Batterries of hearind s are Zinc–air batteries (non-rechargeable; IEC codes: A, P), and zinc–air fuel cells (mechanically rechargeable) are metal-air batteries powered by oxidizing zinc with oxygen from the air. These batteries have high energy densities and are relatively inexpensive to produce.
Traditionally hearing aid batteries were produced using trace amounts of mercury to assist with conductivity and stabilize internal components, but mercury is no longer used in hearing aid batteries. ... Zinc-air batteries remain stable for up to three years when stored in a room temperature, dry environment

Life span of Hearing Aid Batteries on average basis:

Size 10 - three to seven days.
Size 13 - six to 14 days.
Size 312 -three to 10 days.
Size 675 - nine to 20 days

N.B:Please take it from Hearing aid clinic,because they keep genuine things.