the parent units have rechargeable batteries and the bit that you leave with the baby can be mains powered or take 4x AA batteries for use on the move where there's no access to a plug socket, although it will only work for a couple of days on batteries and the parent unit batteries are not rechargable in the unit (you could get rechargable batteries for it, but you'd need to charge them outside of the unit itself).
My friend said that her baby kept wriggling off of this, at which pint the alarm would go off and thus only served to wake everyone and scare the parents. Or doctor also recommended against it. You would be better off following anti-SIDS guidelines (back to sleep, not keeping baby too warm, tight crib sheets, no pillow, proper blanket positioning-I.e. tight and tucked into the foot of the bed, with babies feet near the edge if the bed, no smoking in the house or ideally by any caregiver-even smoke residue carried in from outside increases SIDS risks-no fluffy comforters, pillows or stuffed animals in the crib etc)