Cylindrical Battery Basics

Cylindrical Battery Basics（Battery Pack Production Line）

1. Naming of Cylindrical Batteries

When it comes to cylindrical batteries, battery model naming is unavoidable. Industry insiders are certainly familiar with it, but for friends who are new to the industry, the naming rules may not be so clear. The model name of a lithium-ion cylindrical battery is determined by three aspects:

• the battery’s diameter;
• the battery’s overall height; and
• the battery’s shape (since it is cylindrical, it is usually represented by “0”).

For example, take the most common 18650: its finished diameter is about 18 mm, its height is about 65 mm, and it is cylindrical — hence the name “18650”. The cylindrical battery models you hear most often today, such as 21700, 33140 and 46800, are all named in this way.

Of course, cylindrical batteries also have other naming conventions. In the NiMH battery field, for instance, the rule is different: NiMH battery models are also named by height and diameter, except that the diameter is represented by letters. The corresponding models include “A”, “AA”, “AAA”, “AAAA”, “SC”, “C”, “D” and “F”, etc. An A cell has a diameter of about 16 mm, AA about 14 mm, AAA about 10 mm, AAAA about 7 mm, SC about 21 mm, C about 25.2 mm, D about 32 mm, and F reaches 69 mm. In fact, you may notice that most of the mainstream lithium battery models in the industry today have their counterparts in the NiMH series — this is also for the standardization of consumer batteries.

18650 Battery Pack 3.7V 2600mAh

2. The Cylindrical Lithium Battery Family

With continuous advances in battery technology, prismatic, pouch and cylindrical cells now share the market as the three main form factors. In particular, after Tesla unveiled the 46800 large cylindrical cell at Battery Day on September 20, 2020, the domestic cylindrical battery industry was stirred up considerably.

Based on cell structure and manufacturing process, major domestic battery manufacturers currently offer the following cylindrical cell models.

In recent years, large cylindrical cells have developed rapidly in terms of specifications and models. In my view, the “large” in “large cylindrical” refers not only to the larger cell size, but more importantly to advantages such as high-current charge/discharge capability, low heat generation and low internal resistance. As a result, the assembly process and cell structure of large cylindrical cells have undergone disruptive changes compared with small cylindrical cells such as 18650/21700. Based on current structures, cylindrical cells can be roughly divided into the following types:

• Steel-can products with single or double tabs for current conduction — typically 18650/21700;
• Full-tab steel-can products — e.g., 32140/21700;
• Full-tab aluminum-can products with single- or double-ended designs — e.g., 32130/40130;
• Single-ended steel-can products with cut-and-folded tabs — e.g., Tesla’s 4680.

18650 Battery Pack 3.7V 2600mAh

3. Product Structure of 33140/21700

Products such as 33140 and 21700 mainly consist of the steel can, insulating adhesive, negative current collector plate (busbar), jelly roll, positive/negative current collector plates, cap assembly and electrolyte. The assembly process mainly uses automated equipment to integrate and assemble these structural parts in a low-temperature, low-humidity environment, ultimately producing a semi-finished cell.

	18650 Battery Pack 7.4V 5200mAh

4. Assembly Process Flow of 33140/21700

The assembly process of 33140/21700 starts from winding and ends at cell sealing. It includes the main processes of winding → tab flattening → positive/negative busbar welding → can insertion → bottom spot welding → insulating cap placement → grooving → X-ray inspection → electrolyte filling → cap laser welding → sealing. Combined with the various inspection and material-handling steps, it forms the complete assembly production flow.

5. Key Process Control Points

18650 Battery Pack 7.4V 5200mAh

5.1 Tab Flattening (Tab Forming)

• a. Flattening dimensional accuracy: control of the overall height and the exposed foil length at both the positive and negative ends;
• b. Concentricity: ensure concentricity of the flattening and prevent the center hole from being blocked;
• c. End-face requirements: parallelism of the two end faces and flatness of each end face;
• d. Foil edge control: prevent the foil from flipping outward during flattening;
• e. Dust prevention: measures to prevent dust generation during flattening;
• f. Short-circuit control during flattening;
• g. Main online inspection: CCD inspection of incoming foil length, CCD inspection of the overall height and exposed foil length after flattening, and Hipot test after flattening.

