'Classic' error of parallel connection of maintenance-free batteries

by:Power Kingdom     2021-07-08

  'Classic' error in parallel use of maintenance-free batteries


   When using a specific occasion, batteries usually need to be combined. There are two combinations: series and parallel. The series connection is to obtain the required voltage, and the parallel connection is to obtain the required capacity.


   Parallel is usually a parallel structure of a single unit. When the battery capacity increases, the parallel structure is usually adopted. The simple parallel connection method is to connect the positive and negative electrodes of the battery to form a large-capacity monolithic battery, and the terminal voltage is the voltage of a single battery. In the battery pack of electric vehicles, this method has many applications. Batteries can also be connected in series and in series, and the output voltage is the voltage of the battery string. This method is used for communication power supplies.


   In theory, electric batteries can be used in parallel to obtain any battery capacity. In fact, due to the limitation of process conditions, the capacity of lead-acid batteries on the market is usually limited to less than 500 Ah. The minimum commodity battery. In fact, many electrochemical cells are also used in parallel. The electrochemical cell is the smallest physical size that conforms to the principle of electrochemical reaction, and is actually the smallest single cross-section. This kind of understanding of the battery makes it easy to understand why a small damage in the battery will lead to the failure of the entire battery.


  The reliability of parallel batteries The parallel capacity is theoretically the arithmetic sum of the single-stage capacity. Parallel structure also uses redundant structure, which will increase the effect of safety. This is the technical basis for many electrical designers to choose batteries, but this ideal state does not actually exist.


   In the parallel structure of the battery, the battery will be connected in parallel with multiple individual batteries, and one of the batteries may be damaged, which may cause the entire battery to be damaged. 'The more parallel parts, the lower the reliability of the battery. The joint damage process of parallel batteries is: in Figure 1, due to the failure of the middle three single cells, the terminal voltage gradually decreases. Due to the clamping effect of the parallel circuit voltage, the external current and the charging during the charging process During the process, the impact of charging is small. The other two batteries will discharge the damaged battery until the power is exhausted.


   If 10 batteries are used at the same time, the reliability will be reduced to 10% of a single battery. Some users, using dozens of batteries and connecting to a single battery, believe that reliability can be improved. On the contrary, a single cell of a lithium battery composed of 100 single cells in parallel reduces the reliability of trouble-free working time to 1% of the unit battery life.


   In a lead-acid battery, the separator between the positive electrode and the negative electrode has a small gap of 0.5mm, which short-circuits, resulting in the rejection of the unit battery as a whole, which often happens. In large electric buses, the batteries connected in parallel tend to be higher than 200Ah, and one of them is damaged, causing the entire physical unit to be damaged. In the parallel configuration, four 60Ah batteries are used and connected to a single section, and each battery is damaged by the four batteries without damaging one battery.


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