On a regulated device, the coil resistance is irrelevant to battery amp draw. 50W will consume the same amount of current regardless of what it is being used for. The right way to figure it is 50W/3.7V = 13.5A. Add a bit more on that to account for regulator inefficiency. It also rises as the battery discharges down to cutoff voltage, which is usually 3.0 or 3.2V.
This holds true even if the coil is 0.5Ω