5.2 Positive/Negative Busbar Welding

• a. Dust handling during welding;
• b. Concentricity between the busbar and the jelly roll;
• c. Weld strength assurance and the selection between spot welding and seam (line) welding;
• d. How to avoid damaging the separator during welding (which would cause short circuits), and how to prevent separator shrinkage during welding;
• e. Consistent focal distance on the welding surface;
• f. Selection of the laser type and galvanometer scanner, and handling of high-reflectivity alarms;
• g. At the automation level, combining flying-welding technology with welding time to maximize laser utilization is currently a technical challenge facing the whole industry;
• h. Main inspection: CCD visual inspection after welding, Hipot test, and effective pull-force (peel strength) testing.

5.3 Bottom Welding – Resistance Welding

• a. Center-hole diameter design to meet the weld-area requirement, ensuring the center hole is not blocked during welding, and use of the in-line hole reaming (hot-piercing) function;
• b. Protection against weld splatter (sparking) and incoming-material control of related structural parts;
• c. Weld penetration depth and weld-area assurance;
• d. Poka-yoke (anti-sticking) measures for the welding needle and effective control of welding fixtures;
• e. Consistency of bottom weld appearance;
• f. Temperature stability control of the welding fixtures during the process;
• g. Feedback and data analysis of welding pressure, voltage and current;
• h. Selection of resistance welding machines and related accessories;
• i. Main inspection: CCD inspection of the center hole before welding, current monitoring during welding, and effective pull-force testing.
• 

5.4 Grooving and Sealing

• a. Grooving and sealing dimension design, and stress simulation of the sealing profile;
• b. Control of groove stretch/deformation during grooving, and control of the sealing ring deformation;
• c. Dust handling at the can opening and in the groove during grooving;
• d. Fixture life management for grooving and sealing;
• e. Sealing integrity assurance;
• f. Protection of the nickel-plated layer during grooving and sealing;
• g. Main inspection: in-line CCD dimensional inspection, in-line sealing appearance inspection, in-line Hipot test, and off-line metallographic and air-tightness (leak) testing.
• 

5.5 Electrolyte Filling

• a. Stable filling volume and consistent tab position during filling;
• b. Total residence time of the product in the equipment;
• c. Electrolyte wetting efficiency and uniformity;
• d. Maintainability of the filling fixtures;
• e. Corrosion resistance of the equipment and fixtures;
• f. Safety: detection of flammable gases, quality certification of pressure vessels, fall and collision protection during cell handling, and safe electrolyte transfer;
• g. Automatic handling and position replacement of NG products, such as scanning NG positions to fill gaps and automatic refilling;
• h. Main inspection: weighing system before and after filling, plus data analysis and traceability.

5.6 Cap Welding

• a. Selection of the cleaning method for electrolyte residue on the tabs, such as non-woven wiping or laser cleaning;
• b. Ensuring positional alignment between the cap and the tabs;
• c. Laser and scanner selection and weld strength assurance;
• d. Consistent focal distance during welding;
• e. Dust handling and spark protection during welding;
• f. Main inspection: pull-force testing and CCD inspection before and after welding.

6. The 46-Series Large Cylindrical Battery Structure

From an assembly-process perspective, 46-series cells follow two process routes: one is die crimp (mold) sealing and the other is laser sealing. Today we will briefly introduce the mold sealing route.

The mold-sealing assembly process for 46-series large cylindrical cells evolved from combining the assembly processes of 18650/21700 cylindrical cells and full-tab aluminum-can cells. Technically, it inherits the efficient processes of small cylindrical cells (such as grooving and sealing), continues the full-tab assembly technology of capacitors, and adds the cut-and-fold tab process newly released by Tesla. By integrating these three, domestic battery manufacturers have formed the 46-series mold-sealing assembly structure. Let’s take a look at the structural features of this product.

