1)The composition of lithium batteries
The lithium battery is mainly composed of two blocks, the battery cell and the protection board PCM (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 casing, while the protection board is mainly composed of protection chip (or management chip), MOS tube, resistor, capacitor and PCB board.
2)Advantages and disadvantages of lithium batteries
Lithium batteries have many advantages, high voltage platform, high energy density (light weight, small size), long service life, and environmental protection.
The disadvantage of lithium batteries is that the price is relatively high, the temperature range is relatively narrow, and there are certain safety hazards (protection system required)
| Lead acid battery | Ni-Cd Battery | Ni-MH Battery | Lithium Battery | |
| Nominal Voltage(V) | 2 | 1.2 | 1.2 | 3.2/3.6/3.7 |
| Weight energy density(Wh/kg) | 25~30 | 40~45 | 60~65 | 120~200 |
| Volume energy density（Wh/L） | 65~80 | 150~180 | 300~350 | 350~400 |
| Best working temperature(℃) | -40~70 | -20~60 | -20~45 | 0~45 |
| Safety | Safe | Safe | Safe | Risk |
| Environmental protection | Pb pollution | Cd pollution | Green | Green |
| Cycle life | 200~300 | 500 | 1000 | 800~2000 |
| Battery cost | Low | Medium | High | High |
3) Classification of lithium batteries
Lithium batteries can be divided into two categories: disposable non-rechargeable batteries and secondary rechargeable batteries (also known as storage batteries).
Non-rechargeable batteries such as lithium manganese dioxide batteries, lithium-thionamide batteries.
Secondary rechargeable batteries can be divided into the following classifications according to different situations.
- According to the appearance: square lithium battery (such as ordinary mobile phone battery) and cylindrical lithium battery (such as 18650 for electric tools);
- According to outsourcing materials: aluminum shell lithium battery, steel shell lithium battery, soft pack battery;
- According to the cathode material: lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), ternary lithium (LiNixCoyMnzO2), lithium iron phosphate (LiFePO4), lithium titanate batteries;
- According to the electrolyte status: lithium ion battery (LIB) and polymer battery (PLB);
- Divided by purpose: ordinary batteries and power batteries.
- According to performance characteristics: high-capacity batteries, high-rate batteries, high-temperature batteries, low-temperature batteries, etc.
4) Explanation of commonly used terms
- Capacity (Capacity)
Refers to the amount of electricity that can be obtained from battery lithium under certain discharge conditions.
We know in high school physics that the formula for power is Q=I*t, the unit is coulomb, and the unit of battery capacity is specified as Ah (ampere hour) or mAh (milliampere hour). This means that a 1AH battery can be discharged for 1 hour with a current of 1A when fully charged.
The old NOKIA cell phone battery (like BL-5C) is generally 500mAh, the current smart phone battery is 800~1900mAh, the electric bicycle is generally 10~20Ah, the electric car is generally 20~200Ah, etc.
- Charge-discharge rate (Charge-Rate/Discharge-Rate)
It indicates how much current is used to charge and discharge. It is generally calculated as a multiple of the battery’s nominal capacity, which is generally called a few C.
Like 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, and if it is charged and discharged at 0.1C, it will be charged and discharged at a current of 150mA.
- Voltage (OCV: Open Circuit Voltage)
The battery voltage generally refers to the nominal voltage (also called the rated voltage) of the lithium battery. The nominal voltage of an ordinary lithium battery is generally 3.7V, and we also call its voltage platform 3.7V. The voltage we are talking about generally refers to the open circuit voltage of the battery.
When the battery capacity is 20~80%, the voltage is concentrated around 3.7V (around 3.6~3.9V), the capacity is too high or too low, and the voltage changes greatly.
- Energy (Energy)/Power (Power)
The battery discharges according to a certain standard, the energy (E) that the battery can release, the unit is Wh (watt hour) or KWh (kilowatt hour), and 1 KWh = 1 kilowatt hour.
There is a basic concept in the physics book, E=UIt, 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.
- Internal resistance (Resistance)
Since it cannot be equivalent to an ideal power supply during charging and discharging, it has a certain internal resistance. Internal resistance consumes energy, of course, the smaller the internal resistance, the better.
The internal resistance of the battery is in milliohms (mΩ).
Generally, the internal resistance of a battery is composed of ohmic internal resistance and polarization internal resistance. The size of the internal resistance is affected by the battery’s material, manufacturing process, and battery structure.
- Cycle Life (Cycle Life)
A charge and discharge of a battery is called a cycle, and the cycle life is an important indicator of battery life performance.
The IEC standard stipulates that the mobile phone lithium battery should be discharged to 3.0V at 0.2C and charged to 4.2V at 1C. After 500 cycles of repeated cycles, the battery capacity should remain above 60% of the initial capacity. In other words, the cycle life of a lithium battery is 500 times.
