Li-ion batteries should hold a partial charge like that (15%ish) for six months or longer. You could run into issues if the battery is very cold though, e.g., if you leave it at 15% in May and come back to it in December and it's down to (say) 9% but the ambient temperature is -10C (roughly 23F), that 9% might register as zero.SoCalGuy said:
A month, though, should be a doddle.