18650 Battery Pack 14.8V High Capacity

6.1 Product Introduction

• Cell specifications: cell diameter 46 mm; common height specs 80–120 mm; generally without a separate positive terminal post.
• Positive terminal: currently, domestic power-type 46 cells use an independent aluminum terminal post as the positive terminal.
• Negative terminal: the entire can serves as the negative terminal; both the positive and negative terminals are located on the same side.

	18650 Battery Pack 14.8V High Capacity

6.2 Product Structural Components

18650 Battery Pack 14.8V High Capacity

No.	Component	Main Function	Design Requirements
1	Rivet	Copper rivet used to seal the electrolyte filling hole.	Easy to rivet and easy to form a seal with the cover plate.
2	Cover plate	Seals the open end of the cell; the vent (explosion-proof) valve can be integrated into the cover plate.	SPCC material; the vent valve must be positioned properly; thickness above 0.4 mm; rib reinforcement design required; the outer diameter must match the sealing ring and the sealing process.
3	Sealing ring	Generally made of imported PFA material; used in the mold-sealing process; deforms during mechanical sealing to create the seal.	Must withstand high temperature and corrosion, tolerate 70% deformation while maintaining sealing performance; dimensions must match the cover plate and the sealing process.
4	Negative current collector plate (busbar)	Copper or copper-nickel composite; welded to the negative end of the jelly roll and to the steel can.	Reasonable thickness and outer diameter; the outer diameter must match the inner diameter of the steel can with a small clearance; the lugs should have an elastic design.
7	Jelly roll	Core body of the cell, including the positive electrode, negative electrode, separator and end tape.	Reasonable jelly-roll height, number of folded-tab layers, and exposed length of the cut-and-folded tabs.
9	Positive busbar (current collector)	Aluminum; welded to the positive end of the jelly roll and to the positive terminal post to connect them.	Reasonable thickness and diameter; in the through-weld zone and busbar weld zone, avoid rigid connections that could pull off weld spots or weld lines.
10	Positive insulation sheet	Insulates the positive busbar from the steel can to prevent internal short circuits.	Some customers split the positive insulation into two parts — an insulation sheet and a positive wrap (coating); it must withstand high temperature and corrosion.
11	Negative can (case)	SPCC pre-nickel-plated or post-nickel-plated steel can; used for negative connection welding and serves as the negative case.	Good conductivity, reasonable wall and bottom thickness, and appropriate plating thickness.
12	Positive terminal post	Aluminum/PFA composite; welded to the positive busbar, serves as the positive terminal, and is insulated from the can.	Riveting of the terminal post must ensure sealing, etc.

Note: Since the negative end contacts the can directly, basically no one chooses to wrap it with insulation coating.

6.3 Mold Sealing Assembly Process Route

The following is a mold-sealing process route for 46-series products.

The process above is a common 46 assembly process. However, many processes still cannot meet design requirements in terms of yield and product assurance — for example, preventing foil burn-through during busbar welding, separator shrinkage caused by heat during collector welding, monitorability of through-weld effectiveness, welding of the negative busbar lugs to the can wall, concentricity between the negative busbar and the can during can insertion, plating loss during grooving, and collapse of the sealing groove. Therefore, mass production of 46-series power batteries still has a long way to go. In my personal view, regardless of which sealing process is adopted in the future, optimizing product structural parts and advancing automated equipment technology remain the only path to mass production.

Next, let’s look at the laser sealing process route and the comparison between the two process routes. The image below shows a product produced by Tesla’s second-generation laser sealing process route.

	18650 Battery Pack 11.1V 7800mAh

7. The 46 Laser Sealing Product Structure

The laser sealing process seals the cell with laser welding. Let’s look at the structural components of 46 products that use the laser sealing process.

The image below shows a structural component drawing for illustration only — zoom in to view the original image. For the functions of the structural components, please refer to the article “Assembly of 46-Series Large Cylindrical Tabless Batteries” published on December 4.

18650 Battery Pack 11.1V 7800mAh

7.1 Laser Sealing Process Route

The following is a typical laser sealing assembly process route.

