According to an embodiment, there is provided a battery system. The battery system includes a first battery and a second battery connected in parallel with the first battery. The first battery includes a lead storage battery. The second battery includes a nonaqueous electrolyte battery. The nonaqueous electrolyte battery includes a positive electrode and a negative electrode. The negative electrode includes a negative-electrode-mixture layer, and the negative-electrode-mixture layer contains lithium titanate. The positive electrode contains a positive electrode active material LiMn2-x(a)xO4. A ratio of a battery capacity of the second battery to a battery capacity of the first battery is in a range of 1/133 to 1/2.