Due to the widespread use of lithium batteries in the off-grid solar home systems (SHS), to reduce system costs, it is necessary to accurately determine the required battery capacity instead of choosing a larger battery capacity, as well as ways to increase battery life. For the first time in this paper, instead of a fixed depth of discharge battery, the effect of different depths of discharges battery, that occurs during the day and night, as well as the possible and optimal battery charging method to reduce the required battery capacity in a real off-grid SHS have been investigated. It has been shown that with this method, it is possible to use a battery with a lower capacity than the usual selection method. Moreover, the comparison of both battery charging methods shows that the pulse charging method has a significant performance in increasing the battery life and thus reducing the required battery capacity.