The process flow above is a relatively common main production flow for 46 large cylindrical cell assembly in the industry. For the laser sealing process, air-tightness (leak) testing is required at the sealing step to verify sealing integrity, divided into pre- and post-helium leak tests. For the differences from the mold sealing process, please see the next section.

8. Laser Sealing vs. Mold Sealing

I am not the first to compare the laser sealing and mold sealing assembly processes for 46-series large cylindrical cells. Major power battery manufacturers and smart equipment suppliers have made such comparisons before, and it was only after multiple rounds of validation that they chose their industrialization routes. As an individual, I declare upfront that I do not represent any position: I neither advocate mold sealing nor oppose laser sealing. I worked on mold sealing processes for cylindrical batteries for nearly 20 years and have gained considerable knowledge in laser applications over the past 10 years. Today, from my perspective, I will objectively analyze the comparison between the two approaches. Note that the mold sealing discussed here is not the crimped (rolled-edge) sealing shown in Tesla’s first-generation product.

• Laser sealing: uses laser welding to perform 360° seamless welding between the cover plate and the can, sealing the cell. It is currently widely used for large cylindrical aluminum-can cells and small steel-can coin cells.
• Mold sealing: the equipment pre-forms a groove on the can; the groove serves as the lower-die platform, and the upper die presses the sealing ring to form a sealed closure. It is widely used for small cylindrical cells such as the 18, 21, 26 and 32 series.

Item	Laser Sealing	Mold Sealing	Result
Capacity	—	—	Laser sealing is better by about 1.5%
Diameter machining dimension	Large diameter variation	Diameter machining accuracy 0.10 mm	Mold sealing wins
Sealing monitoring	Helium leak test, 100% inspection required	Off-line metallographic inspection	Mold sealing wins
Structural parts	No sealing ring needed	Sealing ring required	Laser sealing wins
Processing consistency	Meets process requirements	CMK above 1.33	Mold sealing wins
Process difficulty	Rusting of the steel can after laser welding	Development of the sealing ring and process	Mold sealing wins
Processing speed	Slower; cycle time above 5 s	Cycle time below 2 s	Mold sealing wins
Equipment cost	High	Low	Mold sealing wins
Sealing reliability	Needs continued validation for power applications	Proven in 21700 automotive applications	Mold sealing wins

The table above is my personal comparison of mold sealing and laser sealing on the main items. Whether laser sealing or mold sealing is better ultimately depends on what the customer prioritizes. Below I will elaborate on each aspect.

18650 Battery Pack 11.1V 7800mAh

8.1 Capacity (taking the 4695 as an example)

As can be seen from the cross-sections of the two sealing methods, mold sealing adds 2 layers of steel can, 1 sealing gap layer and 1 busbar fold position in the process. The space occupied by the compressed sealing ring can offset the corresponding space of the laser-sealed area, and the total height occupied is no more than 1.5 mm. For a 4695-32Ah cell, this occupies about 500 mAh of capacity; for vehicle application, this equals roughly 2 kWh per vehicle. For this reason, most leading companies currently choose the laser sealing process. But is there still room to improve and optimize mold sealing? The answer is yes — some customers are still developing and optimizing it, while many others, constrained by the overall market trend, have abandoned this route.

8.2 Diameter Machining Dimension

This item is easy to understand. With laser sealing, a ring of weld bead forms around the cell. Depending on weld strength requirements, build-up welding is generally applied, so the diameter becomes relatively larger. During laser welding, because the diameter forms naturally without mold constraints, the diameter fluctuation is larger. Mold sealing has a slight advantage here: the cell’s outer diameter is constrained throughout the process, so the final diameter is generally within 46.1 mm, and this dimension is monitored in-line with good stability. However, laser sealing can also be optimized in this regard; if the cell needs to be used as a single cell in the future and the diameter needs to be optimized, feel free to contact me.

