCV)

When a battery is not discharging, the potential difference between the two poles of the battery is called the open-circuit voltage.

The open-circuit voltage of a battery varies depending on the materials of the positive and negative electrodes and the electrolyte. If the materials of the positive and negative electrodes of a battery are exactly the same, then regardless of the size of the battery or how its geometric structure changes, its open-circuit voltage will be the same.

Cut-off Discharge Voltage

Refers to the lowest working voltage value at which a battery should not continue to discharge when its voltage drops during discharge. Depending on different battery types and different discharge conditions, the requirements for battery capacity and life are also different, so the specified cut-off discharge voltage for batteries is also different.

Depth of Discharge (Depth of discharge DOD)

During the use of a battery, the percentage of the capacity discharged by the battery to its rated capacity is called the depth of discharge.

The depth of discharge is closely related to the charging life of secondary batteries. The deeper the depth of discharge of a secondary battery, the shorter its charging life. Therefore, deep discharge should be avoided as much as possible during use.

Over Discharge

If a battery continues to discharge beyond the cut-off discharge voltage value during the discharge process, it may cause an increase in the internal pressure of the battery, damage the reversibility of the active materials of the positive and negative electrodes, and significantly reduce the capacity of the battery.

Energy Density (Energy density)

The electrical energy released per average unit volume or mass of a battery.

Generally, under the same volume, the energy density of a lithium-ion battery is 2.5 times that of a nickel-cadmium battery and 1.8 times that of a nickel-metal hydride battery. Therefore, when the battery capacities are equal, the lithium-ion battery will be smaller in volume and lighter in weight than nickel-cadmium and nickel-metal hydride batteries.

Self Discharge

Regardless of whether a battery is in use or not, for various reasons, a phenomenon of electricity loss will occur. After a battery is fully charged, it is left for one month. Then it is discharged at 1C to 3.0V, and its capacity is recorded as C2; the initial capacity of the battery is recorded as C0; 1 - C2/C0 is the monthly self-discharge rate of the battery.

Charging Cycle Life (Cycle life)

A battery is fully charged and then fully discharged, and this process is repeated until the capacity decays to 75% of the initial capacity. The number of cycles at this time is the cycle life of the battery.

The cycle life is related to the charging and discharging conditions of the battery. The 1C charging and discharging cycle life of a lithium-ion battery at room temperature can reach 300 - 500 times (industry standard), and can be up to 800 - 1000 times.

Memory Effect (Memory effect)

The memory effect is specific to nickel-cadmium batteries. Due to the sintered negative electrode in the traditional process and the coarse cadmium grains, if a nickel-cadmium battery is recharged before it is completely discharged, the cadmium grains tend to aggregate into blocks, forming a secondary discharge platform when the battery discharges. The battery will store this discharge platform and use it as the end point of discharge in the next cycle, even though the battery's own capacity allows it to discharge to a lower platform. In subsequent discharge processes, the battery will only remember this low capacity. Similarly, in each use, any incomplete discharge will deepen this effect and make the battery's capacity even lower.

There are two ways to eliminate this effect: one is to use a small current for deep discharge (such as discharging to 0V with 0.1C), and the other is to use a large current for charging and discharging (such as 1C) several times.

Neither nickel-metal hydride batteries nor lithium-ion batteries have a memory effect.

V. Basic Knowledge of Lithium Batteries - Principles of Lithium-Ion Batteries

VI. Lithium-Ion Secondary Batteries - Battery Structure and Composition

Lithium-Ion Secondary Batteries - Battery Structure and Composition

Positive Electrode Materials (Cathode)

The commonly used positive electrode materials for lithium-ion batteries are as follows:

Lithium Cobalt Oxide LiCoO2, 3.0V~4.2V/4.35V/4.4V

Lithium Manganese Oxide LiMn2O4, 3.0V~4.2V

Ternary Materials (Lithium Nickel Cobalt Manganese Oxide) LiNixCoyMn1-x-yO2, 3.0V~4.2V

Lithium Iron Phosphate LiFePO4, 2.0V~3.65V

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