PURPOSE: To provide a battery pack in which the voltage of each lithium ion battery can be monitored from the outside of case and a battery pack which delivers a signal to the outside of case when an abnormal state, e.g. overcharge or overdischarge, occurs in any one lithium ion battery.

CONSTITUTION: Cell voltage output terminals 1c, 1d, as well as I/O terminals 1a, 1b, are provided on the side face of a body 1 and the joints between the cell voltage output terminals 1c, 1d and lithium ion batteries 2a-2c are connected through resistors 6a, 6b. Consequently, the voltage between the terminals of each lithium ion battery can be monitored from the outside of case and even if the lithium ion batteries are charged or discharged unevenly, overcharge and overdischarge of the lithium battery can be avoided resulting in the protection of the lithium battery against deterioration or damage.