8.3 Sealing Monitoring

During laser sealing, the high energy of the laser melts the metal and fuses it through penetration. During this process, air, dust impurities and moisture can enter the medium, resulting in pores in the weld seam, and pores can cause leakage. To prevent pores and electrolyte leakage, laser sealing always includes a helium leak test step. Many people may not know that the helium leak tester is the most expensive piece of inspection equipment on the entire line — bar none. First, helium is a noble gas that does not exist in the atmosphere; second, helium must be imported; third, the efficiency of helium leak testing is very low; and fourth, the equipment has very strict leak-rate requirements. As a result, one helium leak tester can only inspect a limited number of products; otherwise, the probability of false judgments becomes very high. For mold sealing, on the other hand, the seal is formed by combining metal with a sealing material and compressing the sealing ring. As long as the mold sealing process is properly designed and the sealing ring material is properly selected, mold sealing will not leak even under continuous operation in harsh environments. The image below shows the metallographic inspection result of mold sealing.

8.4 Core Process Difficulties

Both laser sealing and mold sealing have their own difficulties. Let’s first talk about laser sealing: during sealing welding, the original plating on the steel can and cover plate is deeply damaged, and iron oxides are produced at the same time. After prolonged storage, weld-seam rusting can easily occur — this is probably the biggest challenge at present. As for mold sealing, based on my contact with customers, what they value most is the stability of the sealing ring deformation and the deformation of the cover plate. Because mold sealing applies a large force to the cover plate during processing, if the process and die design are unreasonable, the cover plate can easily deform, which changes the burst pressure value of the vent valve.

8.5 Processing Efficiency

In terms of processing efficiency, laser sealing is directly related to the welding path, material and welding requirements. For a 46 large cylindrical cell, the weld diameter is about 145 mm; to ensure quality, the cycle time per cell conservatively needs to reach 5 s. Many devices on the market claim 100 PPM, 200 PPM or even 300 PPM, mostly achieved by paralleling multiple lasers. Flying welding technology emerged to improve laser utilization, but laser welding itself only eliminates material-handling and positioning time; it does not shorten the laser welding time. On the contrary, flying welding places higher demands on fixture positioning accuracy, posing a greater challenge to equipment assembly precision and laser performance, and placing higher requirements on the level of domestic machine tools. Mold sealing uses the equipment structure of the 21700. Although its efficiency cannot reach the 200 PPM of a single 21700 machine, using three-jaw clamping dies and cam (non-turret) technology, reaching 200 PPM through two- or three-parallel configurations is not difficult.

 

8.6 Equipment Investment Cost

In terms of equipment investment, the two sealing methods differ significantly. The cost of the laser sealing process mainly comes from the lasers and the helium leak tester. Currently, 46 peripheral welding requires high-performance lasers, generally IPG 1000–2000 W; the laser output heads vary by manufacturer, with a single set costing around 300,000 RMB. If one laser has a production capacity of 12 PPM, a 150 PPM line needs at least 13 lasers, meaning the laser cost alone exceeds 4 million RMB. The cost of the other structures is roughly equal to the combined equipment cost of the two mold-sealing processes. Add the helium leak tester cost — as for the other costs, I won’t compare them; the difference is obvious.

 

 

9. The All-Tab Flattening Process

9.1 Overview

The tab flattening process for cylindrical batteries, also called the tab shaping (forming) process, is the process that follows winding. In this process, the positive and negative tabs of the jelly roll are pressed and kneaded by mechanical conical rollers over a rotating travel, consolidating the dispersed copper and aluminum foils into a complete positive current collector and negative current collector.

9.2 Main Process Requirements

1) Incoming jelly roll requirements

• H: the overall height of the jelly roll must be set to an appropriate process dimension with a tolerance range. An oversized overall height usually results from misalignment between the positive and negative coating zones, ineffective edge-guide correction during winding, or spiral winding of the electrode assembly — the failure consequence is that the flattening travel on the positive or negative side is excessive, damaging the electrode and causing short circuits or safety risks. An undersized overall height usually means the exposed dimension at the positive or negative end of the jelly roll is too small — the failure consequence is insufficient flattening travel, so the tabs are not effectively flattened and the separator or electrode may be welded during busbar welding.
• h1/h2: the exposed tab dimensions at the positive and negative ends must be set to appropriate process dimensions with tolerances. The failure consequences of over-long or over-short dimensions are the same as those for overall height deviations.
• D: the jelly roll diameter must be set to an appropriate process dimension with a tolerance range. If the diameter is too large, the grippers may damage the jelly roll surface and cause foil edge flipping at the flattened outer ring; if too small, the grippers cannot hold the jelly roll firmly, resulting in unstable flattening dimensions.

