I've an emergency fan with an AC/DC button on it, but its manual doesn't say on which setting it should be charged. I've tried charging on both settings, and it can be charged either way. On the back of the fan it reads: "AC 220/240 50/60Hz 19w" The fan's box only says: "DC: Operate when power failure. AC: Operate by AC power directly" My question is on which setting (AC/DC) I should charge it for future use when there's a power failure? Thanks in advance! This is not a homework question.