Composition of lithium battery
The lithium battery is mainly composed of two parts, the battery cell and the protection board PCM (the power battery is generally called the battery management system BMS). The battery cell is equivalent to the heart of the lithium battery, and the management system is equivalent to the brain of the lithium battery.
The battery cell is mainly composed of positive electrode material, negative electrode material, electrolyte, diaphragm and shell, while the protection board is mainly composed of protection chip (or management chip), MOS tube, resistor, capacitor and PCB board.
Advantages and disadvantages of lithium batteries
Lithium batteries have many advantages, such as high voltage platform, high energy density (light weight, small size), long service life, and environmental protection.
The disadvantages of lithium batteries are that the price is relatively high, the temperature range is relatively narrow, and there are certain safety hazards (need to add a protection system).
Lithium battery classification
Lithium batteries can be divided into two broad categories: primary non-rechargeable batteries and secondary rechargeable batteries (also known as batteries).
Non-rechargeable batteries such as lithium manganese dioxide batteries, lithium-thionamide batteries.
Secondary rechargeable batteries can be divided into the following categories according to different situations.
1. According to appearance: square lithium battery (such as ordinary mobile phone battery) and cylindrical lithium battery (such as 18650 of electric tools);
2. According to outsourcing materials: aluminum shell lithium battery, steel shell lithium battery, soft pack battery;
3. According to the positive electrode material: lithium cobalt oxide (LiCoO 2 ), lithium manganese oxide (LiMn 2 O 4 ), ternary lithium (LiNi x Co y Mn z O 2 ), lithium iron phosphate (LiFePO 4 );
4. According to the state of electrolyte: lithium ion battery (LIB) and polymer battery (PLB);
5. Divided by use: ordinary batteries and power batteries.
6. According to performance characteristics: high-capacity batteries, high-rate batteries, high-temperature batteries, low-temperature batteries, etc.
Explanation of common terms
Refers to the amount of electricity that can be obtained from lithium batteries under certain discharge conditions.
We learned physics in high school, the formula of electricity is Q=I*t, the unit is coulomb, and the capacity unit of the battery is specified as Ah (Ampere hour) or mAh (milliampere hour). It means that a 1AH battery can be discharged for 1 hour when it is fully charged and discharged with a current of 1A.
The battery of old NOKIA mobile phones (such as BL-5C) is generally 500mAh, and the current smart phone battery is 800-1900mAh, electric bicycles are generally 10-20Ah, electric cars are generally 20-200Ah, etc.
2. Charge and discharge rate (Charge—Rate/Discharge—Rate)
Indicates how much current to charge and discharge, generally calculated as a multiple of the nominal capacity of the battery, generally referred to as several C.
For a battery with a capacity of 1500mAh, it is stipulated that 1C=1500mAh. If it is discharged at 2C, it will be discharged at a current of 3000mA. If it is charged at 0.1C, it will be charged and discharged at a current of 150mA.
3. Voltage (OCV: Open Circuit Voltage)
The voltage of the battery generally refers to the nominal voltage (also called the rated voltage) of the lithium battery. The nominal voltage of ordinary lithium battery is generally 3.7V, we also call its voltage platform as 3.7V. The voltage we are talking about generally refers to the open circuit voltage of the battery.
When the capacity of the battery is 20~80%, the voltage is concentrated at about 3.7V (about 3.6~3.9V), if the capacity is too high or too low, the voltage changes greatly.
The battery is discharged according to a certain standard, and the energy (E) that the battery can release is in Wh (watt-hour) or KWh (kilowatt-hour), and 1KWh=1 kilowatt-hour.
There are basic concepts in physics books, E=U*I*t, which is also equal to the battery voltage multiplied by the battery capacity.
The formula for power is, P=U*I=E/t, which represents the energy that can be released per unit time. The unit is W (watt) or KW (kilowatt).
Like a battery with a capacity of 1500mAh, the nominal voltage is generally 3.7V, so the corresponding energy is 5.55Wh.
5. Internal resistance (Resistance)
Since charging and discharging cannot be equivalent to an ideal power supply, there is a certain internal resistance. Internal resistance consumes energy, of course, the smaller the internal resistance, the better.
The unit of battery internal resistance is milliohm (mΩ).
Generally, the internal resistance of a battery is composed of ohmic internal resistance and polarization internal resistance. The internal resistance is affected by the material, manufacturing process, and structure of the battery.
6. Cycle life (Cycle Life)
A battery charge and discharge once is called a cycle, and the cycle life is an important indicator to measure the battery life performance.
The IEC standard stipulates that the lithium battery of a mobile phone should be discharged at 0.2C to 3.0V, charged at 1C to 4.2 V, and the battery capacity should be kept at more than 60% of the initial capacity after repeated cycles of 500 times. That is to say, the cycle life of the lithium battery is 500 times .
The national standard stipulates that after 300 cycles of cycle life, the capacity should maintain 70% of the initial capacity.
A battery with a capacity lower than 60% of its initial capacity is generally considered to be scrapped.
7. Depth of Discharge (DOD: Depth of Discharger)
Defined as the percentage of the capacity released by the battery to the rated capacity.
Generally, the deeper the discharge depth of lithium batteries, the shorter the battery life.
8. Cut-Off Voltage
Termination voltage is divided into charge termination voltage and discharge termination voltage, that is to say, the voltage at which the battery cannot continue to charge or discharge, and continuing to charge or discharge under the termination voltage has a great impact on the life of the battery.
The end-of-charge voltage of lithium batteries is generally 4.2V, and the end-of-discharge voltage is 3.0V.
It is strictly forbidden to deeply charge or discharge lithium batteries beyond the cut-off voltage.
9. Self-discharge rate (Self-Discharge)
Refers to the rate of decline in battery capacity during storage, expressed as a percentage of capacity decline per unit time.
The self-discharge rate of general lithium batteries is 2%~9%/month.
10. SOC (State of Charge)
Refers to the percentage of the remaining power of the battery to the total power that can be discharged, 0 to 100%. Indicates the remaining power of the battery.