Definition of Series and Parallel Connection of Lithium Batteries

Due to the limited voltage and capacity of the single battery cell, the series and parallel connection is needed in the actual use to obtain higher voltage and capacity, so as to meet the actual power demand of the equipment.

  • Lithium batteries connected in series
    Add the voltage of batteries, capacity remains the same, and internal resistance increases.

  • Lithium batteries connected in paralle
    Constant voltage, added capacity, reduced internal resistance, and extended power supply time.

  • Lithium batteries connected in series and parallel
    There are both parallel and series connection in the middle of the battery pack to increase the voltage and capacity.

  • Series Voltage
    3.7V single battery can be assembled into battery pack with a voltage of 3.7*(N)V as required (N: number of single batteries)
    For example, 7.4V, 12V, 24V, 36V, 48V, 60V, 72V, etc.

  • Capacity of Parallel Connection
    2000mAh single battery can be assembled into a battery pack with capacity of 2*(N)Ah as required (N: number of single batteries)
    For example, 4000mAh, 6000mAh, 8000mAh,5Ah, 10Ah, 20Ah, 30Ah, 50Ah, 100Ah, etc.

Lithium Battery Pack

Lithium battery pack technique refers to the processing, assembly and packaging of lithium battery pack. The process of assembling lithium cells together is called PACK, which can be a single battery or a lithium battery pack connected in series or parallel. The lithium battery pack usually consists of a plastic case, PCM, cell, output electrode, bonding sheet, and other insulating tape, double-coating tape, etc.

  • Lithium cell: The core of a finished battery

  • PCM: Protection functions of over charge, over discharge, over current, short circuit, NTC intelligent temperature control.

  • Plastic case: the supporting skeleton of the entire battery; Position and fix the PCM; Carry all other non-case parts and limit.

  • Terminal lead: It can provide a variety of terminal wire charging and discharging interface for a variety of electronic products, energy storage products and backup power.

  • Nickel sheet/bracket: Connection and fixing component of the cell

Lithium Battery Pack Structure

Lithium Battery Series and Parallel Connection

Due to security reasons, lithium ion batteries need an external PCM used for battery monitoring for each battery. It is not recommended to use batteries in parallel. If connect in parallel, make sure the consistency of the battery parameters (capacity, internal resistance, etc.), the other batteries in series need to have consistent parameters, otherwise, the performance of the battery pack can be much worse than the performance of a single cell.

Lithium Battery Series and Parallel Connection

Lithium battery matching criteria
voltage difference ≤ 10 mv, impedance difference ≤ 5 mΩ, capacity difference ≤20mA

The purpose of lithium battery matching is to ensure that every cell in the battery has consistent capacity, voltage and internal impedance, because inconsistent performances will make lithium battery have various parameters during using. Voltage imbalance will happen. After a long run, the battery will overcharge, over discharge, capacity lost, or even fire to explode.

Two Lithium Batteries Connected in Series (7.4V Lithium Battery)

Two Lithium Batteries Connected in Series
model 18650-2S1P 18650-2S1P 18650-2S2P 18650-2S3P
Voltage 7.4V 7.4V 7.4V 7.4V
Capacity 2200/2500/3000mAh 2200/2500/3000mAh 6000mAh 9000mAh
Dimension 18*105mm 18*36*65mm 37*37*66mm 37*55*66mm
Weight 90g 90g 180g 270g

Three Lithium Batteries Connected in Series (11.1V Lithium Battery)

Three Lithium Batteries Connected in Series
Series and Parallel Connection Mode 18650-3S1 P triangle 18650-3S1P in-line 18650-3S2P 18650-3S3P
Voltage 11.1V 11.1V 11.1V 11.1V
Capacity 2200/2500/3000mAh 2200/2500/3000mAh 6000mAh 9000mAh
Dimension 66.5*36.6*36.6mm 69.8*55.7*18.8mm 66.8*55.0*40.8mm 60.6*68.0*56.1mm
Weight 155g 158g 285g 425g

Four Lithium Batteries Connected in Series (14.8V Lithium Battery)

Four Lithium Batteries Connected in Series
Series and Parallel Connection Mode 18650-4S1P square 18650-4S1P In-line 18650-4S2P
Voltage 14.8V 14.8V 14.8V
Capacity 2200/2500/3000mAh 2200/2500/3000mAh 6000mAh
Dimension 69.6*37.7*37.7mm 69.3*73.4*17.6mm 70.6*74.2*37.1mm
Weight 181g 191g 371g

Six Lithium Batteries Connected in Series (22.2V Lithium Battery)

Six Lithium Batteries Connected in Series
Series and Parallel Connection Mode 18650-6S1P In-line 18650-6S2P 18650-6S3P
Voltage 25.2V 25.2V 25.2V
Capacity 2000/3000mAh 6000mAh 9000mAh
Dimension 114*72*22mm 114*72*41mm 114*72*60mm
Weight 303g 570g 835g

The length of the plug and lead of the lithium battery pack can be customized according to the customer's electrical equipment.

