A battery condition detector configured to detect a micro short circuit of a rechargeable battery is disclosed. The battery condition detector includes a processing part configured to calculate the remaining capacity and the full-charge capacity of a rechargeable battery 200 and to determine the micro short circuit of the rechargeable battery 200 by detecting an overcharge of the rechargeable battery 200 based on a charged capacity charged during the charging of the rechargeable battery 200, the remaining capacity calculated at a calculation time immediately before the start of the charging, and the full-charge capacity calculated before the start of the charging; and a communications part 70 configured to output a signal according to the determination result of the processing part 50.