Paralleling strings together greatly increases the complexity of managing the battery pack and should be avoided unless there is a specific reason to use this configuration. In this setup, each string must essentially be treated as its own battery pack for a variety of reasons. In a below example, 2 strings of 8 cells each are placed in parallel.
The absence of any theoretical limitation to the number of parallel strings is borne out by the experience of telecom operators, and at least one battery manufacturer allows up to 16 parallel strings, depending on system voltage.3
Whenever possible, using a single string of lithium cells is usually the preferred configuration for a lithium ion battery pack as it is the lowest cost and simplest. However, sometimes it may be necessary to use multiple strings of cells. Here are a few reasons that parallel strings may be necessary:
Many telecom operators have a policy of installing adequate capacity to support the system load (i.e. no redundancy), but using a minimum of two parallel strings. This is prudent system design for VRLA batteries, in which cells sometimes fail open or near-open, thus disabling a complete string.
The m series battery pack in parallel are named P 1, P 2 …, P m. The n cells and 2 n + 2 MOSFETs in each series battery pack are named B x1, B x2, …, B xn and S x0, S x1, …, S x(2n+1), where x is the serial number of the parallel battery pack (x = 1, 2, …, m). The inductor is named L. Fig. 1.
Due to the low voltage and capacity of the cells, they must be connected in series and parallel to form a battery pack to meet the application requirements. After forming a battery pack, the inevitable inconsistency between the cells will have a serious impact on its energy utilization and cycle life, and even bring safety hazards , .