The battery can't limit the amps, there's nothing there to do that. So you probably are hitting 15 watts with that resistance, and stressing the battery. It's ratings may be given as continuous discharge (probably that 2.5 you cite, if that's really the battery rating, and not the mod, which CAN limit the amps) or as pulse discharge. You want the continuous rate to be higher than what you'll ever demand from it, or risk battery failure.
If you're using a regulated mod, which will have a controlled maximum amp rating, then with the coil you mention, 1.5 Ω, it may well be "clamping" the voltage lower than you expect to not exceed the mod's amp max. With those values, that would be 3.75 volts.