A battery balancing system comprising: a plurality of multicell battery packs (2, 7, 8, 9), each including a master module, a slave module, a first CPU configured to read terminal voltages of each cell in the master module and the slave module, and a DC-DC converter configured to control current flowing to each cell of the corresponding multicell battery packs (2, 7, 8, 9) based on a balancing target value to adjust a voltage balance; each master module of the multicell battery packs (2, 7, 8, 9) including a first vertical interface and each slave module of the multicell battery packs (2, 7, 8, 9) including a second vertical interface, the first and second vertical interfaces (4-1, 6-1) included in the multicell battery packs (2, 7, 8, 9) being connected with each other; the battery balancing system further comprising a system controller configured to collect and to calculate cell related data from each multicell battery pack (2, 7, 8, 9), said system controller comprising a second CPU configured to receive, from each first CPUs, data of all cells in the multicell battery packs (2, 7, 8, 9) to provide the balancing target value required for each of the battery packs; wherein the second CPU is configured to output a synchronization signal for each of the cells in the master module and the slave module through the first and the second vertical interfaces (4-1, 6-1), said synchronization signal including a reading hold pulse for holding instantaneous voltages of the cells for a same retention time in the multicell battery pack and a reading balance pulse for performing reading and balancing of all cell voltages in the multicell battery pack with a same timing; and in that each master and slave modules further comprise a control unit (4-3, 6-3) configured to control a cell address or perform a hold control, and an interface (4-2, 6-2); wherein the first CPU is configured to output a balance hold pulse to the control units of the master and slave modules in the battery pack through the interfaces (4-2, 6-2) for performing reading of a cell voltage during balancing, the balance hold pulse not being synchronized with the other multicell battery packs to perform a voltage reading with an individual timing. a plurality of multicell battery packs (2, 7, 8, 9), each including a master module, a slave module, a first CPU configured to read terminal voltages of each cell in the master module and the slave module, and a DC-DC converter configured to control current flowing to each cell of the corresponding multicell battery packs (2, 7, 8, 9) based on a balancing target value to adjust a voltage balance; each master module of the multicell battery packs (2, 7, 8, 9) including a first vertical interface and each slave module of the multicell battery packs (2, 7, 8, 9) including a second vertical interface, the first and second vertical interfaces (4-1, 6-1) included in the multicell battery packs (2, 7, 8, 9) being connected with each other; the battery balancing system further comprising a system controller configured to collect and to calculate cell related data from each multicell battery pack (2, 7, 8, 9), said system controller comprising a second CPU configured to receive, from each first CPUs, data of all cells in the multicell battery packs (2, 7, 8, 9) to provide the balancing target value required for each of the battery packs; wherein the second CPU is configured to output a synchronization signal for each of the cells in the master module and the slave module through the first and the second vertical interfaces (4-1, 6-1), said synchronization signal including a reading hold pulse for holding instantaneous voltages of the cells for a same retention time in the multicell battery pack and a reading balance pulse for performing reading and balancing of all cell voltages in the multicell battery pack with a same timing; and in that each master and slave modules further comprise a control unit (4-3, 6-3) configured to control a cell address or perform a hold control, and an interface (4-2, 6-2); wherein the first CPU is configured to output a balance hold pulse to the control units of the master and slave modules in the battery pack through the interfaces (4-2, 6-2) for performing reading of a cell voltage during balancing, the balance hold pulse not being synchronized with the other multicell battery packs to perform a voltage reading with an individual timing.