2) Flattening process requirements

• H: overall height and tolerance requirements after flattening. The pure height variation after flattening is generally within 0.50 mm. However, besides overall height, there are also requirements for perpendicularity between the flattened end face and the cylindrical surface, parallelism between the two end faces, and flatness of each single plane. Reference values: perpendicularity of about 0.1, parallelism between the two planes of less than 0.25 mm; flatness varies with diameter (except for processes with mold-pressing tolerances).
• h1/h2: the exposed foil length and tolerance at the positive and negative ends after flattening; this dimension is related to the overall height.
• Center hole requirements: during flattening, the center hole must not be blocked or damaged, so that subsequent processes can continue; the concentricity of the jelly roll must also be ensured during flattening.
• Flattening appearance requirements: the flattened foil at both ends should not flip outward; the separator surface must be free of scratches and gripper marks; the center hole must be free of dark marks (stains).
• Hipot test requirements: test voltage, test time, and automatic discharge; the test must show no short circuit.
• MES traceability requirements: in current large cylindrical products, the jelly roll surface tape generally carries a QR code. The QR code information must be linked to the product processing information at each process step, such as the foil length before and after flattening at both positive and negative ends, the overall height after flattening, Hipot test data, and equipment running status.
• 

9.3 Equipment Requirements

1) Process flow of the flattening equipment

2) Explanation of the equipment flow

• Positioning & scanning: position the incoming jelly rolls so that all are in the same position, and rotate-scan each jelly roll, storing the scanned product information in the host-computer software.
• Exposed-foil dimension inspection: measure the exposed foil length at both the positive and negative ends; cells outside the process range are rejected without processing. The inspection accuracy is generally 0.05 mm. It can also identify the orientation of the jelly roll’s positive and negative ends to prevent reversed incoming cells.
• Tab flattening: pre-position the jelly roll before flattening; the jelly-roll gripper picks up and lifts the roll to ensure the center hole is concentric with the flattening fixture. During fixture travel, end-face dust removal is performed, with an air velocity above 15 m/s. High requirements are placed on the surface finish, hardness and rotational flexibility of the flattening head. The head’s feed speed, travel length and rotation speed are variably adjustable, and the adjusted speed and length must be displayed quantitatively, as these parameters greatly affect the flattening result. Current equipment generally uses servo motors with reducers, paired with linear guides and precision ball screws, to ensure the accuracy of this station.
• Overall height inspection: measure the overall height of the flattened product and feed the data back to the host software via a displacement sensor.
• Shoulder height inspection: use CCD to inspect the foil height at both ends, with an accuracy of about 0.05 mm.
• Hipot test: a short-circuit test on the product to prevent separator puncture. Common testers include Hioki, Ruijie, Keysight (Agilent) and Chroma. The test voltage for lithium batteries is generally around 100–250 V, with a test time generally under 0.5 s.

9.4 Pain Points of Flattening Automation

Although full-tab cells have been in mass production for years, the flattening process still has its pain points. For example, dust handling: although end-face dust removal and drop-zone dust removal have been installed, due to the particular nature of dust removal and flattening, the dust generated during flattening still cannot be effectively removed. Another pain point is efficiency: as a purely mechanical process, flattening is relatively inefficient — a single flattening station only achieves about 6–8 PPM. To solve this, leading equipment companies have been developing vertical turret flattening equipment, which both reduces dust generation and improves efficiency.

