Battery series and parallel connection
Why it is done?
Series and parallel connections of batteries are done to increase total voltage and increasing Ah capacity. Series connections are done to increase total voltage. Parallel connections are made to increase total Ah of battery bank.
Battery series and parallel connection where it is used:
Initial Charging of Batteries:
Series connection of 18-20 batteries of same type/size are normally carried out in factories to initial charge batteries. When more batteries like 54 /108 batteries are to be charged simultaneously, 18 batteries are connected in series in one string. 3/6 such parallel strings will be able to connect all 108 numbers of 12V batteries
In a series connection, the current flowing through a single string will be the same for all 18 batteries.
When 4 such lines are connected in parallel, the current is divided between all the 3 lines. The current will be equal in all strings when the internal resistance of all four are the same. Otherwise, the current will be distributed non-uniformly. Eg 23 %. 27 %, 26% and 24 % in place of the 25% required. The charging time of all 4 lines will be different.
Connecting Batteries in series in systems.
When the batteries are connected in series as in railway undercarriages, or telephone exchanges, it is essential that the cells are matched for Voltage, Ah, capacity and electrical resistance. These batteries are generally discharged partially or completely. During discharge POLARITY. Reverse charging is totally unacceptable since the expander gets oxidized and the negative plate loses capacity. Therefore it is essential that we should match both flooded and VRLA cells in deep discharge applications., the weaker cells get fully discharged first. On further discharge, these weaker cells undergo reverse charge. (What is commonly called CELL REVERSE).
Battery Series and parallel connections in Lead-Acid battery (LAB) and Lithium-ion batteries, (LIB)
Ah capacity of single cells of lithium batteries is low, of the order of 3000 milliamps – 4000 milliamps (3-4 Ah). This requires a very large number of matched cells.
In practice, the cells are connected in series and parallel into a PACK which now becomes the unit module for making battery banks. These undergo matching. In Electric vehicles, such PACKS are the building blocks to make high voltage, higher Ah capacity batteries. A sophisticated Battery monitoring system becomes inevitable & essential for the proper functioning of lithium-ion batteries. This invariably increases the cost of the lithium-ion battery system.
In the case lead-acid batteries, cells can even have Ah capacity of 1500 Ah, 500 times higher compared to the lithium-ion battery cell. Therefore the manufacture of Li-ion batteries calls for a very high degree of control on processing parameters while manufacturing compared to lead-acid batteries.
In any case, it is necessary to measure the cell voltages and internal resistance of individual cells. Matching of cells is an essential step carried out by manufacturers.
When battery series and parallel connection are required, an equalization of current in all strings is obtained by using the circuit given in the diagram below. The interconnection or cross-connection helps in equalizing the current in all 4 strings.
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