01 Definition of Lithium Battery Series-Parallel Connections

Due to the limited voltage and capacity of individual batteries, series-parallel connections are often required in practice to achieve higher voltages and capacities to meet the power requirements of the device.

Lithium battery series connection: Voltages are added, capacity remains constant, and internal resistance increases.

02 Lithium Battery Pack

• Lithium cell: The core component of the finished battery.
• Protection board: Provides protection against overcharge, over-discharge, overcurrent, short circuit, and NTC temperature control.
• Plastic casing: The supporting framework of the entire battery; it positions and secures the protective plate; and it supports and limits the position of all other non-shell components.
• Terminal leads: Provides various charging and discharging interfaces for various electronic products, energy storage devices, and backup power supplies.
• Nickel sheet/bracket: Components that connect and secure the battery cells.

03 Lithium Battery Series-Parallel Combinations

For safety reasons, lithium batteries must be connected to an external protective plate to monitor each cell. Parallel operation is generally not recommended. If parallel operation is required, battery parameters (capacity, internal resistance, etc.) must be consistent. Furthermore, the parameters of the batteries used in series must also be consistent. Otherwise, the performance of the battery pack will be significantly worse than that of a single cell.

Lithium Battery Pairing Standards: Voltage Difference ≤ 10mV, Internal Resistance Difference ≤ 5mΩ, Capacity Difference ≤ 20mA

2-String Lithium Battery Pack (7.4V Lithium Battery)

• Assuming the specifications of the single cell used are 18650 3.7V 2000mAh
• Assuming the specifications of the single cell used are 18650 3.7V 2000mAh
• Number of cells connected in parallel: 20Ah/2Ah = 10, i.e., 10 cells in parallel
• The entire battery pack is 13 strings x 10 cells = 130 cells

Common series combinations of lithium battery packs

04 Lithium Battery Assembly Process

① Battery Capacity Grading

ACEY-BCT506R-512H lithium battery charge and discharge machine is designed for testing the capacity, voltage, current and charging & discharging performance of cylindrical lithium cells, such as 18650 21700 26650 32650 32700. 

② Battery Insulation Paper Pasting
Apply quick-release paper and Mara tape for insulation.

ACEY-BS01 automatic insulation paper sticking machine is mainly used to stick a layer of barley paper on the positive or negative electrode of 18650/21700/26650/32650/32700 batteries to provide good insulation, wear resistance, waterproofing and other characteristics.

ACEY-AS11S lithium cell sorting machine is an efficient and high-precision device designed for rapid sorting of cylindrical lithium batteries. It uses an advanced internal resistance tester to accurately measure battery voltage and internal resistance. The equipment is compatible with various specifications of batteries (18650/21700/26650/32700/etc), with a sorting speed of up to 5000 PCS/hour, which greatly improves production efficiency and supports data export for easy quality traceability. 

Use molded nickel strips to eliminate cold solder joints, short circuits, low efficiency, and uneven current distribution.

ACEY-5000D manual spot welding machine is used to weld the positive and negative terminals of multiple batteries and materials such as nickel sheets together, ensuring stable current transmission during the charging and discharging process of the battery pack. It is widely used in welding many battery packs such as power batteries, electric tools, and electric bicycles, etc.

⑥ Battery Pack Comprehensive Testing

ACEY-BIT Series battery comprehensive tester can make a quantitative and accurate measurement of some basic parameters of the battery. It can measure the open circuit voltage, internal resistance, charge, discharge, overcurrent protection, short circuit protection and other functions of the battery. It can easily detect bad batteries and increased production efficiency.

ACEY-5035C heat shrink film packaging machine is used in the assembly and packaging of battery packs, particularly for lithium-ion batteries and other types of rechargeable batteries. These machines are designed to securely wrap and seal battery packs using heat shrinkable film, providing protection, insulation, and a clean appearance to the battery packs.

⑧ Inkjet Batch Code Printing

The printing must be clear and legible.

ACEY-CP2 Inkjet batch coding machine is used for printing information such as batch numbers, expiry dates, manufacturing dates, barcodes, and other variable data onto products or packaging in manufacturing and packaging industries. This is essential for product traceability, quality control, and compliance with regulatory .

⑨ Battery Pack Aging
Determine the quality of the battery pack and ensure the stability, safety, and service life of the lithium battery pack during use.

ACEY-BA series battery aging machine is mainly used for battery charging and discharging and related battery characteristics testing. The test channels are completely independent and suitable for aging tests of ternary, lithium iron phosphate, lead-acid, nickel-metal hydride and nickel-cadmium batteries.

• Do not use batteries of different brands together.
• Do not use batteries of different voltages together.
• Do not mix batteries of different capacities or old and new lithium batteries.
• Batteries of different chemistries should not be mixed, such as nickel-metal hydride and lithium batteries.
• When the battery charge is low, replace all batteries.
• Use a lithium battery protection board with corresponding parameters.
• Select batteries with consistent performance. Generally, lithium batteries used in series and parallel require cell pairing. Pairing criteria: voltage difference ≤ 10mV, internal resistance difference ≤ 5mΩ, and capacity difference ≤ 20mA.

• Typical connection methods for lithium battery packs include parallel first, then series, series first, then parallel, and mixed connection methods.
• Lithium battery packs for pure electric buses typically use a parallel-first, then series connection method.
• Lithium battery packs for grid energy storage often use a series-first, then parallel connection method.

Advantages of Parallel-First, Then Series-Second Connection for Lithium Batteries

• A lithium battery cell failure automatically shuts down. Aside from a reduction in capacity, this does not affect the ability to operate in parallel.
• If a lithium battery cell shorts in a parallel connection, the parallel circuit will experience very high currents, which is usually prevented by fuse protection technology.

• Due to differences in internal resistance and uneven heat dissipation among lithium battery cells, the cycle life of a parallel-connected lithium battery pack can be affected.

Advantages of Connecting Lithium Batteries in Series First and Parallel Last

• Connecting lithium battery cells in series first, based on their capacity, such as 1/3 of the total capacity, and then in parallel last, reduces the probability of failure in large-capacity lithium battery modules. Connecting in series first and then in parallel also significantly improves the consistency of lithium battery packs.

• Lithium batteries with or without protection boards cannot be charged in parallel. Batteries without protection boards are prone to overcharging and damage.
• Batteries charged in parallel generally require the battery's built-in protection board to be removed and a single protection board used.
• If batteries charged in parallel do not have a protection board, the charging voltage must be limited to 4.2V, and a 5V charger cannot be used.

• Lithium batteries must be charged using a dedicated charger. Otherwise, they may not reach full capacity, affecting their performance.
• Lithium batteries do not need to be fully discharged before charging.
• Do not leave the charger plugged in for extended periods. Remove the battery from the charger as soon as it is fully charged.
• Remove the battery from any unused electrical appliances and drain it before storing.
• Do not insert the battery with the polarity reversed, as this may cause the battery to swell or rupture.
• Do not use nickel and lithium chargers interchangeably.