A mechanical mod is an unregulated type of device. There is no circuitry in the mod to keep the voltage steady throughout the life of the battery. Since you are running directly off the battery you can't increase or decrease the voltage and the vape will slowly become weaker over the lifespan of the battery.
These types of mods are unregulated unless you add your own regulator. This means you need to be very careful when using these especially with home made coils as there is nothing to shut off the power if there is a short in the atomizer. We recommend using a coil ohm meter to test your coils before using them on a mechanical mod.