The purpose of the present invention is to appropriately discharge the internal current of a battery pack and improve the maintainability. A battery system comprises a plurality of battery packs (200) and second ICs (18). The battery packs (200) each have: a lithium-ion battery (16); a main circuit connected in series with the lithium-ion battery (16) to form a power supply path; a second protection circuit (62) connected in parallel with the lithium-ion battery (16) and consuming the internal power under a predetermined condition; and a switching means for cutting off between the lithium-ion battery (16) and the main circuit and connecting the lithium-ion battery (16) and the second protection circuit (62). The second ICs (18) are each connected to the main circuit and second protection circuit of each of the battery packs (200) and each control the input/output of each of the battery packs (200). When detecting the output current or voltage from the protection circuit of any of the battery packs (200), the second ICs (18) notify the user of the abnormalities of the battery packs.