The Difference between Series Connection And Parallel Connection of Batteries

Definition of series and parallel connection of lithium batteries: Due to the limited voltage and capacity of individual cells, series and parallel combinations are required in practical applications to achieve higher voltage and capacity to meet the actual power supply requirements of devices.

Series connection of lithium batteries: Voltage is added, capacity remains unchanged, and internal resistance increases.

Parallel connection of lithium batteries: Voltage remains unchanged, capacity is added, internal resistance decreases, and the power supply time is extended.

Series-parallel connection of lithium batteries: There are both parallel and series combinations in the middle of the battery pack, increasing the voltage and capacity.

Series voltage: 3.7V single cells can be assembled into battery packs with voltages of 3.7*(N)V (N: number of single cells), such as 7.4V, 12V, 24V, 36V, 48V, 60V, 72V, etc.

Parallel capacity: 2200mAh single cells can be assembled into battery packs with capacities of 2*(N)mAh (N: number of single cells), such as 4400mAh, 6600mAh, 8800mAh, 5Ah, 10Ah, 20Ah, 30Ah, 50Ah, 100Ah, etc.

Lithium battery pack (Pack): Lithium battery pack refers to the processing, assembly, and packaging of lithium battery packs. The process of assembling lithium cells into groups is called PACK, which can be a single battery or a series-parallel lithium battery pack, etc. A lithium battery pack typically consists of a plastic shell, a protection board, cells, output electrodes, connection tabs, and other insulating tapes and double-sided tapes.

Lithium cell: The core part of a finished battery.

Protection board: It provides functions such as overcharge, overdischarge, overcurrent, short circuit, and NTC temperature control intelligent protection.

Plastic shell: The supporting framework of the entire battery; it positions and fixes the protection board; it carries all non-shell components and limits their movement.

Terminal leads: Various terminal wires for charging and discharging interfaces can be provided for use in various electronic products, energy storage products, and backup power supplies.

Nickel sheet/support: The connection and fixation components of the cells.

Series-parallel combination of lithium batteries: Due to safety reasons, lithium batteries must be externally connected to a protection board for monitoring each cell. Generally, parallel use of batteries is not recommended during use. If parallel connection is necessary, it is essential to ensure the consistency of each battery's parameters (capacity, internal resistance, etc.). Additionally, the batteries used in series must also have consistent parameters; otherwise, the performance of the battery pack will be much worse than that of a single cell.

The purpose of lithium battery matching is to ensure that the capacity, voltage, internal resistance, and performance of each battery in the battery pack are consistent. Inconsistency will cause the parameters of the lithium battery pack to drift further apart during use, leading to voltage imbalance, overcharging, overdischarging, and inability to fully utilize the capacity, which may result in explosion and fire hazards.

Series-parallel combination methods of lithium batteries

3-series lithium battery combination (11.1V lithium battery)

4-series lithium battery combination (14.8V lithium battery)

6-series lithium battery combination (22.2V lithium battery)

Lithium battery pack wires/terminals

The plugs and lead lengths of lithium battery packs can be customized as needed, based on the customer's electrical equipment.

Calculation of series-parallel lithium battery packs: We know that the voltage increases in series and the capacity increases in parallel. How do we calculate how many series and parallel combinations a lithium battery pack has and how many cells it consists of? Before calculating, we need to know the specifications of the cells used to assemble the battery pack, as different cells have different voltages and capacities, and the required series and parallel numbers are different. Common types of lithium cells on the market include 3.7V lithium cobalt oxide, 3.6V ternary, 3.2V lithium iron phosphate, and 2.4V lithium titanate, with capacities varying depending on the size, material, and manufacturer of the cells.

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

Assume the specification of the single cell used is 18650 3.7V 2000mAh Number of cells in series: 48V / 3.7V = 12.97, which means 13 cells in series (13 cells connected in series)

Number of cells in parallel: 20Ah / 2Ah = 10, which means 10 cells in parallel (10 cells connected in parallel)

The entire battery pack consists of 13 cells in series × 10 cells in parallel = 130 cells

Common lithium battery pack series combinations

Lithium battery assembly process

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

Precautions for lithium battery series and parallel connection

· Do not use batteries of different brands together

· Do not use batteries of different voltages together

· Do not mix batteries of different capacities or new and old lithium batteries together

· Batteries of different chemical materials cannot be mixed, such as nickel-metal hydride and lithium batteries

· When the battery is low on power, replace all the batteries

· Use lithium battery protection boards with corresponding parameters

· Select batteries with consistent performance. Generally, lithium batteries need to be matched when used in series and parallel. The matching standards are: voltage difference ≤ 10mV, internal resistance difference ≤ 5mΩ, capacity difference ≤ 20mA

Series connection of lithium batteries with different voltages: Due to consistency issues, even within the same system (such as ternary or lithium iron phosphate), when lithium batteries are connected in series and parallel to form a group, batteries with consistent voltage, internal resistance, and capacity should be selected. If batteries with different voltage platforms and internal resistances are connected in series, a certain battery will be fully charged and discharged first in each cycle. If there is a protection board and it does not fail, the overall capacity of the battery pack will decrease. Without a protection board, it will inevitably lead to overcharging or overdischarging of that battery, causing damage.

