How to Calculate the Number of Lithium Batteries in Series or Parallel?

When to Connect Lithium Batteries in Series or Parallel？

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

Before performing the calculation, we need to know what specifications of batteries are used in the assembly of this lithium battery pack. Because different batteries have different voltage and capacity, they are assembled into lithium battery packs of specific specifications, and the number of series and parallel required is different.The common types of lithium batteries on the market are 3.7V for lithium cobalt oxide, 3.6 V for ternary, 3.2V for lithium iron phosphate, and 2.4V for lithium titanate. The capacity varies depending on the cell size, material, and manufacturer.

Due to the limited voltage and capacity of single batteries, series and parallel combinations are required in actual use to obtain higher voltage and capacity in order to meet the actual power supply needs of the equipment.

3 ways to connect Lithium Batteries

series: the voltage is added, the capacity remains the same, and the internal resistance increases.
parallel: the voltage remains the same, the capacity is added, the internal resistance is reduced, and the power supply time is extended.
series and parallel: There are both parallel and series combinations in the middle of the battery pack, which increases the voltage and increases the capacity.
Series voltage: 3.7V single battery can be assembled into a battery pack with a voltage of 3.7*(N)V as needed (N: Number of single batteries) Such as 7.4V, 12V, 24V, 36V, 48V, 60V, 72V, etc.
Parallel voltage: The 2000mAh single battery can be assembled into a battery pack with a capacity of 2*(N)Ah as needed (N: number of single batteries) Such as 4000mAh, 6000mAh, 8000mAh, 5Ah, 10Ah, 20Ah, 30Ah, 50Ah, 100Ah and so on.

Take 48V 20Ah lithium battery pack as an example

Assuming that the single cell specification used is 18650 3.7V 2000mAh
Number of parallel cells: 20Ah/2Ah=10, that is, 10 parallel (10 cells are connected in parallel to increase battery capacity)
Number of series: 48V/3.7V=12.97, that is, 13 parallel (13 batteries need to be connected in series to increase the voltage)
The whole set of batteries is 13 strings ×10 batteries =130 batteries

Parallel to increase capacity, the voltage remains the same
For example: 2 and connect to 3.2V20AH
3 parallel power supply 3.2V30AH
And so on

Series and combine to increase voltage and capacity at the same time
For example: 4 strings of 2 parallel power supply 12V20AH
8 strings of 4 parallel power supply 24V40AH
And so on

Lithium Batteries PACK

Lithium battery PACK refers to the processing, assembly and packaging of lithium battery packs.The process of assembling lithium batteries into groups is called PACK, which can be a single battery or a lithium battery pack in series and parallel.Lithium battery packs are usually composed of plastic housings, protective plates, batteries, output electrodes, connecting pads, and other insulating tape, double-sided tape, etc.

Assembly Process

Lithium battery: the core part of the finished battery
Protection board: It has functions such as overcharge, over discharge, overcurrent, short circuit, and intelligent protection of NTC temperature control.
Plastic shell: the supporting skeleton of the entire battery; positioning and fixing of the protective plate; carrying all other non-shell parts and limiting positions.
Terminal leads: Various terminal line charge and discharge interfaces can be provided for various electronic products, energy storage products, and backup power supplies.
Nickel sheet/bracket: the connection and fixing components of the battery cell.

: LITHIUM BATTEYIES PACK

There is a “barrel effect” in series. During the charging process, the charging current is the same. The low-capacity single battery will be the first to be fully charged, but the overall voltage of the battery pack does not reach the charging cut-off voltage. However, the charging voltage of the charger is set according to the total voltage of the battery pack. Continued charging will cause the fully charged single battery to be overcharged, while the large-capacity single battery is in an undercharged state. In the long run, the service life of the battery pack will be affected to a certain extent.Therefore, try to achieve the same parameters in all aspects (the factors of the battery itself and the factors of the assembly process)
When the single batteries in the battery pack are connected in parallel, there is a problem of mutual internal consumption between the single batteries.Since the internal resistance of each single battery is different in size, when the internal resistance deviation of the battery occurs during use, the battery with large self-discharge will drag down the battery with small self-discharge, which seriously reduces the charge and discharge efficiency of the battery pack.

Lithium battery pairing

Lithium battery pairing standard voltage difference ≤10mV, internal ≤5mW, capacity difference ≤20 mAh
The purpose of lithium battery pairing is to ensure that the capacity, voltage, internal resistance, and effect of each battery in the battery pack are consistent. Inconsistency will cause the various parameters of the lithium battery pack to be pulled farther and farther away during use, resulting in voltage imbalance. After a long time, it will be overcharged, over-discharged, and the capacity cannot be used, causing the risk of explosion and fire.

