Most manufacturers recommend an overnite charge, but the power you require to recharge a phone is very minimal, so I would recomend at least 4 hours, unless the battery back up is new then you can charge your phone while charging the battery back up for the full 12 hrs.
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Most manufacturers recommend an overnite charge, but the power you require to recharge a phone is very minimal, so I would recomend at least 4 hours, unless the battery back up is new then you can charge your phone while charging the battery back up for the full 12 hrs.