Cells in Series and Parallel

Cells in Series and Parallel obtain a higher voltage and capacity in order to meet the actual power supply requirements of the equipment. Due to the limited voltage and capacity of single cells.

• Lithium cells in series: voltage is summed, capacity remains the same, and internal resistance increases.
• Lithium batteries in parallel: the voltage remains the same, the capacity is added, the internal resistance is reduced and the power supply time is extended.
• Lithium cells series and parallel connection: There are both parallel and series combinations in the middle of the battery pack so that the voltage is increased and the capacity is increased.
• Increase voltage in series: 3.7V single cells can be assembled into battery packs of 3.7*(N)V (N: number of single cells) such as 7.4V, 12V, 24V, 36V, 48V, 60V, 72V, etc. as required.
• Increased capacity in parallel: 2000mAh single cells can be assembled into packs with a capacity of 2*(N)Ah (N: number of single cells) such as 4000mAh, 6000mAh, 8000mAh, 5Ah, 10Ah, 20Ah, 30Ah, 50Ah, 100Ah, etc. according to requirements.

Lithium Battery Pack

Lithium battery PACK refers to the processing and assembly of lithium battery packs and packaging. The process of assembling lithium cells into a group is called PACK, which can be a single cell or cells in series and parallel lithium battery pack, etc. Lithium Battery Pack usually consists of plastic shell, protection plate, battery cell, output electrode, connection with touch piece, and other insulating tape, double-sided tape, etc.

• Lithium cell: the core part of the battery pack.
• Protection plate: plays the function of overcharge, overdischarge, overcurrent, short circuit, NTC temperature control intelligent protection, etc.
• Plastic shell: support skeleton of the whole battery; positioning and fixing of the protection plate; carrying all other non-shell parts and limiting the position.
• Terminal leads: can provide a variety of terminal line charging and discharging interface for various electronic products, energy storage products, backup power supply.
• Nickel tabs/brackets: connecting and fixing components for electric cells.

Lithium cells in series and parallel combinations

Due to the safety of lithium batteries, an external protection board must be used for the monitoring of each cell, and the use of cells in parallel is generally not recommended. If the batteries are connected in parallel, the consistency of the parameters (capacity, internal resistance, etc.) must be ensured, in addition to the consistency of the parameters of the cells used in series, otherwise the performance of the battery pack will be much poorer than the performance of the individual cells.

Lithium cell pairing standard :

Voltage difference ≤ 10mV, internal resistance difference ≤ 5mΩ, capacity difference ≤ 20mAh

The purpose of the lithium cell pairing is to ensure that the capacity, voltage, internal resistance, and effect of each cell in the battery pack are consistent, inconsistency will lead to the use of the lithium battery pack in the process of various parameters further and farther away, resulting in voltage imbalance, over time will be over-charged, over-discharged, capacity loss, causing the risk of explosion and fire.

How to connect Lithium Cells in Series and Parallel?

2 series lithium battery cells combination (7.4V lithium battery)

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

3 series lithium battery cells combination (11.1 lithium battery)

Cells in Series and Parallel Method	18650-3S1P 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

4 series lithium battery cells combination (14.8 lithium battery)

Series-Parallel Method	18650-4S1P Cubic	18650-4S1P Inline	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

6  series lithium battery cells combination (22.2 lithium battery)

Cells in Series and parallel Method	18650-6S1P	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

Lithium battery pack wire/terminal

The lithium battery pack plug and lead length can be customised at will, depending on the customer’s electrical equipment.

How to calculate Lithium battery cells in series and parallel?

We all know that the series voltage of lithium batteries increases, and the parallel capacity increases, so how to calculate how many strings and parallels a lithium battery pack is composed of?

Before performing the calculation, we need to know what specifications of battery cells are used for the assembly of this lithium battery pack, because different battery cells have different voltage capacities, and the number of serial parallels required to assemble a lithium battery pack of a specific specification is different. The common types of lithium batteries on the market are 3.7V for lithium cobaltate, 3.6V for ternary, 3.2V for lithium iron phosphate, and 2.4V for lithium titanate(3V Battery). The capacity varies with the size, material and manufacturer of the battery.