Lithium Battery Wire/Terminal

We all know that lithium battery voltage increases after series connection, capacity increases after parallel connection, then how to calculate a lithium battery quantity of series or parallel connection, and how many cells?

Before the calculation, we need to know which cell specification of the battery pack is adopted for the assembly, because different cells have different voltage and capacity. The cell quantity of series and parallel connection required to assemble a specific lithium battery pack varies. The common lithium cell types on the market are:3.7V LiCoO2, 3.6V ternary, 3.2V LFePO4, 2.4V lithium titanate. The capacity is different because of the cell size, material and manufacturers.

Take 48V 20Ah Lithium Battery Pack for Example

  • Suppose the size of the single cell used is 18650 3.7V 2000mAh
  • Cell quantity of series connection: 48V/3.7V=12.97. That is 13 cells in series.
  • Cell quantity of parallel connection: 20Ah/2Ah=10. That is 10 cells in parallel.
  • The whole battery pack is 13 cells in series × 10 cells in parallel = 130 cells

Commonly Used Lithium Battery Connected in series

Nominal Voltage Battery Category Common Quantity of series connection Charging Voltage
12V 3.7V LiCoO2 3S 12.6V
3.2V LiFePO4 4S 14.6V
24V 3.7V LiCoO2 7S 29.4V
3.2V LiFePO4 8S 29.2V
36V 3.7V LiCoO2 10S 42.0V
3.7V LiCoO2 11S 46.2V
3.2V LiFePO4 11S 40.2V
3.2V LiFePO4 12S 43.8V
48V 3.7V LiCoO2 13S 54.6V
3.7V LiCoO2 14S 58.8V
3.2V LiFePO4 15S 58.8V
3.2V LiFePO4 16S 58.8V
60V 3.7V LiCoO2 17S 71.4V
3.2V LiFePO4 20S 73.0V
72V 3.7V LiCoO2 20S 84.0V
3.2V LiFePO4 24S 87.6V

Lithium Battery Assembly Process

18650-3S6P/11.1V/15600mAh Lithium Battery Assembly Process

  • Cell Capacity Grading

    Cell Capacity Grading
    Capacity Difference≤30mAh
    After capacity grading, stay still for 48-72h and then distribute.

  • Voltage Internal Impedance Sorting and Matching

    Voltage Internal Impedance Sorting and Matching
    Voltage Difference≤5mV
    Internal Impedance Difference≤5mΩ 8 cells with similar voltage internal impedance are distributed together.

  • Cell Spot Welding

    Cell Spot Welding
    The use of formed nickel strip eliminates the problems of spurious joint, short circuit, low efficiency and uneven current distribution

  • Welded PCM

    Welded PCM
    Make sure that the circuit board has no leakage components, and the components have no defective welding.

  • Battery Insulation

    Battery Insulation
    Paste the fibre, silicone polyester tape for insulation.

  • Battery Pack Aging

    Battery Pack Aging
    For the quality of the battery, improve the stability, safety and service life of the lithium battery.

  • PVC Shrink Film

    PVC Shrink Film
    Position the two ends after heat shrinking,
    then heat shrink the middle part.
    Put PVC film in the middle. No whiten after stretching. No hole.

  • Finished Product Performance Test

    Finished Product Performance Test
    Internal Impedance:≤150mΩ
    Charge-discharge and overcurrent performance test.

  • Battery Code-spurting

    Battery Code-spurting
    Code-spurting cannot be skewed, and it needs legible handwriting

Precautions for Lithium Batteries in Series and Parallel

  • Don't use batteries with different brands together.
  • Do not use batteries with different voltages together.
  • Do not use different capacities or old and new lithium batteries together.
  • Batteries with different chemical materials cannot be used together, such as nickel metal hydride and lithium batteries.
  • Replace all batteries when electricity is scarce.
  • Use the lithium battery PCM with corresponding parameters.
  • Choose batteries with consistent performance. Generally, distributing of lithium battery cells is required for series and parallel connection. Matching standards: voltage difference≤10mV, impedance difference ≤5mΩ, capacity difference ≤20mA

Due to the consistency issue of lithium batteries, when the same system (such as ternary or lithium iron) is used for series or parallel connection, it is also necessary to select the batteries with the same voltage, internal impedance and capacity for matching. Batteries with different voltage platforms and different internal impedance used in series will cause a certain battery to be fully charged and discharged first in each cycle. If there is a PCM and no fault occurs, the capacity of the whole battery will be reduced. If there is no PCM, the battery will be overcharged or over discharged, which will damage the battery.