Parallel connection of lithium batteries with different capacities

If batteries of different capacities or new and old lithium batteries are mixed together, leakage and zero voltage may occur. This is because during charging, the capacity difference causes some batteries to be overcharged and some not fully charged. During discharging, some batteries with higher capacity are not fully discharged, while those with lower capacity are overdischarged. This vicious cycle causes damage to the batteries, resulting in leakage or low (zero) voltage.

Should lithium batteries be connected in parallel or series first?

Typical connection methods for lithium battery packs include connecting in parallel first and then in series, connecting in series first and then in parallel, and hybrid connection. Pure electric buses usually use the connection method of connecting in parallel first and then in series.

Lithium battery packs for grid energy storage often use the connection method of connecting in series first and then in parallel.

Advantages of connecting lithium batteries in parallel first and then in series: When a single cell fails, it automatically exits, only reducing the capacity without affecting the use after parallel connection; when a single cell in the parallel circuit shorts, the current in the parallel circuit becomes very large, and usually a fuse protection technology is added to avoid this.

Disadvantages of connecting lithium batteries in parallel first and then in series: Due to the difference in internal resistance of the lithium battery cells and uneven heat dissipation, the cycle life of the lithium battery pack after parallel connection will be affected.

Advantages of connecting lithium batteries in series first and then in parallel: According to the capacity of the single cell, they are connected in series first, such as 1/3 of the total capacity, and then connected in parallel, reducing the failure probability of large-capacity lithium battery modules; connecting in series first and then in parallel is very helpful for the consistency of the lithium battery pack. From the perspective of the reliability of the lithium battery pack connection and the development trend and performance impact of voltage inconsistency, the connection method of connecting in parallel first and then in series is superior to the connection method of connecting in series first and then in parallel, and the lithium battery topology of connecting in series first and then in parallel is conducive to the detection and management of each single cell in the system.

Charging of lithium batteries in series and parallel

Charging of lithium batteries in series: Currently, the charging of lithium battery packs generally adopts series charging, mainly because the series charging method has a simple structure, low cost, and is relatively easy to implement. However, due to the differences in capacity, internal resistance, degradation characteristics, self-discharge and other performance aspects among individual lithium-ion cells, when charging a lithium-ion battery pack in series, the cell with the smallest capacity in the pack will be fully charged first, while the other cells are still not fully charged. If the charging continues, the fully charged cell may be overcharged. Overcharging of lithium-ion cells can seriously damage the battery's performance and may even cause an explosion, resulting in personal injury. Therefore, to prevent overcharging of individual cells, lithium-ion battery packs are generally equipped with a battery management system (BMS) during use. The BMS protects each individual cell from overcharging. When charging in series, if the voltage of one cell reaches the overcharge protection voltage, the BMS will cut off the entire series charging circuit and stop charging to prevent this cell from being overcharged. However, this will cause other cells to not be fully charged.

Lithium-ion battery parallel charging: When charging lithium-ion batteries in parallel, each cell should be ensured to be charged evenly; otherwise, it will affect the performance and lifespan of the entire battery pack during use. Commonly used balancing charging techniques include: constant shunt resistor balancing charging, on-off shunt resistor balancing charging, average battery voltage balancing charging, switch capacitor balancing charging, buck converter balancing charging, and inductor balancing charging, etc.

Several issues need to be noted when charging lithium-ion batteries in parallel:

1. Lithium-ion batteries with protection boards and those without cannot be charged in parallel. Batteries without protection boards are prone to overcharging and damage.

2. Batteries charged in parallel usually need to remove their own protection boards and use a unified protection board.

3. If there is no lithium-ion battery protection board when charging in parallel, the charging voltage must be limited to 4.2V and a 5V charger cannot be used.

After lithium-ion batteries are connected in parallel, there will be a charging protection chip to protect the batteries. Lithium-ion battery manufacturers have fully considered the changes after batteries are connected in parallel when making them and have designed the current and selected the cells according to the above requirements. Therefore, users need to follow the instructions of the parallel lithium-ion battery pack step by step when charging to avoid possible damage to the battery caused by incorrect charging.

Lithium-ion battery charging precautions:

1. Lithium-ion batteries must use dedicated chargers; otherwise, they may not reach the saturated state and affect their performance.

2. Lithium-ion batteries do not need to be completely discharged before charging.

3. Do not leave the charger plugged in for a long time. Once the battery is fully charged, it should be removed from the charger as soon as possible.

4. Batteries should be removed from long-unused electrical appliances, discharged, and stored.

5. Do not insert the battery's positive and negative terminals in reverse; otherwise, it may cause the battery to swell or rupture. Nickel chargers and lithium chargers cannot be used interchangeably.