Take 48V 20Ah lithium battery pack as an example：

• Assume that the specification of the single cell used is 18650 3.7V 2000mAh
• Number of cells in parallel: 20Ah/2Ah=10, that is, 10 in parallel (10 cells in parallel)
• The whole set of batteries is 13 strings x 10 cells = 130 cells

Common lithium battery series combination

Nominal Voltage	Battery Type	Common Number of Strings	Charging Voltage
12V	3.7V Lithium cobaltate	3S	12.6V
	3.2V  lithium iron	4S	14.6V
24V	3.7V  Lithium cobaltate	7S	29.4V
	3.2V lithium iron	8S	29.2V
36V	3.7V  Lithium cobaltate	10S	42.0V
	3.7V  Lithium cobaltate	11S	46.2V
	3.2V lithium iron	11S	40.2V
	3.2V lithium iron	12S	43.8V
48V	3.7V  Lithium cobaltate	13S	54.6V
	3.7V  Lithium cobaltate	14S	58.8V
	3.2V lithium iron	15S	58.8V
	3.2V lithium iron	16S	58.8V
60V	3.7V  Lithium cobaltate	17S	71.4V
	3.2V lithium iron	20S	73.0V
72V	3.7V  Lithium cobaltate	20S	84.0V
	3.2V lithium iron	24S	87.6V

Lithium battery assembly process

18650-3S6P/11.1V/15600mAh Lithium battery assembly process

Capacity division

Capacity difference≤30mAh

After the capacity division is completed, let it stand for 48-72H and then make a group

Voltage internal resistance selection matching group

Voltage difference≤5mV

Internal resistance difference ≤ 5mΩ, 8 batteries with similar voltage internal resistance are paired into a group

Cell Spot Welding

Use formed nickel strips to eliminate problems such as virtual welding, short circuit, low efficiency, and uneven current distribution.

Welding protection plate

Confirm that the circuit board has no leaking components, components have no defects such as solder joints and continuous soldering

Battery insulation

Paste Kuaba paper and Mala glue for insulation

Battery pack aging

Determine the quality of the battery pack to ensure the stability, safety and service life of the lithium battery pack during use

PVC shrink film

First heat shrink both ends to position, then blow the middle heat shrink, PVC film centered, no whitening, leakage.

Finished product performance test

Voltage: 10.8～11.7V

Internal resistance: ≤150mΩ

Charge and discharge, over-current performance test

Battery coding

The code should not be skewed and the handwriting should be clear

What are the Precautions for Cells series and parallel?

• Do not use batteries of different brands together.
• Do not use batteries of different voltages together.
• Do not mix old and new lithium batteries with different capacities.
• Batteries of different chemical materials cannot be mixed, such as NiMH and lithium batteries.
• When the battery power is low, replace all the batteries.
• Use a lithium battery protection board with corresponding parameters.
• Choose batteries with consistent performance. Generally, lithium battery cells need to be paired for cells in series and parallel use. Matching standards: voltage difference ≤ 10mV, internal resistance difference ≤ 5mΩ, capacity difference ≤ 20mA

Can Lithium batteries with different voltages be connected in series?

Due to the consistency problem of lithium batteries, if they are connected in series under the same system (such as ternary or iron-lithium), it is also necessary to select groups with the same voltage, internal resistance, and capacity. The series connection of batteries with different voltage platforms and different internal resistances will cause a certain battery to be fully charged and discharged first in each cycle. If there is a protection board and there is no failure, the capacity of the entire group will be reduced. Cause the battery to be overcharged or overdischarged and cause damage.

Can Lithium batteries with different capacities be connected in parallel?

If different capacities or new and old lithium batteries are used together, liquid leakage and zero voltage may occur. This is due to the difference in capacity during the charging process, which causes some batteries to be overcharged during charging, and some batteries are not fully charged. When discharging, some batteries with high capacity are not fully discharged, while those with low capacity are over-discharged. In such a vicious cycle, the battery is damaged and leaks or low (zero) voltage.