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Precautions for Battery Series And Parallel Connection

Precautions for Battery Series And Parallel Connection

November 11, 2022

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Precautions for Lithium Batteries in Series And Parallel

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


Lithium Batteries of Different Voltages are Connected in Series

  Due to the consistency problem of lithium batteries, if the same system (such as ternary or iron-lithium) is used in parallel and series, it is also necessary to select the same voltage, internal resistance, and capacity for matching. Batteries with different voltage platforms and different internal resistances are used in series, which will cause a certain battery to be fully charged first and then discharged in each cycle. If there is a protection board and there is no failure, the capacity of the whole group will be reduced. Damage to the battery caused by overcharging or overcharging.


Parallel Connection of Lithium Batteries of Different Capacities

  If different capacities or old and new lithium batteries are used together, there may be liquid leakage, zero voltage and other phenomena. 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 overdischarged. Such a vicious cycle, the battery gets damaged and leaks or has low (zero) voltage.


What is the difference between the sequence of lithium batteries in series and parallel?


1.The advantages of lithium batteries in parallel first and then in series

The lithium battery will automatically exit when it fails, except for the capacity reduction, it will not affect the use after parallel connection;
When a lithium battery cell in parallel is short-circuited, the parallel circuit current is very large, which is usually avoided by adding fusing protection technology.


2.Disadvantages of lithium batteries in parallel first and then in series

Due to the difference in the internal resistance of the lithium battery cells and the uneven heat dissipation, the cycle life of the lithium battery pack after being connected in parallel will be affected.


3.The advantages of lithium batteries in series first and then in parallel

According to the capacity of the lithium battery cells, first connect in series, such as 1/3 of the capacity of the entire group, and finally connect in parallel, which reduces the failure probability of large-capacity lithium battery modules; first series and then parallel are very helpful for the consistency of lithium battery packs.


Lithium Battery Series and Parallel Charging


1.Lithium Battery Charging in Series

At present, the charging of lithium battery packs generally adopts series charging, mainly because the series charging method is simple in structure, low in cost, and relatively easy to implement. However, due to the differences in capacity, internal resistance, attenuation characteristics, self-discharge and other performances between single lithium batteries, when the lithium battery pack is charged in series, the single lithium battery with the smallest capacity in the battery pack will be fully charged first. 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 of a lithium battery will seriously damage the performance of the battery, and may even cause an explosion and cause personal injury. Therefore, in order to prevent overcharging of a single lithium battery, a lithium battery pack is generally equipped with a battery management system (Battery Management System for short). BMS), 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 overcharged. The battery cannot be fully charged.


2.Parallel Charging of Lithium Batteries

When lithium batteries are charged in parallel, each lithium battery should be charged equally, otherwise the performance and life of the entire group of lithium batteries will be affected during use. Commonly used equalization charging techniques include: constant shunt resistance equalization charging, on-off shunt resistance equalization charging, average battery voltage equalization charging, switched capacitor equalization charging, buck converter equalization charging, inductor equalization charging, etc.


There are several issues that need to be paid attention to when charging lithium batteries in parallel:
- Lithium batteries with and without a protective plate cannot be charged in parallel. Batteries without protective plates are easily damaged by overcharging.
- Batteries charged in parallel usually need to remove the battery protection board and use one battery protection board uniformly.
- 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.


After the lithium battery is connected in parallel, there will be a charging protection chip to protect the lithium battery from charging. The lithium battery manufacturer has fully considered the change characteristics of the parallel lithium battery when making the parallel lithium battery, and the current design and cell selection are also carried out according to the above requirements. Therefore, users need to follow the instructions of the parallel lithium battery to charge step by step to avoid possible damage to the battery caused by incorrect charging.


3.Lithium Battery Charging Precautions
- Lithium batteries must use special chargers, otherwise they may not reach saturation and affect their performance.
- Lithium batteries do not need to be fully discharged before charging.
- Do not leave the charger plugged into the socket for a long time, and remove the battery from the charger as soon as possible after the battery is fully charged.
- The battery should be removed from the electrical appliance that will not be used for a long time, and the battery should be emptied and stored.
- Do not insert the positive and negative terminals of the battery in reverse, otherwise the battery will swell or rupture.
- Nickel chargers and lithium chargers cannot be mixed.

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