Battery state of charge determination method and battery state of charge determination apparatus

A power generation voltage of an alternator capable of supplying a charging current to a battery is boosted (Step 104). A determination is then made as to whether or not a condition in which the charging current supplied to the battery equals or exceeds a predetermined threshold has been established continuously for a predetermined time after the power generation voltage of the alternator is boosted (Step 112). The battery is determined to be in a low state of charge condition, in which a state of charge of the battery is equal to or smaller than a predetermined state of charge, when it is determined that the condition in which the battery charging current equals or exceeds the predetermined threshold has been established continuously for the predetermined time (Step 114).