The national standard stipulates that after a cycle life of 300 times, the capacity should maintain 70% of the initial capacity. If the battery capacity is less than 60% of the initial capacity, it is generally necessary to consider scrapping.
7.Depth of Discharge (DOD: Depth of Discharger)
Defined as the percentage of the battery’s discharged capacity to the rated capacity.
Generally, the deeper the depth of discharge of lithium batteries, the shorter the battery life.
8.Cut-Off Voltage (Cut-Off Voltage)
The termination voltage is divided into the charge termination voltage and the discharge termination voltage, that is to say the voltage at which the battery cannot continue to be charged or discharged. Continued charging or discharging under the termination voltage has a great impact on the battery life.
The charge termination voltage of lithium batteries is generally 4.2V, and the discharge termination voltage is 3.0V.
Deep charging or deep discharging of lithium batteries exceeding the termination voltage is strictly prohibited.
9.Self-Discharge rate (Self-Discharge)
Refers to the rate of decrease in battery capacity during storage, expressed as a percentage of capacity decrease per unit time.
The self-discharge rate of general lithium batteries is 2%-9%/month.
10.SOC(state of Charge) to the percentage of the remaining power of the battery to the total power that can be discharged, 0~100%. Reflect the remaining battery capacity.
5) Battery naming rules
Different manufacturers have different naming rules, but we all follow a unified standard for general batteries. According to the name of the battery, you can know the size of the battery.
IEC61960 stipulates that the rules for cylindrical and square batteries are as follows:
1. Cylindrical battery, 3 letters followed by 5 numbers, 3 letters, the first letter represents the negative electrode material, I represents built-in lithium ion, L represents lithium metal or lithium alloy electrode. The second letter represents the cathode material, C represents cobalt, N represents nickel, M represents manganese, and V represents vanadium. The third letter is R for cylindrical shape. There are 5 digits, the first 2 digits indicate the diameter, the last 3 digits indicate the height, the unit is mm.
2. Square battery, after 3 letters, there are 6 numbers and 3 letters. The first two letters have the same meaning as a cylindrical shape, and the last letter is P to indicate a square shape.
6 numbers, the first 2 numbers indicate the thickness, the middle 2 indicate the width, the last 2 indicate the height (length), the unit is also mm.
For example: ICR 18650 is a general-purpose 18650 cylindrical battery with a diameter of 18mm and a height of 65mm;
ICP 053353 is a square battery with a thickness of 5mm, a width of 33mm, and a height (length) of 53mm
6) Lithium battery technology
There are certain differences in the process flow of different batteries and different manufacturers. The detailed process flow will be very complicated. The basic process flow, the process flow of battery cell production and the process flow of PACK production are listed below.
The battery cell production process mainly includes pole piece manufacturing, battery cell production, battery assembly, liquid injection, formation, and sorting.
From ingredients to winding, the positive and negative electrodes are made in different workshops at the same time. After the positive and negative electrodes are completed, the subsequent processes are done together. Different quality inspection QA links will be inserted in the middle.
7) Battery grouping and series-parallel connection
Applied in different fields, the requirements for batteries are different, and the system has some special requirements for voltage, capacity, internal resistance, etc. Often a single-cell battery cannot meet the requirements, and the battery needs to be connected in series and parallel to supply external power.
The performance of batteries connected in series and parallel is determined by the performance of the worst battery, which is what we often call the “barrel principle”. Therefore, the most important point of a battery pack is the consistency of battery performance parameters.
Such as notebooks, electric bicycles, electric vehicles, energy storage systems, etc., all need to consider the series and parallel connection of batteries to form a battery pack.
Notebook battery voltage is generally 11.1V or 14.8V, mainly using 18650 batteries, so generally 2 series 3 parallel or 2 series 4 parallel.
Apple iPAD is 3 polymer batteries connected in parallel with a capacity of about 25Wh.
Electric bicycles and electric motorcycle systems are generally 24V, 36V, 48V, 60V, 72V systems, and the specific group conditions are shown in the following table (S stands for series connection).
Pure electric vehicles and hybrid electric vehicles (EV/PHEV) have higher voltages, around 250~500V, and can be connected in series to more than 150 knots.
In addition, there are many things to consider when batteries are grouped in series and parallel, such as the consistency of the battery voltage platform, the consistency of the battery capacity, and the consistency of the battery’s internal resistance.
The consistency of the parameters after the battery is connected in series and parallel has a great impact on the performance and life of the battery
Battery Voltage VS Battery Type VS Series Number
| Battery Pack Voltage | Ni-MH Battery/NCM Battery | LiFePO4 Battery | LTO Battery |
| 12V | 4S | 4S | 5S |
| 18V | 5S | 6S | 8S |
| 24V | 7S | 8S | 10S |
| 36V | 10S | 12S | 15S |
| 48V | 13S | 15S/16S | 20S |
| 60V | 16S | 19S | 25S |
| 64V | 18S | 20S | 27S |
| 72V | 20S | 23S | 30S |