Precautions for lithium batteries in series and parallel

1.Do not use different brands of batteries together.
2.Do not use batteries of different voltages together.
3.Do not mix different capacities or old and new lithium batteries together.
4.Batteries of different chemical materials cannot be mixed, such as nickel-metal hydride and lithium batteries.
5.When the battery is low, replace all the batteries.
6.Use the lithium battery protection board with the corresponding parameters.

Lithium batteries of different voltages in series

Due to the problem of consistency of lithium batteries, they are grouped in series under the same system (such as ternary or lithium iron), and they also need to be selected with the same voltage, internal resistance, and capacity.Batteries with different voltage platforms and different internal resistance are used in series, which will cause a battery to be fully charged and discharged first in each cycle. If there is a protection board and there is no failure, it will cause the capacity of the entire set to be reduced. Without a protection board, it is bound to cause the battery to be overcharged or overcharged and cause damage.

lithium batteries SERIES OR PARALLEL

Lithium batteries of different capacities are connected in parallel

If different capacities or old and new lithium batteries are mixed together, there may be leakage, zero voltage and other phenomena.This is due to the difference in capacity during the charging process. Some batteries are overcharged when charging, and some batteries are not fully charged. When discharging, there are high-capacity batteries that are not fully charged, while those with low capacity are overcharged.In such a vicious circle, the battery is damaged and leaks liquid or low (zero) voltage.

Lithium battery charging in series and parallel

1. Lithium battery charging in series
At present, the charging of lithium battery packs generally adopts series charging, mainly because the series charging method has a simple structure, low cost and easier to implement. However, due to the differences in capacity, internal resistance, attenuation characteristics, self-discharge and other properties between single lithium batteries, when charging the lithium battery pack in series, the single lithium battery with the smallest capacity in the battery pack will be the first to be fully charged. At this time, other batteries have not been fully charged. If you continue to charge in series, the fully charged single lithium battery may be overcharged.

Overcharging lithium batteries can seriously damage the performance of the battery, and may even cause explosions and cause injuries. Therefore, in order to prevent overcharging of single lithium batteries, lithium battery packs are generally equipped with a battery management system (BMS) when used, which protects each single lithium battery from overcharging through the battery management system.When charging in series, if the voltage of a single lithium battery reaches the overcharge protection voltage, the battery management system will cut off the entire series charging circuit and stop charging to prevent the single battery from being overcharged, which will cause other lithium batteries to be unable to be fully charged.

2. Parallel charging of lithium batteries
When lithium batteries are charged in parallel, each lithium battery should be charged in a balanced manner, otherwise the performance and life of the entire set of lithium batteries will be affected during use.Commonly used balanced charging technologies are: constant shunt resistor balanced charging, on-off shunt resistor balanced charging, average battery voltage balanced charging, switching capacitor balanced charging, step-down converter balanced charging, inductance balanced charging, etc.

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

1.Lithium batteries with and without protection plates cannot be charged in parallel.Batteries without protective plates are easily damaged by overcharging.
2.Batteries that are charged in parallel usually need to remove the protection board that comes with the battery and use a unified battery protection board.
3.If the battery charged in parallel does not have a lithium battery protection board, the charging voltage must be limited to 4.2V, and a 5V charger cannot be used.
4.After the lithium batteries are connected in parallel, there will be a charging protection chip to charge and protect the lithium batteries. When making parallel lithium batteries, lithium battery manufacturers have fully considered the characteristics of the changes after the lithium batteries are connected in parallel, and the current design and battery selection are also carried out in accordance with the above requirements. Therefore, users need to follow the instructions for parallel lithium batteries to charge step by step to avoid damage that incorrect charging may cause to the battery.

Precautions for charging lithium batteries

1.Lithium batteries must use a special charger, otherwise they may not reach the saturation state and affect their performance.
2.The lithium battery does not need to be fully discharged before charging.
3.Do not plug the charger into the socket for a long time, and remove the battery from the charger as soon as possible after it is fully charged.
4.The battery should be removed from electrical appliances that have not been used for a long time, and the battery should be emptied and stored.
5.Do not insert the positive and negative electrodes of the battery in reverse, otherwise it will cause the battery to bulge or break.
6.Nickel chargers and lithium chargers cannot be mixed.