10.Cylindrical Battery Basics FAQS

No.	Question & Answer
Q1	Q: What is a cylindrical lithium battery? A: A cylindrical lithium battery is a lithium-ion cell built in a round metal casing. Active materials, separator and electrolyte are rolled into a jelly roll and sealed inside a steel or aluminum can, featuring standardized dimensions and mature manufacturing technology.
Q2	Q: What are the most common cylindrical cell sizes? A: Main models include 18650, 21700, 26650, 32140. The naming rule: first two digits = diameter (mm), next three digits = length (mm).
Q3	Q: What chemistries are used for cylindrical lithium cells? A: NMC (nickel manganese cobalt), NCA and LFP (lithium iron phosphate). NMC/NCA delivers high energy density; LFP offers longer cycle life and better thermal safety.
Q4	Q: What is the nominal voltage of a single cylindrical cell? A: NMC/NCA: 3.7V (full charge 4.2V); LFP cylindrical cell: 3.2V (full charge 3.65V).
Q5	Q: What is the difference between energy-type and power-type cylindrical cells? A: Energy-type cells maximize capacity for long runtime with low continuous current. Power-type cells support high-rate discharge, sacrificing some capacity for high peak current output.
Q6	Q: What are the core internal components of a cylindrical cell? A: Positive electrode foil, negative electrode foil, separator, electrolyte, safety vent, PTC protection device, positive terminal cap and metal can casing.
Q7	Q: What is the function of the safety vent on cylindrical cells? A: It is a pressure relief valve. When internal pressure rises due to overheating or abuse, the vent ruptures to release gas and prevent violent cell explosion.
Q8	Q: What does the PTC component inside a cylindrical cell do? A: PTC is a positive temperature coefficient thermistor. It increases resistance sharply under overcurrent or high temperature to limit current and reduce thermal runaway risk.
Q9	Q: How do you connect cylindrical cells to build a battery pack? A: Series connection raises voltage; parallel connection increases total capacity and discharge current. A pack uses a combination of series and parallel configurations to match target voltage and capacity.
Q10	Q: Why are cell holders widely used for cylindrical pack assembly? A: Plastic cell holders fix cells in position, maintain safe gaps between cells, provide electrical insulation and reserve space for thermal interface materials.
Q11	Q: What welding methods are used for cylindrical cell interconnection? A: Nickel strip spot welding is the mainstream. Ultrasonic metal welding is selected for high-current packs with thicker busbars. Laser welding is less common for small cylindrical tabs.
Q12	Q: What is cell sorting for cylindrical cells, and why is it needed? A: Cell sorting groups cells with closely matched OCV, internal resistance and capacity. Mismatched cells in one pack cause uneven aging, imbalance and shortened pack service life.
Q13	Q: What is the typical cycle life range for cylindrical cells? A: NMC: 500–1000 cycles to 80% capacity retention. LFP cylindrical cells can reach 2000 cycles or more under proper operating conditions.
Q14	Q: What are the main advantages of cylindrical lithium cells? A: Standardized dimensions, mass mature production, low cost, built-in safety vent, good mechanical pressure resistance and wide global supply chain.
Q15	Q: What are the disadvantages of cylindrical cells? A: Lower space utilization inside battery packs; many cells are required for high-capacity packs, creating more welding points and higher assembly workload.
Q16	Q: What typical applications use cylindrical lithium cells? A: E-bikes, power tools, portable power stations, drones, vacuum cleaners, backup power, small consumer electronics and low-speed mobility devices.
Q17	Q: What basic safety precautions apply to bare cylindrical cells? A: Prevent short circuit between positive and negative terminals, avoid cell casing puncture, prohibit overcharging, store away from high temperature and metal conductive scraps.
Q18	Q: What common tests are performed on bare cylindrical cells? A: OCV, internal resistance, capacity test, AC impedance, high-temperature storage, overcharge test, crush test and UN38.3 transport safety test.
Q19	Q: What is the difference between bare cylindrical cells and finished cylindrical battery packs? A: Bare cells are single raw units. A finished pack combines multiple cells with BMS, busbars, insulation and housing, adding protection and forming a ready-to-use power source.
Q20	Q: What workshop requirements are needed for cylindrical cell handling and assembly? A: ESD anti-static protection, good ventilation, temperature & humidity control, fireproof cell storage cabinets and tools to prevent accidental short circuits.