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.
This could be a 9 volt battery or 6 volt dry cell for rejuvenation of small, low voltage battery packs, or a 12 volt car battery or charged up power tool battery for higher voltage power packs. If you need more than 12 volts, you can connect two 12 volt batteries in series as shown below to get 24 volts.