Battery state deciding method, and automobile

Provided is a battery state deciding method capable of performing the degradation decision of an automobile-mounted lead battery highly precisely. The battery state deciding method comprises the first degradation degree calculating step of calculating the degradation degree due to the discharge of a lead battery by using the discharge quantity of the lead battery and the deterioration degree increment per unit discharge quantity, the second degradation degree calculating step of calculating the degradation degree due to the overcharge of the lead battery by using the overdischarge quantity of the lead battery and the deterioration degree increment per unit overcharge quantity, or the third degradation degree calculating step of calculating the degradation degree of the lead battery by using the degradation degrees calculated at both the first and second degradation degree calculating steps.