A power tool battery pack includes a lower side case, an upper side case fixed to the lower side case, a battery cell in the lower side case and a circuit board connected to the battery cell. A plurality of electrical signal terminals are arranged in parallel on the circuit board, and the circuit board includes at least one slit between adjacent pairs of the plurality of signal terminals to make it difficult for any water that reaches the circuit board to electrically short two of the signal terminals together. Alternately or in addition, a wall structure may be provided between the terminals for reducing the likelihood that water reaching one of the signal terminals will be able to electrically connect two of the signal terminals.