Full voltage not available

If different capacities or old and new lithium batteries are used together, there may be leakage, zero voltage and other issues, because during the charging process, capacity differences make some batteries overcharge, some batteries not, while during discharge process, high capacity batteries do not run out of power, but low capacity batteries over discharge. In such a vicious cycle, the batteries will be damaged by leakage or low (zero) voltage.

Full Capacity not available

To assemble lithium batteries, connect them in parallel or in series first?

  • Topological Structure of Lithium Battery Connected in Series and Parallel
  • The typical connection modes of a lithium battery pack are connecting first in parallel and then in series, first in series and then in parallel, and finally, mixing together.
    Lithium battery pack for pure electric buses is usually connected first in parallel and then in series.
    Lithium battery pack for power grid energy storage is tend to be connected first in series and then in parallel.

    • First Parallel and Then Series of Power Battery Module Topological Structure
      First Parallel and Then Series of Power Battery Module Topological Structure
    • First Series and Then Parallel of Power Battery Module Topological Structure
      First Series and Then Parallel of Power Battery Module Topological Structure
    • First Parallel, Then Series and Parallel Again of Power Battery Module Topological Structure
      First Parallel, Then Series and Parallel Again of Power Battery Module Topological Structure
    • Advantages of Lithium Batteries First Connected in Parallel and Then in Series
      If a lithium battery cell automatically exits, except the capacity reduction, it does not affect parallel connection;
      In parallel connection, a short circuit of a lithium battery cell may cause short circuit due to large current, which is usually avoided by using fuse protection technology.

    • Disadvantages of Lithium Batteries First Connected in Parallel and Then in Series
      The difference of internal impedance and heat dissipation will affect the cycle life of parallel connection lithium battery.

    • Advantages of Lithium Batteries First Connected in Series and Then in Parallel
      First connecting the batteries in series according to the capacity, for example, 1/3 of the whole battery capacity are connected in series, and then connecting the rest in parallel, will reduce the failure probability of high-capacity lithium battery modules. First series and then parallel connection help the consistency of the lithium battery pack.

    • From the perspective of the reliability of the lithium battery connection, the development trend of voltage inconsistency and the influence of performance, the connection mode of first parallel and then series is better than that of first series and then parallel, and the topology structure of first series and then parallel lithium battery is conducive to the detection and management of each lithium battery cell in the system.

    Lithium Batteries Charging in Series and Parallel

    At present, lithium battery tends to be charged in series, which is mainly due to its simple structure, low cost and easy realization. But as a result of different capacity, internal impedance, aging characteristics and self-discharge performance, when charge lithium battery in series, battery cell with the smallest capacity will be fully charged first, and at this point, the other battery cell is not full of electricity. If continue to charge in series, the fully charged battery cell may be overcharge.

    Lithium Battery overcharge will damage the battery performance, and even lead to explosion and injuries, therefore, to prevent battery cell overcharging, lithium battery has equipped with Battery Management System (BMS). The Battery Management System has overcharge protection for every single lithium battery cell, etc. When charging in series, if the voltage of a single lithium battery cell reaches the overcharge protection voltage, the battery management system will cut off the whole series charging circuit and stop charging to prevent the single lithium battery cell from being overcharged, which will cause other lithium batteries unable to be fully charged.

    In parallel charging of lithium batteries, each lithium ion battery needs equalizing charge, otherwise, the performance and life of the whole lithium ion battery pack will be affected. Common charging equalization technologies include: constant shunt resistance equalizing charge, on-off shunt resistance equalizing charge, average battery voltage equalizing charge, switch capacitor equalizing charge, step-down converter equalizing charge, inductance equalizing charge, etc.

    Several problems need to be paid attention to in parallel charging of lithium batteries:

    • Lithium batteries with and without PCM cannot be charged in parallel. Batteries without PCM can easily be damaged by overcharging.
    • Batteries charged in parallel usually need to remove the built-in PCM of the battery and use a unified battery PCM.
    • If there is no PCM in parallel charging battery, the charging voltage must be limited to 4.2V and 5V charger cannot be used.

    After lithium ion batteries connecting in parallel, there will be a charging protection chip for lithium battery charging protection. Lithium battery manufacturers have fully considered the change characteristics of lithium battery in parallel before battery production. The above requirement of current design and choice of batteries are very important, so that users need to follow the instructions of parallel lithium batteries charging step by step, so as to avoid the possible damage for incorrect charge.

    • Special charger must be used for lithium battery, or battery may not reach saturation state, affecting its performance.
    • Before charging the lithium battery, it does not need to discharge completely.
    • Do not keep the charger on the socket for a long time. Remove the charger as soon as the battery fully charged.
    • Batteries shall be taken out of electric appliances that have not been used for a long time and stored after they are fully discharged.
    • Do not plug the anode and cathode of the battery into the opposite direction, otherwise, the battery will swell or burst.
    • Nickel charger and lithium charger cannot be used together.

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