modality - a method of therapy that involves physical or electrical therapeutic treatment. rehabilitation - the treatment of physical disabilities by massage and electrotherapy and exercises.
Alkaline AA batteries are rated 1.5 volts, while NiMH AA batteries are rated at 1.2 volts. NiMH generally refers to a battery being rechargeable and to the materials from which the battery is constructed.
That means overall, your supply must be between 2.7 and 3.5V so you would probably choose 3.0V e.g. two series connected AA batteries. As the battery starts to go flat, the voltage will decrease and when it gets to around 2.7V the first electronic component might start to operate incorrectly.