Precautions for Lithium Batteries in Series and Parallel

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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

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

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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