Dual battery pack and operating method thereof

A vehicle battery system having a multi battery system according to an exemplary embodiment of the present invention includes: a first battery pack connected in parallel with the alternator and including a plurality of first battery cells connected in series; and a second battery pack connected with the alternator and the first battery pack in parallel and having a capacity that is greater than that of the first battery pack, and a negative electrode of the first battery cell includes a negative electrode active material and the negative electrode active material includes a carbon-based material of which interlayer spacing (d) of a (002) plane is 0.34 nm to 0.50 nm in X-ray diffraction measurement using the Cu-Kα line.